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Molecular Semiconductor Fullerite 60
.. Strzhemechny Verkin Institute for Low Temperature Physics and Engineering, Kharkov, Ukraine Outline New allotropic forms of carbon Fullerene 60 molecule; scientific issues and possible applications Fullerite 60: energetics, thermodynamics, rotational dynamics Roto-electric effect Two types of disorder Effect of orientational ordering on other properties Diffusion phenomena Saturation kinetics


Collaborators:
Verkin Institute, Kharkov, Ukraine Dr. Yu.E. Stetsenko Dr. K.A. Yagotintsev Prof. A.I. Prokhvatilov Dr. S.V. Lubenets Prof. L.S. Fomenko Prof. A.A. Avdeenko I.V. Legchenkova
Dr. Eugene Katz, Dr. D. Faiman

Ben-Gurion University of the Negev, Israel

Queens College, CUNY, USA
Leuven Lab, Heverlee, Belgium Umea University, Sweden Vienna University

Prof. Steven Schwarz
Dr. Konstantin Iakubovskii Prof. Bertil Sundquist Prof. Michael Zehetbauer Dr. Erhard Schafler

Funding: Science and Technology Center in Ukraine (grant 2669) Ukrainian-Austrian Collaboration Program (grant M/140)


Fullerenes have been discovered in 1985 by Robert Curl, Harold Croto, and Richard Smalley and named after Richard Buckminster Fuller fullerenes or bucky-balls. In 1996 these scientists were awarded the Nobel prize

C60 molecule is an ideal truncated icosahedron, formed by 12 pentagons and 20 hexagons with the respective number 5-fold and 3-fold (not 6-fold !) axes.



New allotropic forms of carbon
1. Fullerenes
2. Nano-tubes

3. Graphene, a one-atom-thick planar sheet of sp-bonded carbons 4. Astorlenes, self-connected tubular torus, a doughnut (or Russian bublik)


Possible applications
Fullerene C60 can be utilized in applications for these areas: · Combined with porphyrin, ferrocene, etc. in supramolecular structures for various electrical technologies (molecular wiring in batteries, switching devices, variable capacitors) · Conversion of photon energy to electricity (wide response spectrum, high light-to-current transformation efficiency above 50% etc.) · Medicine (transporting carrier of medical molecules) · Hydrogen energetics (as part of the media for storing molecular hydrogen)

One of the important properties of the fullerene molecule in various combinations is its ability to slow down the annihilation of separated charges. This is true for separate molecules in suprastructures and for fullerite, especially in film forms.


Charge disttribution
Bonds within a hexagon are nor equivalent: the bonds shared by two neighboring hexagons are double; the bonds shared by a hexagon and a pentagon are single. It means that every hexagon has an excess of electrons while every pentagon has a deficit of electrons.

Charge distribution anisotropy Anisotropy of intermolecular charge transport


60, , ,
At room temperature, C60 molecules rotate fast (reorientation times below 8 pcsec): the charge distribution anisotropy at atomic level averages out to zero and C60 forms a fcc (Fm3) lattice, mainly owing to vdW forces, with a lattice parameter of a 14.16 е and a nearest neighbor distance of about 10 е.

At 260 K the almost free rotation ceases, the crystal transforms from fcc (Fm3) to simple cubic (Pa3) lattice with 4 sublattices, in which the 5-fold axes are directed along the cube diagonals. The molecules go on rotating, yet not freely but around certain axes, performing jump-like reorientations. Projection along (100)


The crystallographic motive is such that the double bond (excess of electrons) of a C60 molecule, which below = 260 performs infrequent reorientations, can face a hexagon (excess of electrons) of the neighboring molecule or its pentagon (deficit of electrons). Hence, the energetic preference of the so called pentagon mutual orientation. Slightly below the hexa and penta states are occupied almost equally. As the temperature is lowered, the fraction of pentas increase but the reorientation frequency, which is proportional to exp(-E/kT), drops and at 95 the reorientations stop: the system finds itself in the state of orientational glass.

E

E/ 11



hex a penta


As follows from calculations for T = 0 and experimental data, given a complete preferable mutual molecular orientations in pure bulk crystalline C60, the fullerite is a typical semiconductor with a forbidden band width of about 2,3 eV and half filling.

In films, donor and acceptor systems of different nature are inevitably present.
E , eV

C o n d u c tio n m o b ility e d g e C o n d u c tio n b a n d b o tto m A c c e p to r-lik e s ta te F e rm i le v e l D o n o r-lik e s ta te

Mobility gap 2.3

Optical gap 1.6

0 .9 - 1 .1

1 .3

V a le n c e b a n d to p V a le n c e m o b ility e d g e


Roto-Electrical Effects
In a bulk high-quality polycrystal [Chiu et al., Appl. Phys. Lett. (1996)] the conductivity suffers an appreciable (40-70%) positive jump when going down in temperature across the critical temperature Tc

«Our result clearly indicates that the underlying mechanism of the PPC effect involves the motion of the C60 molecule.»


Results concerning the effect of orientational ordering on charge transport in C60 films were in shear contradiction between themselves, depending on sample neatness morphology, film preparation techniques, measurement schemes, etc.

As in crystals (positive jump)

No effect

Opposite effect

{ { {

Mort et al., Appl.Phys.Lett. (1996) H.Yamaguchi et al. J.Phys.Soc.Jpn. (1995) Balberg et al, Fuller.Sci.Technol. (1998) Hosoya et al., Phys.Rev.B (1994) Kazaoui et al., Solid State Commun. (1994) Balberg et al., Fuller.Sci.Technol. (1998)

Kaiser et al., Solid State Commun. (1993)
H.Yamaguchi et al., J.Phys.Soc.Jpn. (1995) Chiu et al., Jpn.J.Appl.Phys. (2002)



Katz, Faiman, Isakina, MAS, et al. J.Appl.Phys. 93 (2003) High-quality films Good quite narrow Bragg reflections. Clear-cut lattice parameter jump across Tc (a/a = 0,22% instead of a/a = 0.32% in bulk samples)
T
13.96 0 50
g

14.16

14.12

a, е

14.08

14.04

14.00

To
150 200 250

Tc
300

100

T, K
14.16

a, е

Low-quality films Broadened reflections, intensive halo due to amorphic fraction. No lattice parameter jump at the transition

14.12

14.08 To 100 150 200 Tc1 Tc2 250 300

T, K


Electrical measurements
High-quality films
Temperature dependence of the dark current (1) and photocurrent (2) for a textured large-grain C60 film on mica. A positive surge near Tc .
340 320 300 280 10
5

260

T (K) 240 220

200

180

DC, PC (arb. units)

10

4

10

3

2 1 3
3.0 3.5 4.0 4.5 1000/T (1/K) 5.0 5.5 6.0

10

2

10

1

Curve 3 is for the same film sample after a 15 minute saturation with oxygen at 100 bar.
Katz, Faiman, Isakina, MAS, et al. J.Appl.Phys. (2003)


Low-quality film samples
290 10
4

280

T (K)

270

260

250

DC, PC (arb. units)

10

3

2

10

2

10

1

1

10

0

3.4

3.5

3.6

3.7

3.8

3.9

4.0

1000/T, (1/K)

When cooled below the orientational ordering point, the currents drop faster with lowering temperature 1 ­ dark current 2 ­ photo current


C60 SPV Spectra Surface Photo Voltage Spectroscopy
Sub-gap photoexcitation

I

III

Valence-to conduction inter-band excitation

0.20 0.16 0.12

SPV (V)

0.08 0.04 0.00 -0.04 0.55 1.10 1.65 2.20 2.75

300 270 260 250 240 230 200 160 120

K K K K K K K K K

3.30

Photon Energy (eV)


All SPV regions are in correspondence with the well resolved relevant features in photo-conductivity spectra as found by different groups [cf. A.Hamed, in Organic Conductive Molecules and Polymers, ed. S.Nalva, Wiley (1997)]. This correspondence suggests that SPV spectra can be employed to monitor the bulk electronic properties of films.

0 .0 6

0 .0 5

V 2 (V )

0 .0 4

0 .0 3

0 .0 2

Non-monotone behavior of SPV signal across the transition qualitatively reproduces the temperature dependence of the dark conductivity
150 200 250 300

0 .0 1 100

T e m p e r a tu r e ( K )


Nature of anomalies
Since the characteristic energy of the intermolecular tunneling is 0.020.04 eV, while the intramolecular attraction energy is 0.1-0.3 eV, solid C60 can be treated as a system with strong coupling and with relatively strong electron correlations.
Ignoring the correlations, the Hamiltonian of an electron in the conduction band is H=T+V Here V describes (quasi)-static disorder, i.e. scattering on phonons and lattice irregularities, and T is the kinetic term of tunneling from site i to site j

T t ( w w n )a a
ij i j ij i

j

The jump probability t essentially depends on the mutual orientation of neighbor molecules i and j. This term is the source of dynamic disorder. Summing up, the unusual property of fullerite as a current-carrying solid consists in that the mutual orientations (which govern intermolecular transport) are controlled by an independent rotational dynamics
MAS and Katz, Fullerenes (2004)


Variation of the conductivity is determined by the varying relation of the static and dynamic disorders.
In pure systems (high-quality films), static disorder is irrelevant. So, transition to the orientationally ordered state, in which mutual orientations favor intermolecular charge transport, causes a conductivity surge. In «dirty» systems (low-quality films), scatterers are numerous and strong. Transitions to any ordered state, in which all motions are progressively hindered, will generally entail strengthening (previously rotationally averaged close to low levels) of irregularities. This, in particular, was documented through an appreciable broadening of Bragg reflections across the ordering transitions in our "dirty" samples or when entering the region of the orientational glass in "good" samples. Then one could expect a certain increase of the efficiency of scatterers and, as a consequence, a decrease in the conductivity.

Presence of oxidizers (oxygen or iodine) not only drastically changes the band structure but also tends to fix molecular positions and orientations.


Intermediate conclusions
A complex of specially planned studies of the roto-electric effect in pure C60 films have been carried out. As a result, a model was suggested which consistently explains the basics of the phenomena observed as well as the existing controversies. In high-quality films the conductivity is essentially higher in the orientationally ordered state. In films containing numerous defects an opposite effect is observed. Our explanation is based on the fact that in C60 semiconductor two types of disorder can compete, the effect of which depend differently on the temperature and the structure morphology and defects of C60 films. Utilization of the promising properties of C60 in any form, especially as a basis of various semiconductor devices encounters certain problems


Problems to tackle
Theory
Overlap integral t is not known as a function of mutual molecular orientations In the orientationally ordered state the conduction problem is clear [cf. Loktev et al., Fiz. Nizk. Temp. (2002)], whereas for the ordered or intermediate cases no theory is available

Polymerization
Two double bonds in molecules in a hexagon configuration can open up to form a covalently bonded (polymeric) dimer or longer polymeric chains. High temperatures and/or pressure and ultraviolet are the hazards promoting polymerization The dimer state is negligible higher in energy compared to the ground state of two non-bonded molecules; these two state are also close in space [Suzuki et al. (2000) Phys. Rev. B].

Mechanical softness
The material is inconveniently soft


Tackling the technologic problems with 60 (oxygen poisoning, ultraviolet hazard, softness) involved a few protection measures (coating, implantation of strengthening elements, etc.), which all proved inefficient. Our main idea was: To saturate fullerite 60 with a dopant which would prevent oxidized poisoning, would strengthen mechanically and ­ simultaneously ­ would not essentially deteriorate its electric properties. Sequence of studies: 1. Detailed saturation kinetics for various chemically neutral species 2. Strength characteristics and the means to affect them 3. Comparative resistometric measurements


Joint penetration from two sides of 60 films of 2 molecules and u atoms
0. 8 0. 7 0. 6
Cu O C

0. 5 0. 4 0. 3 0. 2 0. 1 0 0 5 10 15 20 25 Sputtering Time, min 30 35

Distribution of atomic fractions of Cu, O, and C (in units corresponding Ar ion impact time) in a C60 film on Cu substrate after a 10 months of exposure to air.

Atomic Fractiom

0. 25
Copper Atomic Fraction

0. 2 0. 15 0. 1 0. 05 0 0 0. 02 0. 04 0. 06 0. 08 0. 1 0. 12 0. 14 0. 16 Oxygen Atomic Frac tion

Correlation between atomic fractions of Cu and O as measured at a fixed depth (after I min sputtering) at several points of the film Conclusion: because of the counter-flow up-diffusion (double electric layer) the fullerite with an oxygen inflow from the "external" film side tends to "eat up" copper and other metal substrates at a rate 10-100 е per month

Katz, Faiman, Froumin, Polak, Isakina, Yagotintsev, MAS, Schwarz , Physica B (2001).


Saturation with helium: two-stage process
14,22 14,20

Intercalation of with He

60

0,30 0,27

a, A

d2Q, degr.

14,18 14,16 14,14

0,24 0,21 0,18 0,15
0 100 200 300 400 2500 3000 3500

0

400

800

1200

3800

4000

111 220 311

Time, h

Lattice parameter vs. saturation time; pressure 1 bar; room temperature

Time, h

Reflection width vs. intercalation time

Conclusion: since the reflections are appreciably and systematically narrower upon filling than in the initial state and remain so after degassing, saturation tends to "purge" part of the defects (dislocations) from crystallites.


Filling of voids
a, A

14,22 14,20 14,18 14,16

Intercalation of with He

60

1.0

Da /Dainf, A

0.8 0.6 0.4 0.2 0.0 0

diffusion theory

14,14

0

400

800

1200

3800

4000

Time, h

Filling of octahedral voids T = 310 h D = 7.5 10
100
-14

14,22
2

Filling of tetrahedral voids

cm /s

14,21 14,20 14,19 14,18 14,17
-t / t0

a, A

200

y = y0 - y e

t, h

Conclusion: saturation with helium proceeds in two stages: first, octavoids are filled (inhomogeneously, from surface into inside) and then tetra doids are filled, homogeneously over the volume.

y0 = 14.238 A y = 0.065 A t0 = 3120 h
0 1000 2000 3000 4000

t, h

Effective infusion to tetra-voids is about 10-19 cm2/s, i.. by 5-6 orders slower, than to octa-voids

rattle


Comparison of luminescence spectra
a) pure C
60

b) C60 saturated by helium during 440 h under normal conditions. Octa-system is virtually completely filled c) Difference luminescence spectrum (a) - (b) Finding: luminescence intensity of saturated C60 is lower exactly in the range where emission from polymeric dimers is expected.

Conclusion: C60 with He in octa-voids contains less dimers than the starting material. Remember also that saturation purges defects out of sample.


Correlation between hardness of 60 and orientational ordering
Temperature dependence of the microhardness close to the orientational phase transition (260 ) . Curve 1: polished non-annealed sample; curve 2: polished and then annealed (for 24 h at 10-3 Torr and 300 ) sample. The correlation is evident; it is more clearly seen in a pur (without oxygen) sample

Saturation with chemically neutral particles entails an essential hardening of 60


Conclusions
1. Investigation into the effects of orientational ordering on the conductivity of C60 film allowed us to explain the relevant rotoelectrical phenomena in this molecular semiconductor. The explanation is based on the idea about two types of disorder. 2. The saturation process of C60 with helium and hydrogen was studied, showing that for He two stages exist. Room-temperature diffusion coefficients have been determined. The cause behind the anomalous volume incease of 60 during infusion of particles (), smaller than the size of octahedral voids, was explained. 3. Saturation with helium and neon was found to purge part of defects and to enhance the ability of C60 to counteract polymerization by light.

4. It was found that orientational ordering essentially influences the hardness of 60. Saturation of this material with chemically neutral particles results in a substantial increase of its hardness


Thank you for your attention


Experimental
Sample preparation
Two types of films were grown, both about 100 nm thick. One (high crystallinity) on mica, the other (low crystallinity) on glass, as in [Yagotintsev et al., Physica B (2003), Katz et al., Thin Solid Films (2000)]

Electrical measurements
Dark-current and photo-current measurements at 130-310 K on samples that underwent preliminary in situ at 150 C during 2 hr. The low voltages used ensured Ohmic regimes [Katz et al., J. Appl. Phys. (March 2003), Yagotintsev et al., Physica B (2003)]. Surface Photo Voltage Spectrometry was used [Katz et al., J. Appl. Phys. (December 2003)] in order to check the conclusions of the current measurements for consistency.

Structure studies
Temperature resolved (80 to 300 K) powder x-ray diffraction


Motivation
Basic aim: to establish correlation between the morphology / structure state and charge transport properties, especially, with varying orientational order

What to avoid: (i) Oxygen poisoning of the film surface (ii) Metal substrates Both factors influence drastically the electrical properties of films and even their structure, especially in combination

11


Trapping present



DC

=enF

F (< 1) is the dimensionless factor that accounts for carrier trapping

What changes across Tc - F or ?
In single crystals is known to increase by 40-70% in the ordered state. Of course, the trapping parameters that determine the magnitude of F (concentration, capture probability, energy distribution) may change in the same way, though a clear understanding how this could occur is absent.



8.0 6.0

L , 1 0 , c

2

-6

4.0 2.0 0.0 -2.0 -4.0

1

3

0

200

400

t,

600

800

1000

60 , , . , ( ), , -, 60 V.G. Manzhelii, A.V. Dolbin, V.B. Esel`son, V.G. Gavrilko, D. Cassidy, G.E. Gadd, S. Moricca, and B. Sundqvist, (2006).