Effect of nanoscale geometry on molecular conformation: Vibrational Sum-Frequency Generation of alkaynethiols on metal nanoparticles Achani Yatawara, Andrey N. Bordenyuk and Alexander V. Benderskii* Department of Chemistry Wayne State University Detroit, MI by Champika N. Weeraman

Nanoscale geometry – molecular conformation ► Expect a relationship between the substrate geometry (nano scale) and conformation of the adsorbed molecule

vs. Nanoscale geometry – molecular conformation Flat surface Curved surface

Molecular conformation in nanostructured materials Bio sensors-bio markers Aerosols Gratzel-solar cells

Sum-frequency generation (SFG) originates from induced polarization Virtual State In media without inversion symmetry  SFG =  IR +  vis S()S() SFG - CCD image Spectrally narrow (temporally long) Vis pulse IR  IR vis  vis Broad-band IR pulse (temporally short) + In media with inversion symmetry Vibrational sum frequency generation (VSFG) ► Broad band VSFG

Case II: gauche defects Case I: all trans (zig-zag) strong CH 3 transitions, weak CH 2 transitions Flat Surface Experiment More CH 2 transitions Curved surface Increase surface curvature Change of Molecular conformation SFG sensitivity ? d+d+ r+r+ d-d- r + FR r-r- ► Strong CH 3 stretch transitions (r +,r + FR,r - ) ► Weak CH 2 stretch transitions (d +,d - ) ► predominantly trans zig-zag molecule arrangement with few gauche defects Chain length ~1.6 nm

TEM images:1-DDT capped Au and AgNPs (I) Gold nanoparticles (II) Silver nanoparticles 20 nm (b) 20 nm (c) 100 nm (d) 20 nm (a) =1.8 nm =1.8 nm =2.9 nm =2.9 nm =7.4 nm =7.4 nm =23 nm =23 nm 20 nm (a) 20 nm (b) 100 nm (c) 100 nm (d) 20 nm (a) 20 nm (b) =1.8 nm =1.8 nm =3.6 nm =3.6 nm =7.9 nm =7.9 nm =24.6 nm =24.6 nm Meliorum Technologies, Inc.

d+d+ r+r+ d-d- r - op VSFG 1-DDTcapped AuNps (SSP polarization) ► 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 B j = amplitude Γ j = Lorentzian line width ω j = transition frequency A NR = nonresonant contribution Intensity of mode j; Intensity of SFG signal (SSP polarization, SFG-Vis-IR) Effective non linear susceptibility: Multi-Lorentzian app. ГjГj ГjГj

Size dependent SFG spectra gauche defects Chain length ~1.6 nm d + /r + Chain length ~1.6 nm d - /r -

VSFG 1-DDT capped AgNPs (SSP polarization) d+d+d+d+ r - (op) d-d-d-d- r+r+r+r+ 3.6 nm 7.9 nm 24.6 nm 1.8 nm ► Transitions are broader than AuNPs (AuNPs: 10-12cm -1 AgNPs: 15-23cm -1 ) ► Weaker thiol-Ag bond compared to thiol-Au bond d + /r + d - /r -

R L R Φ Cylindrical volume for alkyl chain Conical volume between spheres of radii R and R+L Additional volume for gauche defects Φ = solid angle L = chain length R = particle radius a = area per head group Size dependent gauche defects Φ Trans- gauche free energy difference and Isomerization barrier are comparable to kT Geometrical model

Size dependent gauche defects R L R Φ d + /r + d - /r -

d + /r + d - /r - d+d+d+d+ r - (op) d-d-d-d- r+r+r+r+ d + /r + d - /r - VSFG spectra: Au and AgNPs (PPP polarization) Gold nanoparticlesSilver nanoparticles AuNPs AgNPs d+d+d+d+ r - (op) d-d-d-d- r+r+r+r+

z y C 3v θ α ψ IR vis SFG P S φ x Euler matrixMolecular hyperpolarizability Orientation angle ( θ ) Azimuthal angle ( ψ ) Torsional angle ( φ ) Distribution function Assumed random distribution for φ and ψ angles SFG intensity From experimental geometry and beam polarization Molecular orientation analysis

A. SSP B. PPP Molecular orientation analysis 0 π f(θ) θ θcθc z θcθc θ C3C3 z AuNPs AgNPs 0 π θ θcθc z θcθc θ C3C3 z

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 The GroupAdvisor: Prof. Alex V. Benderskii Dr. Andrey N. Bordenyuk Dr. Igor Stiopkin Himali D. Jayethilake Achani K. Yatawara Fadel Y. Shalhout Adib J. Samin WSU start-up grant WSU research grant ACS-PRF Grant No G6 NSF CAREER Grant No Funding Professor Winters’ group-WSU Dr. Charles Dezalah ECE-WSU Prof. G. Auner Dr. J. Smolinski CIF-WSU Dr. Yi Liu Dr. Sam Shinozaki

Surface curvature Chain length VSFG spectra: 5nm AuNPsVSFG spectra: 50nm AuNPs C2-thiol C12-thiol C18-thiol C6-thiol C2-thiol C12-thiol C18-thiol

AuNPs AgNPs

d+d+d+d+ r+r+r+r+ r - (op) d-d-d-d- d+d+d+d+ r+r+r+r+ d-d-d-d- IR and Raman spectra: 1-DDT gold nanoparticles IR spectra Raman spectra No size dependent spectral features

Possible interpretations: 1.Increasing fraction of gauche defects (SFG propenisty rules) 2. Heterogeneity of local fields E(ω) E(ω, r) r  No size-dependence in Raman spectra (should have same EM enhancement)  Far off resonance: λ SPR ~520 nm λ SFG =665 nm SFGRaman  IR  vis  SFG  pump  Stokes |v=0  |v=1 

SSP polarization CdSe

Molecular orientation analysis Non-vanishing molecular hyperpolarizability C 3v (r + ) Non-vanishing molecular hyperpolarizability C 2v (d + ) Effective for polarization combination sfg-P, vis-P and IR-P Effective for polarization combination sfg-S, vis-S and IR-P

Molecular orientation analysis Intensity ratio (d + /r + ) θ c degrees SSP polarization PPP polarization θ c degrees ► Qualitatively explains less intense d + modes in PPP spectra for broad distribution of tilt angle θ.

800 nm 40 nm bandwidth 803 nm 26 nm bandwidth 40 fs 2 mJ/pulse, 1 kHz Reflection geometry IR output: 3-8  m fs 300 cm -1 bandwidth 1-2  J/pulse 0.1  m precision Shaped vis pulse: bandwidth  6 cm -1 Chirp control Fs oscillator Regenerative amplifier 2-pass amplifier OPA with DFM Optical delay stages Sample Monochromator LN 2 -cooled CCD Vis pulse shaping Experimental Setup OPA Delay stages Vis pulse shaping Sample CCD