Athermal Photofluidization of Glasses or Controlling Viscosity with Light

Transcription

1 Athermal Photofluidization of Glasses or Controlling Viscosity with Light Guanjiu Fang, Joe Maclennan, Youngwoo Yi, Matt Glaser Department of Physics University of Colorado, Boulder Matt Farrow, Eva Korblova, Dave Walba Department of Chemistry and Biochemistry University of Colorado, Boulder Tom Furtak Department of Physics Colorado School of Mines Dmitry Bedrov Department of Materials University of Utah Nat. Comm. (2013)

2 azobenzene trans hν, k B T cis

3 azo based photo-active glassy polymers Barrett Broer Ikeda Wright- Patterson

4 azo-based tethered molecular monolayer (dmr) u precursor (D. Walba) (D. Bedrov M. Glaser)! u u covalently bonded to glass surface photo-orients in-plane Weigert mechanism: u photo-orientable 2D XY system!

5 optical absorbance of the dmr monolayer 514 nm wavelength u compare to solution (monolayer ~70% coverage) u a fluence of 20mJ/cm 2 gives 1photon absorbed/molecule (pa/mol)

6 Photo-orientation t=0 After 9ns

7 writing dynamics

8 dynamic measurement of in-plane birefringence u u u red probe light (632 nm, focused to 40µm) green write light (514 nm focused to 1mm) 100 µsec time resolution u high extinction contrast between crossed polarizers (2.4 x )

9 a 2D nematic (smectic C monolayer) φ(r) degennes (1972) D = K/ γ 100 µm Δn(t) ~(t/τ) α α = T/8πK τ = a 2 /D = a 2 γ/k τ 10 psec

10 comparing SmC* and dmr monolayers t (sec)

11 tethered monolayer is a glass t (sec) γ DMR ~ γ SmC

12 erasure by k B T at T = 300K τ U t ~ k B T ln(10 11 ) = 7,500K

13 dmr monolayer is an orientational glass τ γ dmr ~ γ LC γ ~ Gτ, τ ~ 1sec

14 The barrier gap (activation energy) τ τ (U) = τ r exp(u / k B T ) T m = 860K η = k B T / U m = T / T m U t ~ k B T ln(10 11 ) = 7,500K

15 erasure by k B T at T = 300K Δn(t) 1/[1 + t/τ] η

16 T = 300K and η = T/T m so T m = 860K the power law tail

17 erasure by circularly polarized light T = 300K and η = T/T m so T m = 860K

18 crossover to photo-controlled erasing

19 T m increases with writing fluence

20 increasing erase intensity

21 τ occurs at F = 1 photon absorbed/molecule QE ~ 1

22 erasure by circularly polarized light T = 300K and η = T/T m so T m = 860K T m = 860K and η = 0.83 so T = 710K

23 every absorbed photon induces a local glass transition U t = 7,500 T = 800K gives QE ~ exp(-7,500/800) but we find QE ~ 1! QE ~ 1 This means that each 800K photo event melts (fluidizes) the local 7,500K barrier, i.e. produces a local glass transition

24 the photo-absorption event u hν = 2.4 ev u T = hν/k B = 29,000K ~2 ev u each molecule: ~100 motional degrees of freedom Tiberio, Muccioli, Berardi, & Zannoni, ChemPhysChem (2010)

25 the photo-absorption event

26 writing dynamics

27 T m increases with writing fluence

28 orientational work hardening at large fluence δδn ~ log(t) δt m ~ log(t) 1 P(t) = tν w exp(-δt m (t)/t) Nabarro Mat. Sci. and Eng. A (2001)

29 azobenzene trans hν, k B T cis

30 summary u photo-indiced relaxation is essentially isothermal u thermal and photo-induced relaxation events occur by random sequences of discrete local relaxation events. u isomerizing molecules attack their confining barriers with an effective T ~ 800K u the T ~ 800K attacks produce local glass transitions, such that every photon generates a barrier escape event

31 the end

32 Q(t) = A barrier height distributions τ(u) = τ r exp(u /k B T) e (t /τ) H(τ)dτ = e (t /τ(u )) f (U)dU 0 0 τ = τ r exp(u(t) / k B T ) U() = k B T log(/τ r )

33 it s an orientational glass T m = 860K U t ~ k B T ln(10 11 ) = 7,500K

34 dependence on writing and erasing local collective

35 azo-based tethered molecular monolayer u precursor (a derivative of methyl red - dmr) u u covalently bonded to glass surface photo-orients in-plane Weigert mechanism: u photo-orientable 2D XY system

36

37 a different model u orientational diffusion with a divergent distribution of waiting times: Q(t) = exp(-t/τ) H(τ) dτ u τ = τ ο exp (Δ/kT), the rate is determined by activated hopping with barrier Δ u f(δ) = (kt o ) -1 exp(- Δ/kT o ), the barrier is distributed with a mean width kt o u With these assumptions Schlesinger shows H(τ) (τ ο /τ) β with β -1 = T/T o

38 barrier model results u integration gives Q e (t) = [1 + (t/ τ ο ) ] (1-β), with β > 1 u power-law writing and erasing: β controls erasing: Δn(t) ~ Q e (t) = (t/τ ο ) (1-β) = (t/τ ο ) -T/To at large t writing: Δn w (1 - Q e (t)) (t/ τ ο ) at small t u β depends on net energy that generated the written state through mean barrier height T o β -1 = T/T o u T o increases as log(dose) T o δ(t)

39 coverage measured by absorbance 1 σ iso 2 N = s 3 2 A A : absorbance θ : tilt angle ln10 sin θ N s : number of molecule/area σ iso = cm 2 (d-mr in chloroform at 40 nm) nm N s monolayer (~ 70% coverage)* * AFM shows uniform layer morphology

40 aligns nematics ~2 absorbed 450 nm photons/molecule TN cell transmission Yi & Furtak, APL 2004

41 reorienting the writing polarization (+45º to -45º) 0.41 (counts/msec)

42 model (motivated by data) u Q(t) = in-plane nematic-like order parameter Δn u transmission (thin layer) = T Δn 2 Q 2 P A u model: orientational diffusion with a divergent distribution of waiting times: relaxation: Q e (t) = H(τ) * exp -(t/τ) α dτ where data indicates: H(τ) ~ (τ o /τ) β * exp -(τ o /τ) α

43 erasing the birefringence with circular polarized light

44 statistics of extreme values of random functions Schlesinger, Ann. Rev. Phys. Chem. (1988) Scher Shlesinger distribution Schleshinger, Ann. Rev. Phys. Chem. (1988) f G (U) = {β/[u m Γ(1/β)]}{exp-[U/U m + exp-(βu/u m )]} H G (τ) = [α/γ(h/β)τ t ] [exp-(τ t /τ) α ] [τ/τ t ] -(η+1) Gumbel distribution Δn(t) Q G (t) = 1/[1 + (t/τ t ) α ] η/α

45 dmr monolayer is an orientational glass τ γ dmr ~ γ LC γ ~ Gτ, τ ~ 1sec