Andrea Benassi!EMPA - Materials Science and Technology (Zurich)

Does rotational melting make molecular crystal surfaces more slippery?

Phase transitions to control friction

Substrate

Tip

Nano-manipulation!The properties of the materials/adsorbates are modified acting

with the tip

Active friction control!The tip frictional properties and its sliding motion are controlled

by the substrate

Substrate

Tip/Slider

Substrate

Tip

Nano-manipulation!The properties of the materials/adsorbates are modified acting

with the tip

Active friction control!The tip frictional properties and its sliding motion are controlled

by the substrate

Substrate

Tip/Slider

Phase transitions to control friction

Phase transitions to control friction

SubstrateSubstrate

Tip

Nano-manipulation!The properties of the materials/adsorbates are modified acting

with the tip

Active friction control!The tip frictional properties and its sliding motion are controlled

by the substrate

Substrate

Tip/Slider

Phase transitions to control friction

In order for the substrate to take action, it must have some flexibility in the physical properties involved in the interaction with the tip. !!Such a flexibility can be provided by the occurrence or simply by the presence of a phase transition of some kind. !!If we can control the order parameter of the phase transition with some external field or with the temperature we can reversibly adjust the frictional and sliding properties of the tip on-the-fly. Substrate

Active friction control!The tip frictional properties and its sliding motion are controlled

by the substrate

Substrate

Tip/Slider

Phase transitions to control friction

In order for the substrate to take action, it must have some flexibility in the physical properties involved in the interaction with the tip. !!Such a flexibility can be provided by the occurrence or simply by the presence of a phase transition of some kind. !!If we can control the order parameter of the phase transition with some external field or with the temperature we can reversibly adjust the frictional and sliding properties of the tip on-the-fly.

Charge density waves in NbSe2 T<70K Langer et al. Nature Materials 13 (2014) 173

!!!!!

Antiferro transition in SrTiO3 T=120K M. Kisiel talk

Pb superconducting transition T=7K Dayo et al. PRL 80 (1998) 1690

Ala-Nissila et al. PRL 68 (1992) 1866 Ala-Nissila et al. Adv. Phys. 51 (2002) 949 !

In linear response theory the diffusion coefficient of an adsorbate interacting with a substrate is related to the Structure Factor of the substrate. !!!In presence of a structural phase transition the Structure Factor goes to zero and the adsorbate dissipation has a divergence! !

Benassi et al. PRL 106 (2010) 256102 !With MD (beyond linear response) we showed that also the sliding friction of an AFM tip experiences a strong dissipation peak at the critical point of a structural phase transition occurring in the substrate.

A general theory

Two examples: - Rotational melting transition in molecular crystals to control nano-scale friction - Magnetic domains to control meso-scale sliding motion

prototype structural phase transition

Tip

generic substrate

adsorbate

1

D/ � / h⇢(r)⇢(r0)i

Ala-Nissila et al. PRL 68 (1992) 1866 Ala-Nissila et al. Adv. Phys. 51 (2002) 949 !

In linear response theory the diffusion coefficient of an adsorbate interacting with a substrate is related to the Structure Factor of the substrate. !!!In presence of a structural phase transition the Structure Factor goes to zero and the adsorbate dissipation has a divergence! !

Benassi et al. PRL 106 (2010) 256102 !With MD (beyond linear response) we showed that also the sliding friction of an AFM tip experiences a strong dissipation peak at the critical point of a structural phase transition occurring in the substrate.

A general theory

Two examples: - Rotational melting transition in molecular crystals to control nano-scale friction - Magnetic domains to control meso-scale sliding motion

prototype structural phase transition

Tip

generic substrate

adsorbate

1

D/ � / h⇢(r)⇢(r0)i

Rotational melting to control nano-friction

fcc

sc

The ideal stable material hosting a simple 1st order phase transition close to room temperature is Fullerite, a molecular crystal made of C60 molecules.

T= 50K T= 300K

The (111) surface

SEM$$$$

Op'cal$Microscope$$$$

AFM$on$(111)$surface:$topography$(a)$and$dissipa'on$(b)$

$$$

STM$on$(111)$surface$$

empty$states$im

age$$

BS$$$$

BS$$$$

BS$$$$

At the (111) surface the BS molecules are less bounded and they start to rotate early followed by the rest of the surface molecules and only later by the bulk molecules.

Monte Carlo simulation from Laforge et al.

[PRL 87, 085503 (2001)]

Experimental evidences

fcc

sc

The ideal stable material hosting a simple 1st order phase transition close to room temperature is Fullerite, a molecular crystal made of C60 molecules.

In the sc phase the double bonds in each C60 preferentially face the pentagons of the neighboring molecules. Searching the literature for already existent friction measurements on Fullerite surfaces we found:

Liang et al. PRL 90 (2003) 146102 Liang et al. J. Phys. Chem. B 110 (2006) 403

Their explanation: due to the fast C60 rotation, the angular dependence of the intermolecular potential is averaged out resulting in a weakening of their interaction. Using many models available in literature for the C60-C60 interaction, it is easy to prove that this weakening is not sufficient to justify a drop of a gator two in the cohesive energy.

Our simulations

U(ri,j) = 4�

✓⇤

ri,j

◆12

�✓

⇤

ri,j

◆6�+

1

4⇥�0

qiqjri,j

For our simulations we used the potential by Sprik et al. (J. Phys. Chem. 96 (1991) 2027) treating every molecule as a rigid ball with 90 interaction centers having both short and long range interactions:

we first used MD to reproduce the bulk and surface phase transitions. Then we started to simulate sliding friction and pull-off experiments.

~ 400000 atoms 24 h runs on 4096 cores with LAMMPS MD code

Our simulations

U(ri,j) = 4�

✓⇤

ri,j

◆12

�✓

⇤

ri,j

◆6�+

1

4⇥�0

qiqjri,j

For our simulations we used the potential by Sprik et al. (J. Phys. Chem. 96 (1991) 2027) treating every molecule as a rigid ball with 90 interaction centers having both short and long range interactions:

we first used MD to reproduce the bulk and surface phase transitions. Then we started to simulate sliding friction and pull-off experiments.

Change of commensurability

U(ri,j) = 4�

✓⇤

ri,j

◆12

�✓

⇤

ri,j

◆6�+

1

4⇥�0

qiqjri,j

For our simulations we used the potential by Sprik et al. (J. Phys. Chem. 96 (1991) 2027) treating every molecule as a rigid ball with 90 interaction centers having both short and long range interactions:

we first used MD to reproduce the bulk and surface phase transitions. Then we started to simulate sliding friction and pull-off experiments.

3"thermostat"layers""

Change of commensurabilityWhich is the reason for such a commensurate-incommensurate transition at the critical point? The only missing ingredient in our simulations is the tip (no explicit simulation is possible due to lack of information)

Tip

C60 substrate

C60 flake

The C60 flake is anchored to the tip. To tilt it, a static friction torque Tt must be exerted on it. !The phase transition can change the total torque Ts exerted on the flake by the substrate. !If Ts(<250K) < Tt < Ts(>250K) the phase transition can trigger a change in commensurability.

…an entropy gain could also be the cause of the flake tilting. !!This work is done in collaboration with:

A. Vanossi E. Tosatti C.A. Pignedoli D. Passerone

More information at: https://sites.google.com/site/benassia/

Thank you!

Modeling material properties !

at different length scales