G. Palasantzas, W. Broer, B. J. Kooi, J. KnoesterZernike Institute for Advanced Materials, University of Groningen, The Netherlands

V. B. SvetovoyMESA+ Institute for Nanotechnology, University of Twente, The Netherlands

M. WuttigI. Physikalisches Institut (IA) and JARA-FIT, RWTH Aachen University, Germany

PASI Frontiers of Casimir Physics 2012, Ushuaia, Argentina

Casimir force control between real materials

for MEMS applications

60 groups 11

countries 

Network CASIMIR

Nature inspires…..humans copy …!

Boesel et al., Advanced Materials 2010

Stanford researchers with the Stickybot

If all 6.5 million setae of one gecko toes attached simultaneously it lifts 30-40 kg

….am I a “”Drude”gecko or .… a “plasma”….one…

Nature inspires…humans copy …!

…..glue .with .vdW . forces …..!

A photo showing a stainless steel adapter of 473 g hanging on a SiO2/Si-wafer supported vertically aligned SWCNT dry adhesive film (4mm x 4mm) http://www.nanowerk.com/spotlight/

Geim et al., Nature Materials 2003

Kwak et al, Advanced Materials 2011

Micro/nano technology ……!

Device / engines concepts……..

Stiction & Levitation

Quantum engine cycle between two Casimir force states – tap energy from vacuum (F. Pinto PRB 1999)

Engines

MEM Switches

Studies aimed to tame the Casimir force …with optical properties manipulation……

• Switchable mirrors (2003)• Si –carrier enhancement (2007)• PCM systems (2010--)• ITO / UV-ITO (2011---)

Quantum Engines

Intersting idea but…..no clear force contrast !

Chen et al, Opt. Express 2007, PRB 2007

Si-illunination

Chang et al., PRL 2011, PRB 2012

ITO / UV-ITO

Control Optical properties: Phase change materials (PCM): Amorphous crystalline phase transitions / 2010-now…….

…more than a “billion switches per year”….

A laser beam heats the PCM above the melting temperature & Rapid quenching glass-like amorphous state

heating the amorphous phase with moderate intensity laser beam: amorphous state more stable crystalline phase

This principle has already been successfully employed in three generations of rewriteable (RW) optical data storage (CD, DVD, BD)

Pandian et al., Adv. Materials 2007

Pandian et al.,Adv. Materials 2007

Phase change materials Amorphous (A) crystalline (C) phase transitions

Crystallization is accompanied by the formation of resonance bonds

Change of bonding mechanism affects the optical properties in the visible & UV range

So far no other materials have similar change of optical properties upon crystallization

Crystallization generates of a large amount of free carriers affecting the IR range

PCM candidates

The situation in which a single, half-filled p-band forms two bonds to the left and right more than allowed by the 8-N rule (N valence) was coined as resonant bonding by Pauling.

electron delocalization characterizing resonant bonding increased electronic polarizability - a fingerprint of resonance bonding

Resonant bondingFree carriers /Drude relevant for optical data storage: photon energies between 1.5 eV (compact discs) - 3 eV (blu-ray disk) are employed /)( eV, 9.8

)/()( eV, 07.0

3

2

A

iC p

Phys. Rev. A 2010,  New Scientist July 2, 2010 (interview with Eugene Reich)

Force measurement in UHV (Dynamic AFM)

Casimir force upon crystallization is due to the change of the dielectric function

• in the infrared regime (free carriers) ……?

• or in the visible-UV range (resonance bonding)….?

• Or both and how much they contribute ?

/)( eV, 9.8

)/()( eV, 07.0

3

2

A

iC p

Extrapolation

1600 |/ :Au toCompare

17 |/ :ITO toCompare

eV 07.0for 6.10|/

ITO

2

2

2

Au

AIST

p

p

p

Crystalline PCM: free carriers in many cases (1020/cm3)

But have very small mean free paths, even below 3 nm

We substracted form the crystalline phase the Drude contribution…….

How to achieve high force contrast in PCM ?

Two strategies to maximize the force contrast upon crystallization of phase change materials: • employ materials with particularly strong

resonant bonds (PCM with low iconicity and low hybridization of s- and p-states)

• small disorder and large carrier concentration to ensure a large Drude contribution

Future …

…plethora of issues to be resolved to make a

device…..

MEM Switches

Take roughness contributions for both Casimir and electrostatic forces……

Broer et al, EPL 2011, PRB 2012

Broer et al, EPL 2011, PRB 2012

Flat surface Pertubative term Peak contribution

….compare Electrostatic – Casimir forces….

Cas: F ( ) ( )eq elV z F z

No electrostatic force (V=0)

: Bifurcation parameter

in certain situations typically two equilibria occur: • a stable center around which periodic solutions exist,• unstable saddle point

=0

(z)=(1-z/Lo)(FL(Lo)/FCas(z)

[(z)/ cas-1]Fcas(z)=0(z)/= cas

electrostatic force absent (V=0 V)

Stable regionLo=100 nm

….average power laws of the force…..

….Good concensus among various groups…..…!

…..Conclusions……

Surface roughness strongly influences actuation dynamics and proper roughening can benefit systems against stiction/pull-in instabilities (see poster W. Broer) .... ............take into account hydrodynamic forces for rough surfaces... Work in progress

PCMS are promissing for Casimir engines .....more research is needed to fabricate materials to have larger force contrast - Two strategies to maximize force contrast of PCMs:

• employ materials with particularly strong resonant bonds (low ionicity and low s-p hybridization)

• small disorder and large carrier concentration to ensure a large Drude contribution