Liquid/Semiconductor Interfaces through Vibrational Spectroscopy R. Kramer Campen Physical Chemistry Department Fritz Haber Institute of the Max Planck Society 1 / 37 25/3/13 w/ Maria Sovago, Cho-Shuen Hsieh, Mischa Bonn, Ana Vila Verde

Outline I.Justification: why vibrational spectrocopy? II.Background: making vibrational spectroscopy interface specific: vibrational sum frequency spectroscopy III.Interfacial Solvent (Water): structure and dynamics IV.Semiconductor Interface: optical detection of surface phonons 2 / 37 25/3/13 Outline

Why vibrational spectroscopy? 3 / 37 25/3/13 Vibrations report, label free, on inter/intramolecular forces (and structure) I. Justification Barth and Zscherp (2002) Quarterly Reviews of Biophysics, 35, 4 from Zscherp and Heberle (1997) Journal of Physical Chemistry B, 101, Spectrum of local oscillators modified by environment, 1.Through-bond coupling 2.Hydrogen bonding 3.Transition dipole coupling Macromolecules (proteins)

4 / 37 25/3/13 Why vibrational spectroscopy? Vibrations report, label free, on interatomic/molecular forces (and structure) Liquids (water) I. Justification OH stretch modulated by anharmonic coupling to low frequency modes.

5 / 37 25/3/13 II. Background Interface specific vibrational spectroscopy Vibrational Sum Frequency Spectroscopy ir vis sfg ν=0 forbidden in bulk water  (2) =0 ν=1

6 / 37 25/3/13 II. Background What is detected? The field radiated by the second order induced polarization Induced polarization can be expanded in a Taylor series Assuming two incident plane waves and just considering the second order term,

7 / 3725/3/13 Origin of Interfacial Specificity Even order nonlinear susceptibilities Second order nonlinear susceptibilities are third rank tensors … Changing the sign of all indices is equivalent to inverting the axes: the material response must change sign… But, with inversion symmetry, all directions are equivalent so… For materials with inversion symmetry (χ (2) =0), inversion symmetry is always broken at interfaces. II. Background

8 / 3725/3/13 Origin of Chemical Specificity Raman scattering off an excited, coherent, vibration II. Background VSF active modes must be Raman and IR active.

9 / 3725/3/13 III. Solvent to Interface: Structure III. Approaching the Interface from the Solvent Side: Interfacial Water Structure

10 / 3725/3/13 The OH stretch at the air/water interface Two (or three) qualitative populations Free OH IR frequency (cm -1 ) 3000 IR Abs: Liquid Water Hydrogen Bonded OH I SFG (arbitrary units) free III. Solvent to Interface: Structure VSF spectral features are apparent that are absent in bulk IR.

11 / 3725/3/13 III. Solvent to Interface: Structure Is the double peaked feature general? Yes! Double peaked feature appears at all interfaces Kataoka,…, Cremer (2004) Langmuir, 20(5), 1662 Becraft and Richmond (2005) Journal of Physical Chemistry B, 109(11), 5108

12 / 3725/3/13 III. Solvent to Interface: Structure Is the free OH feature general? It appears at all hydrophobic interfaces CCl 4 / H 2 O Hexane / H 2 O Air/ H 2 O Scatena, Brown, Richmond (2001) Science, 292, 908 Silica/OTS/Water Interface Ye, Nihonyanagi, Uosaki (2001) PCCP, 3(16), 3463

13 / 3725/3/13 III. Solvent to Interface: Structure IR frequency (cm -1 ) liquid water 3000 I SFG Du,…, Shen(1993) Physical Review Letters, 70(15), 2313 ice weak ‘water-like’ strong ‘ice-like’ What causes the double peaked feature ? One idea: interfacial structural heterogeneity

14 / 3725/3/13 What causes the double peaked feature ? A second idea: symmetric/asymmetric stretch O-D asym symm (same water molecule) III. Solvent to Interface: Structure

15 / 3725/3/13 Two peaks should collapse to one SS AS Sym/Asym hypothesis D2OD2O HDO SSAS Strong Weak D2OD2O HDO ‘Ice/Liquid-like’ hypothesis The amplitude of the whole spectrum should change. How can we distinguish these scenarios experimentally Isotopic dilution III. Solvent to Interface: Structure

16 / 3725/3/13 Two peaks collapse to one double peaked spectral feature ≠ water structure SFG intensity (arb. units) IR frequency (cm -1 ) O-D C-H water / lipid ÷10 water/air H 2 O:D 2 O 2:1 pure D 2 O H 2 O:D 2 O 2:1 pure D 2 O III. Solvent to Interface: Structure

17 / 3725/3/ IR frequency (cm -1 ) water/air bend overtone (  OH =2) O-D asym symm  OH =2 * ** D 2 O  HOD switches off coupling H D * Two peaks have same symmetry! # # Gan,…, Wang (2006) Journal of Chemical Physics, 124, Are the two peaks sym/asym? III. Solvent to Interface: Structure

18 / 3725/3/13 frequency stretch mode DOS coupling off coupling on Evans window bend overtone (  OH =2) IR frequency (cm -1 ) water/air bend overtone (  OH =2) ** Hypothesis one: two peaks are from a Fermi Resonance with the Bend Overtone III. Solvent to Interface: Structure

19 / 3725/3/13 Hypothesis two: low frequency peak from collective effects (intermolecular coupling) III. Solvent to Interface: Structure Buch et al. (2007) Journal of Chemical Physics, 127(20), coupling switched off Computation suggests the low frequency peak is a the result of intermolecular coupling (vibrational delocalization).

20 / 3725/3/13 In either case using HDO allows more straightforward access to structure Fitting the HDO data III. Solvent to Interface: Structure

25/3/13 III. Solvent to Interface: Dynamics IR frequency (cm -1 ) 3000 IR Abs: Liquid Water I SFG (arbitrary units) free Free OH 21 / 37

25/3/13 III. Solvent to Interface: Dynamics Probing dynamics: IR pump – VSF Probe IR (100 fs) vis (100 fs) SFG v = 0 v = 1 IR pump (100 fs)  wait SFG signal decreases due to depletion of ground state. Recovery reflects vibrational relaxation and possibly reorientation.  wait SFG pump 22 / 37

25/3/13 III. Solvent to Interface: Dynamics Probing reorientation of the free OH 23 / 37

25/3/13 III. Solvent to Interface: Dynamics different rates the same rate Signals relax at… At long times… signals do not convergesignals converge Developing intuition for the signal 24 / 37

25/3/13 III. Solvent to Interface: Dynamics Free OH relaxation is intermediate τ p, pump = 640 ± 20 fs τ s. pump = 870 ± 50 fs 25 / 37

25/3/13 III. Solvent to Interface: Dynamics A brief simulation interlude Simulation box is 30*30*60 Å. Periodic boundary conditions. NVE at T = 300 K. Simulation run for 2 ns (step = 1 fs). SPC/E potential. 26 / 37

25/3/13 III. Solvent to Interface: Dynamics  = yes θ = maybe (diffusion within a potential) 〈ϕ2〉〈ϕ2〉 free OH bulk H 2 O 〈ω2〉〈ω2〉 Free OH reorientation is diffusive (in MD) 27 / 37

25/3/13 III. Solvent to Interface: Dynamics Consistent with diffusion in a potential Free OH has a preferred distribution in θ 28 / 37

25/3/13 III. Solvent to Interface: Dynamics Model reorientation as diffusive in a potential 29 / 37

25/3/13 III. Solvent to Interface: Dynamics D  = 0.32 rad 2 /ps, D  = 0.36 rad 2 /ps Fitting 2D diffusion model to simulation output 30 / 37

25/3/13 III. Solvent to Interface: Dynamics MD results describe data without adjustable parameters 31 / 37

Bulk (arb orient) D ϕ = 0.1 rad 2 /ps D θ = 0.1 rad 2 /ps Free OH D ϕ = 0.32 rad 2 /ps D θ = 0.36 rad 2 /ps On average, free OH reorient ≈ 3x faster than bulk 25/3/13 III. Solvent to Interface: Dynamics Free OH reorientation relative to bulk 32 / 37

33 / 37 IV. Solid to Interface Phonon spectroscopy in bulk α-qtz Gonze, Allan, Teter (1992) Physical Review Letters, 68(24), 3603 Probing these modes optically (IR or Raman) depends on: 1.Mode symmetry 2.Tranverse (couples to IR) v. longitudinal (does not) 25/3/13

34 / 3725/3/13 VSF probes both IR and Raman active transverse optical phonons: a simplified phonon spectrum that reflects bulk symmetry. VSF phonon spectroscopy in bulk α-qtz Liu and Shen (2008) Physical Review B, 78(2), IV. Solid to Interface 795 cm cm cm -1

35 / 3725/3/13 VSF surface phonon spectroscopy on α-qtz and amorphous SiO 2 IV. Solid to Interface Resonances appear in VSF response at, 1.nonbulk frequencies 2.nonbulk symmetries 3.for amorphous material 4.and are perturbed by surface chemistry Liu and Shen (2008) Physical Review Letters, 101(1),

36 / 3725/3/13 IV. Solid to Interface VSF surface phonon spectroscopy on α-qtz for tracking surface reconstruction wavenumber (cm -1 ) after being baked at 100°C rehydroxylated in ambient air after being baked at 500°C rehydroxylated in ambient air rehydroxylated in boiling water SiSi SiSi O SiSi OH Liu and Shen (2008) Physical Review Letters, 101(1), SiSi OH 2 SiSi SiSi O H2OH2O + surface damage = quartz = amorphous silica +

Conclusions 37 / 3725/3/13 Conclusions 1.Using VSF spectroscopy one can probe both interfacial solvent (most work on water) and surface phonons at buried interfaces. 2.Adding additional pulses to VSF experiments makes it possible to probe structural and energy relaxation dynamics with interfacial specificity. 3.Future work: optically probing the association of interfacial molecules with particular surface sites.

Related Publications (from our previous work the results of which were discussed above) 1.Sovago, Campen, Wurpel, Müller, Bakker, Bonn (2008) Vibrational Response of Hydrogen-Bonded Interfacial Water is Dominated by Interfacial Coupling, Physical Review Letters, 100, Sovago, Campen, Bakker, Bonn (2009) Hydrogen bonding strength of interfacial water determined with surface sum-frequency generation, Chemical Physics Letters, 470, 7 3.Hsieh, Campen, Vila Verde, Bolhuis, Nienhuys, Bonn (2011) Ultrafast Reorientation of Dangling OH Groups at the Air-Water Interface Using Femtosecond Vibrational Spectroscopy, Physical Review Letters, 107(11), Vila Verde, Bolhuis, Campen (2012) Statics and Dynamics of Free and Hydrogen- Bonded OH Groups at the Air/Water Interface, Journal of Physical Chemistry B, 116(31), 9467 bonus page25/3/13