Effect of nanoscale geometry on molecular conformation Vibrational
Effect of nanoscale geometry on molecular conformation: Vibrational Sum-Frequency Generation of alkaynethiols on metal nanoparticles by Champika N. Weeraman Achani Yatawara, Andrey N. Bordenyuk and Alexander V. Benderskii* Department of Chemistry Wayne State University Detroit, MI 48202
Nanoscale geometry – molecular conformation ►Expect a relationship between the substrate geometry (nano scale) and conformation of the adsorbed molecule
Nanoscale geometry – molecular conformation vs. Flat surface Curved surface
Molecular conformation in nanostructured materials Bio sensors-bio markers Gratzel-solar cells Aerosols
Vibrational sum frequency generation (VSFG) Sum-frequency generation (SFG) originates from induced polarization Virtual State In media with inversion symmetry In media without inversion symmetry ►Broad band VSFG S( ) vis IR SFG - CCD image + IR Broad-band IR pulse (temporally short) vis Spectrally narrow (temporally long) Vis pulse SFG= IR+ vis
Experiment Flat Surface Curved surface + - r+ d FR r - Chain length ~1. 6 nm Case I: Case II: gauche defects Increase surface curvature all trans (zig-zag) strong CH 3 transitions, weak CH 2 transitions More CH 2 transitions ►Strong CH 3 stretch transitions (r+, r+FR, r-) ►Weak CH 2 stretch transitions (d+, d-) Change of trans Molecular ►predominantly zig-zagconformation molecule arrangement with few SFG sensitivity ? gauche defects
TEM images: 1 -DDT capped Au and Ag. NPs (I) Gold nanoparticles <d>=1. 8 nm (a) Meliorum Technologies, Inc. <d>=2. 9 nm (b) 20 nm <d>=7. 4 nm (c) 20 nm <d>=3. 6 nm <d>=7. 9 nm <d>=23 nm (d) 100 nm (II) Silver nanoparticles <d>=1. 8 nm (a) (b) 20 20 nm nm (c) 20 20 nm nm <d>=24. 6 nm (d) 100 nm
VSFG 1 -DDTcapped Au. Nps (SSP polarization) d + r+ d- r-op ► Both CH 2 stretch and CH 3 stretch transitions easily observable ► Increase of relative intensity of CH 2 stretch vs. CH 3 stretch with decreasing particle size
SFG Spectral Fitting: Quantitative analysis of SFG spectra Intensity of SFG signal (SSP polarization, SFG-Vis-IR) Effective non linear susceptibility: Multi-Lorentzian app. Гj Bj = amplitude Γj = Lorentzian line width ωj = transition frequency ANR= nonresonant contribution Intensity of mode j; Гj
Size dependent SFG spectra gauche defects d+/r+ Chain length ~1. 6 nm d-/r- Chain length ~1. 6 nm
VSFG 1 -DDT capped Ag. NPs (SSP polarization) + d+ r d- r-(op) 24. 6 nm d-/r 7. 9 nm d+/r+ 3. 6 nm 1. 8 nm ►Transitions are broader than Au. NPs (Au. NPs: 10 -12 cm-1 Ag. NPs: 15 -23 cm-1) ► Weaker thiol-Ag bond compared to thiol-Au bond
Size dependent gauche defects Trans- gauche free energy difference and Isomerization barrier are comparable to k. T Geometrical model Φ = solid angle Cylindrical volume for alkyl chain Conical volume between spheres of radii R and R+L Additional volume for gauche defects Φ L = chain length R = particle radius a = area per head group L Φ R R
Size dependent gauche defects L Φ R R d+/r+ d-/r-
VSFG spectra: Au and Ag. NPs (PPP polarization) Gold nanoparticles Silver nanoparticles r-(op) d+ r+ d+ r + d- Au. NPs r-(op) d+/r+ dd-/r- Ag. NPs d+/r+ d-/r-
Molecular orientation analysis z θ C 3 v Euler matrix P S α φ SFG vis IR Molecular hyperpolarizability Distribution function ψ x Assumed random distribution for φ and ψ angles y Orientation angle (θ) Azimuthal angle (ψ) Torsional angle (φ) SFG intensity From experimental geometry and beam polarization
Molecular orientation analysis z Au. NPs z z z θ θ C 3 A. SSP C 3 θc θc B. PPP Ag. NPs f(θ) 0 0 θc θc π θθ π
Conclusions ►Alkylthiols on nanoscale materials (gold and silver Nps) possess significant amount of gauche defects comparing with the SAM on planar gold ► Increasing amount of gauche defects with decreasing particle size can be understood in terms of the conical volume available for the chain on a curved surface ► Vibrational sum frequency generation is a powerful spectroscopic tool to characterized the molecular conformation on nanostructured materials
Acknowledgements The Group Advisor: Prof. Alex V. Benderskii Dr. Andrey N. Bordenyuk Dr. Igor Stiopkin Himali D. Jayethilake Achani K. Yatawara Fadel Y. Shalhout Adib J. Samin Professor Winters’ group-WSU Dr. Charles Dezalah ECE-WSU Prof. G. Auner Dr. J. Smolinski CIF-WSU Dr. Yi Liu Dr. Sam Shinozaki Funding WSU start-up grant WSU research grant Nano@Wayne ACS-PRF Grant No. 40868 -G 6 NSF CAREER Grant No. 0449720
Surface curvature Chain length VSFG spectra: 5 nm Au. NPs VSFG spectra: 50 nm Au. NPs C 18 -thiol C 12 -thiol C 6 -thiol C 2 -thiol
Au. NPs Ag. NPs
IR and Raman spectra: 1 -DDT gold nanoparticles Raman spectra d+ r+ d- r-(op) IR spectra dd + r+ No size dependent spectral features r-(op)
Possible interpretations: 1. Increasing fraction of gauche defects (SFG propenisty rules) 2. Heterogeneity of local fields E(ω) Ø No size-dependence in Raman spectra (should have same EM enhancement) E(ω, r) Ø Far off resonance: λSPR~520 nm λSFG=665 nm r SFG Raman vis SFG pump IR |v=1 |v=0 Stokes
Cd. Se SSP polarization
Molecular orientation analysis Non-vanishing molecular hyperpolarizability C 3 v (r+) Non-vanishing molecular hyperpolarizability C 2 v (d+) Effective for polarization combination sfg- S, vis-S and IR-P Effective for polarization combination sfg- P, vis-P and IR-P
Molecular orientation analysis + + Intensity ratio (d /r ) SSP polarization PPP polarization 0. 110 0. 105 0. 100 0. 095 0. 090 0. 085 0 50 100 150 θc degrees ►Qualitatively explains less intense d+ modes in PPP spectra for broad distribution of tilt angle θ.
Experimental Setup CCD Fs oscillator 800 nm 40 nm bandwidth Regenerative amplifier 2 -pass amplifier Sample Delay stages OPA with DFM OPA Vis pulse shaping 803 nm 26 nm bandwidth 40 fs 2 m. J/pulse, 1 k. Hz Shaped vis pulse: bandwidth 6 cm-1 Chirp control Vis pulse shaping IR output: 3 -8 m 65 -75 fs 300 cm-1 bandwidth 1 -2 J/pulse Optical delay stages 0. 1 m precision Sample Monochromator LN 2 -cooled CCD Reflection geometry
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