Molecular Orientation in Organic Thin Films

1. Molecular Orientation in Organic Thin Films Prof. Kathy Rowlen Department of Chemistry and Biochemistry University of Colorado, Boulder

2. Why study molecular orientation in thin films? • interfacial properties (optical, electronic and mechanical) • molecular interactions • organizational model for complex systems

3. Questions to be addressed: • best means to probe molecular orientation? • does the substrate affect thin film characteristics? • how does molecular structure affect thin film characteristics? • does molecular orientation and organization vary with time and (or) coverage?

4. Organization at Low Surface Coverage?

5. Photoacoustic Spectroscopy A + hn A* A* A +heat

6. Angle-Resolved Absorbance with Photoacoustic Detection (ARAPD) Lab Z-axis Incident Beam Molecular z'-axis qz'Z  Lab Y-axis 90 - g Lab X-axis Surface Plane For a long-axis transition moment:

7. Katz et al. Science (1991) 254, 1485-1487 • Exhibited second harmonic generation • No linear dichroism • Apparent orientation angle (by SHG) ~ 45° • No change in orientation as layers added

8. Evolution of Orientation in Multilayer Film

9. Molecular Long Axis Orientation (as a function of number of layers)

10. One “layer” 32°± 2° 15°± 1° Six “layers” “Self-Healing” ? • Questions: • How does the angular distribution change? • What is the effect of surface roughness?

11. Local Surface Normal, s Lab Z-axis qsZ qsz' DZ L Substrate Roughness

12. Effect of surface roughness on ARAPD (linear dichroism) measurements in which each value of Kij is equal to <cos2qij> and the subscripts indicate the relevant angle, such that qz'Z is the angle between the molecular orientation axis, z', and the macroscopic surface normal, Z.

13. Error as a function of length scale

14. Second Harmonic Generation 1064 nm 532 nm

15. Z-axis Molecular Long Axis cosq q bzzz X-axis sinq Molecular Orientation by SHG cZZZ = Ns<cos3q>bzzz cZXX = (1/2)Ns<sin2q cosq >bzzz

16. Effect of surface roughness on SHG

17. Influence of Angular Distribution (on SHG determination of zZ) Common assumption:

18. Influence of Angular Distribution (on SHG determination of zZ) Common asumption: Assume Gaussian distribution:

19. SHG “Magic Angle”

20. Legendre Polynomial (cos)

21. SHG Magic Angle Sequence of events as angular distribution broadens • as <P3> approaches zero, SHG apparent tilt angle converges to 39.2° • as <P2> approaches zero, loss of linear dichroism • as <P1> approaches zero,loss of SHG intensity

22. Reported orientation angles (by SHG) within 2 degrees of 39.2° 1) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Rev. A1983, 28, 1883. 2) Grubb, S. G.; Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir1988, 4, 452. 3) Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990, 94, 3681. 4) Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys.1990, 29, 750. 5) Li, DeQ.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. 6) Bubeck, C.; Laschewsky, A.; Lupo, D.; Neher, D.; Ottenbreit, P.; Paulus, W.; Prass, W.; Ringsdorf, H.; Wegner, G. Adv. Mater.1991, 3, 54. 7) Liu, X.; Liu, L.; Chen, Z.; Lu, X.; Zheng, J.; Wang, W. Thin Solid Films1992, 219, 221. 8) Bell, A. J.; Frey, J. G.; VanderNoot, T. J. J. Chem. Soc. Faraday Trans.1992, 88, 2027. 9) Yitzchaik, S.; Roscoe, T. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem.1993, 97, 6958. 10) Nalwa, H. S.; Watanabe, T.; Nakajima, K.; Miyata, S. Thin Solid Films1993, 227, 205. 11) Higgins, D. A.; Naujok, R. R.; Corn, R. M. Chem. Phys. Lett.1993, 213, 485. 12) Yokoyama, S.; Yamada, T.; Kajikawa, K.; Kakimoto, M.; Imai, Y.; Takezoe, H.; Fukudo, A. Langmuir1994, 10, 4599. 13) Kezhi, W.; Chunhui, H.; Guangxian, X.; Xinsheng, Z.; Xiaming, X.; Lingge, X.; Tiankai, L. Thin Solid Films1994, 247, 1. 14) Naujok, R. R.; Higgins, D. A.; Hanken, D. G.; Corn, R. M. J. Chem. Soc. Faraday Trans.1995, 91, 1411. 15) Lin, W.; Yitzchaik, S.; Lin, W.; Malik, A.; Durbin, M. K.; Richter, A. G.; Wong, G. K.; Dutta, P.; Marks, T. J. Angew. Chem. Int. Ed. Engl.1995, 34, 1497. 16) Marks, T. J.; Ratner, M. A. Angew. Chem. Int. Ed. Engl.1995, 34, 155. 17) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.; Lin, W.; Wong, G. K.; Dutta, P. Langmuir1996, 12, 4128. 18) Yokoyama, S.; Kakimoto, M.; Imai, Y.; Yamada, T.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Thin Solid Films1996, 273, 254. 19) Zhang, T.; Feng, Z.; Wong, G. K.; Ketterson, J. B. Langmuir1996, 12, 2298.

23. DM Nd:YAG DM PD QF GLP HWP DM Nd:YAG V DM PMT PD IR P IF QF GLP HWP L Combined SHG and ARAPD Total Internal Reflection Cell 2w w Piezo Inlet Outlet ARAPD SHG

24. Physisorbed Stilbene Dye (DPB)

25. Ellipsometry yields an average orientation angle of ~75° Assuming a 45 Å rod-like molecular length

26. Angle-Resolved Photoacoustic Detection If a narrow distribution is assumed: Mean tilt angle ~ 72°± 3° (monolayer)

27. SHG for Monolayer DPB Monolayer DPB If a narrow angular distribution is assumed the calculated orientation angle is 73°± 3°

28. Mean and Angular Distribution For the DPB monolayer: ARAPD yields a tilt angle of 72°± 3° SHG yields a tilt angle of 73°± 3°

29. For DPB, molecular long axis tilted ~70° with respect to surface normal, fairly narrow angular distribution.

30. DPB Multilayer by ARAPD If a narrow distribution is assumed: Mean tilt angle ~ 72°± 3° (monolayer) Mean tilt angle ~ 53°± 0.9° (multilayer)

31. DPB Multilayer by SHG If a narrow angular distribution is assumed the calculated orientation angle is 70°± 3°

32. Mean and Angular Distribution For the DPB monolayer: ARAPD yields a tilt angle of 72°± 3° SHG yields a tilt angle of 73°± 3° For DPB multilayer: ARAPD yields a tilt angle of 53°± 0.9° SHG yields a tilt angle of 70°± 3°

33. Covalent Molecular System: Azo Dye

34. Water Contact Angle and Ellipsometry Ellipsometry indicates only 6.5 Å thickness, estimated 0.1 monolayer, 37 Å2 per molecule

35. Combined SHG and ARAPD Results

36. Mean and Angular Distribution Assuming a narrow distribution for ARAPD: 58° ± 2° for SHG: 46° ± 2°

37. Mean and Angular Distribution Assuming a narrow distribution for ARAPD: 58° ± 2° for SHG: 46° ± 2° Linear Dichroism SHG

38. Mean and Angular Distribution Assuming a narrow distribution for ARAPD: 58° ± 2° for SHG: 46° ± 2°

39. Azo Dye with aminopropyl silane linker 57 °± 30°

40. SHG for adsorption isotherms and adsorption / desorption kinetics • SHG intensity depends on both the number density and molecular orientation • Experimental geometry can be used to minimize sensitivity to orientation cZZZ = Ns<cos3q>bzzz cZXX = (1/2)Ns<sin2q cosq >bzzz

41. Conventional SHG Adsorption Measurements • Single polarization combination (e.g., p-polarized  and 2 ), assume molecular orientation does not change with surface coverage • Orientation angle corrected (OAC) • Measurement of several polarization combinations at each concentration • Calculation of molecular orientation, , at each concentration (assuming narrow angular distribution) • Signal normalization • Construction of isotherm

42. Theoretical p-polarized SHG response curves as a function of orientation angle (bzzz dominant)

43. RMS deviation for p-polarized 2w (bzzz dominant) g*

44. Test Case: Disperse Red 1 (in methylene chloride) Fused Silica

45. Adsorption isotherm for DR-1 as determined by a variety of polarization conditions

46. Apparent orientation angle as a function of concentration

47. Adsorption isotherm for DR-1 after correcting for change in orientation

48. Experimental constants obtained from Langmuir fit to adsorption isotherms IppIpsIs45Ip63 OAC Keq (M-1) 940 410 470 540 500 ± 40 ± 40 ± 50 ± 60 ± 40 DGads (kJ/mol) -16.8 -14.7 -15.1 -15.4 -15.2 ± 0.1 ± 0.3 ± 0.3 ± 0.3 ± 0.2 OAC = orientation angle corrected

49. Future Directions Adsorption / Desorption Kinetics by SHG

50. Thin Film Growth Mechanism