1. Epitaxial nucleation and growth of organic crystals on inorganic substrates Supervisors:Willem van Enckevort Sander Graswinckel Mirjam Leunissen

2. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

3. Introduction Subject Epitaxial three-dimensional nucleation and subsequent growth of organic substances on inorganic substrates from the solution and vapor phase What is epitaxy? Present use Oriented growth of one crystal upon another Etymology Greek derivation ‘under order’ phalanx Introduction

4. Epitaxy is the oriented crystal growth of a substance on a crystal surface of the same (‘homo-epitaxy’) or another substance (‘hetero- epitaxy’) in which the structure of the substrate determines the orientation of the guest crystals Definition Introduction

5. homo -- hetero monolayer -- three-dimensional crystal inorganic -- organic polar -- apolar melt / solution / vapor / vacuum evaporation Types of epitaxy Introduction

6. flat substrate 2D unit cell of substrate matches 2D cell of overlayer (not necessarily 1:1) substrate doesn’t dissolve similar type of force in substrate and overlayer (polar/apolar) Requirements Introduction

7. Examples of epitaxy ‘natural’: minerals growing together crystal growth/seeding GaN on sapphire (Al 2 O 3 ): optical and electronic devices Introduction

8. Goal of present study To study aspects of epitaxial crystal growth from the solution and vapor phase (e.g. growth mechanism, symmetry aspects) Why are we interested in epitaxy? Applications grow thick monocrystalline layers of organic compounds on a substrate grow ‘on command’ new polymorphs and crystals of substances which won’t crystallize under ‘normal’ conditions Approach Grow oriented three-dimensional nuclei on top of a substrate, which, on continued growth, coalesce and grow together. Introduction

9. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

10. Systems & experimental methods *HOPG: Highly Ordered Pyrolytic Graphite Systems & experimental methods

11. substrate cooled to 0 ºC (saturated) solution of 45-55 ºC of the guest substance 2-10 minutes dry with a tissue Solution deposition Systems & experimental methods

12. nitrogen atmosphere substrate same temperature as vapor: 165 ºC 15-20 hours Vapor deposition Systems & experimental methods

13. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

14. Alizarin Alizarin: general General

15. Crystal structure according to Guilhem (1967): Pa a = 21.04 Å b = 3.75 Å c = 20.12 Å β = 104.5º Alizarin: general

16. needle shaped crystals long, thin hexagonal or rectangular sometimes hollow SEM Crystallization without substrate Alizarin: general

17. Solution deposition: reproduction results of prof. Neuhaus No great differences between solution and vapor deposition considering general aspects (morphology, orientation) Alizarin on NaCl {100} Solution: toluene + 2.5 mass% absolute ethanol General Vapor and solution deposition Alizarin: NaCl {100}

18. NaCl:cubic Fm-3m {100} tetragonal symmetry Orientation Alizarin: NaCl {100}

19. Alizarin length axis // [011] and [0-11] of NaCl Vapor Solution Polarization microscope Alizarin: NaCl {100}

20. Morphology ‘Roof’ like, pointed or topped of SEM AFM Alizarin: NaCl {100}

21. Contact face Based on morphology: (001) Alizarin: NaCl {100}

22. Verification with X-ray powder diffraction Diffraction vector  substrate surface for all diffraction angles Enhanced reflection from (hkl) planes // contact face Randomly oriented crystallites (dashed) Oriented crystallites on NaCl (solid) (003) Alizarin: NaCl {100}

23. Contact face (001): alizarin molecules  interface strong interaction protruding oxygen atoms with ionic substrate Alizarin: NaCl {100}

24. Epitaxial nucleation on faces other than {100} What is the influence of the substrate orientation on the orientation of the guest crystals? ‘Hole experiment’ Alizarin: hole experiment

25. Asymmetric unit of point group m3m Alizarin: hole experiment

26. oriented crystallites grow on all faces strong dependence substrate orientation and preferred directions transition from tetragonal to trigonal symmetry on going from {100} to {111} no relationship between size and amount of crystallites and specific substrate orientation Alizarin: hole experiment

27. Alizarin on as-grown NaCl {111} NaCl:{111} trigonal symmetry Alizarin: NaCl {111}

28. Alizarin length axis // [-101], [1-10] and [0-11] of NaCl Alizarin: NaCl {111}

29. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

30. Anthraquinone Alizarin structure analogue Less complicated molecular structure Monoclinic: P2 1 /c a = 7.87 Å b = 3.96 Å c = 15.78 Å β = 102.7 º Vapor and solution deposition General Anthraquinone: general

31. Orientation Anthraquinone on NaCl {100} Anthraquinone length axis // [011] and [0-11] of NaCl Optical microscope Vapor Solution Anthraquinone: NaCl {100}

32. Morphology ‘Roof’ like, pointed or topped of AFM SEM Anthraquinone: NaCl {100}

33. Contact face Contact face (10-2): anthraquinone molecules  interface strong interaction protruding oxygen atoms with ionic substrate Anthraquinone: NaCl {100}

34. Dock ‘candidate’ faces onto the NaCl {100} lattice Size a x b x c : a = # molecules in a row b = # rows per layer c = # layers Rotate and translate plane until stage of minimal energy is reached 5x2x1 Molecular mechanics: prediction contact face and orientation ‘Candidate’ faces from morphology prediction (Eatt): (100), (002), (10-2) Anthraquinone: molecular mechanics

35. Plot energy as function of orientation angle φ 1 row of molecules orientation 45 ° Orientation of face (10-2) distance between protruding O-atoms within 0.2% identical to distance between Na-ions Anthraquinone: molecular mechanics

36. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

37. Paraffins Dotriacontane: C 32 H 66 (orthorhombic) Tritriacontane: C 33 H 68 (orthorhombic) Tetracosane: C 24 H 50 (triclinic) General Paraffin = n-alkane = C n H 2n+2 Apolar All-trans structure Diluted n-heptane solutions Paraffins: general

38. n-Alkanes adsorb on graphite Highly Ordered Pyrolytic Graphite (HOPG): apolar stacking of layers (0001) hexagonal Paraffins: general

39. Three preferred directions in trigonal pattern Orientation Paraffins on HOPG (0001) Differently oriented domains Cryo-SEM Paraffins: HOPG (0001)

40. Crystal length axis oriented // HOPG periodic bond chain directions Determination precise orientation by atomic force microscopy Paraffins: HOPG (0001)

41. ‘plate’ like crystals steep and high flat top faces  substrate surface Morphology Paraffins: HOPG (0001)

42. Prediction based on morphology crystals without substrate: (100) or (110) Contact face Extinction directions Verification: reflection polarization microscopy - different extinction conditions C 32 H 66 and C 33 H 68 : (100) Paraffins: HOPG (0001)

43. Growth mechanism Onset of hetero-epitaxial growth: formation monolayer (2 types) Paraffins: HOPG (0001)

44. Assumption: same crystal structure with and without substrate Dock (100) bulk face on HOPG: no existent monolayer structure is obtained Chain directions in monolayer and bulk crystal differ ~30º (or equivalent -30º and 90º ) Crystal Paraffins: HOPG (0001) Monolayer

45. Monolayer  bulk crystal: subsequent layers of n-alkane molecules have to be rotated Gilbert et al. (1994): bilayer 1 st and 2 nd layer mutually rotated by 90 º 1 st layer consists of rows of parallel n-alkane chains with molecule plane perpendicular to HOPG surface Stranski-Krastanov: monolayer followed by three-dimensional nucleation Paraffins: HOPG (0001)

46. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

47. Nucleation theory Nucleation theory: general General Competition three-dimensional nucleation in bulk phase and on substrate surface larger number of nucleation sites (bulk)  lower activation barrier (substrate) Assumption: spherical nuclei Rate of nucleus formation (J) surface area critical nucleus rate of addition of monomers concentration critical nuclei

48. Homogeneous: Heterogeneous: Surface area substrate f(α): correction factor for relative volume change critical nucleus f’’(α): correction factor for reduced surface area critical nucleus Kinetic factor Activation barrier/ free enthalpy critical nucleus Volume fluid Volume growth unit Surface energy Monomer concentration Nucleation theory: general

49. Contact angle α depends on the surface energy of: substrate crystal interface γ int γ cryst γ sub Σ F i,hor = 0 Nucleation theory: general

50. Nucleation competition ratio (NCR) Temperature bulk Temperature substrate Nucleation theory: general

51. Conditions for epitaxial nucleation to be favored over bulk nucleation: 1) rate of formation of nuclei on the substrate surface must be higher than in the bulk: J het > J homo (NCR>1) 2) rate of formation of nuclei on substrate surface must be reasonable: J het > 10 5 m -2 sec -1 (= 10 2 nuclei per mm 2 in 10 3 sec) Nucleation theory: general

52. Nucleation theory: vapor Application to vapor deposition J het increases dramatically for decreasing γ and α values

53. NCR decreases for decreasing surface energy and increasing contact angle Nucleation theory: vapor

54. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

55. Prediction number of overlayer domains with different orientation (n): two-dimensional point group symmetry of the two contacting faces N(S) = number of symmetry operators applying to the two-dimensional point group of the surface S substrate = {S s,1 ; S s,2 ; S s,3 ; ….; S s,n } S crystal = {S c,1 ; S c,2 ; S c,3 ;…..; S c,n } S s/c,1 = E Symmetry operators = transformations (x,y,z)  (x,y,z)E (x’,y’,z’)S 2 (x n,y n,z n )S n Symmetry considerations

56. Example: alizarin on NaCl {100} Alizarin: Pa Contact face (001): m N(S alizarin ) = 2 NaCl: Fm-3m Face {100}: 4mm N(S NaCl ) = 8 N(S NaCl  S alizarin ) = 2 Symmetry considerations

57. Single domain monocrystalline layer: substrate with lowest symmetry possible, i.e. S substrate = {E} Symmetry considerations

58. Outline IntroductionWhat is epitaxy? Why study it? Systems & experimental methods AlizarinGeneral On NaCl {100} ‘Hole experiment’ On NaCl {111} AnthraquinoneGeneral On NaCl {100} Molecular mechanics ParaffinsGeneral On HOPG (0001) Nucleation theory Symmetry considerations Discussion & conclusionWhat have we learned? Future

59. expensive ultra high vacuum equipment not necessary close relationship between symmetry substrate surface and preferred orientations understanding of processes underlying formation of oriented three- dimensional nuclei and their subsequent growth general conditions for the formation of epitaxial three-dimensional nuclei to be favored over bulk nucleation What have we learned? Discussion & conclusion

60. first onset epitaxial growth exact role of all factors by precise measurements nucleation theory for anisotropic nuclei let the separate nuclei grow together into a domain with a single orientation general rules to predict suitable combinations guest and substrate compounds induction of polymorphism Future Discussion & conclusion