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Self-LimitedGrowth in Pentacene Thin Films

View Article: PubMed Central - PubMed

ABSTRACT

Pentaceneis one of the most studied organic semiconducting materials. Whilemany aspects of the film formation have already been identified invery thin films, this study provides new insight into the transitionfrom the metastable thin-film phase to bulk phase polymorphs. Thisstudy focuses on the growth behavior of pentacene within thin filmsas a function of film thickness ranging from 20 to 300 nm. By employingvarious X-ray diffraction methods, combined with supporting atomicforce microscopy investigations, one crystalline orientation for thethin-film phase is observed, while three differently tilted bulk phaseorientations are found. First, bulk phase crystallites grow with their00L planes parallel to the substrate surface; second,however, crystallites tilted by 0.75° with respect to the substrateare found, which clearly dominate the former in ratio; third, a differentbulk phase polymorph with crystallites tilted by 21° is found.The transition from the thin-film phase to the bulk phase is rationalizedby the nucleation of the latter at crystal facets of the thin-film-phasecrystallites. This leads to a self-limiting growth of the thin-filmphase and explains the thickness-dependent phase behavior observedin pentacene thin films, showing that a large amount of material ispresent in the bulk phase much earlier during the film growth thanpreviously thought.

No MeSH data available.


SpecularX-ray reflectivity data of pentacene films of various nominal thicknesses;curves are vertically shifted for clarity. The dashed lines show thetheoretical Bragg peak positions of the pentacene thin-film phase(TF) and Campbell phase (C).12,15
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fig1: SpecularX-ray reflectivity data of pentacene films of various nominal thicknesses;curves are vertically shifted for clarity. The dashed lines show thetheoretical Bragg peak positions of the pentacene thin-film phase(TF) and Campbell phase (C).12,15

Mentions: Specular X-ray diffraction patterns from samples of different nominalpentacene thicknesses are depicted in Figure 1. The curves for all samples show an increasein intensity at low angles, which represents the area of total externalreflection and reducing footprint as the scattering angle (vector)increases. At about 0.03 Å–1 the intensitystarts to decrease, typical for X-rays starting to penetrate the substratesurface. In the curve of the 20 nm sample, there are two distinctfeatures superimposed. First, Kiessig fringes34 of two different periodicities occur, meaning that there are atleast two types of layers in this system. Higher periodic Kiessigfringes (hardly noticeable in this image representation) correspondto the 150 nm silicon dioxide layer of the substrate. Fringes of lowerperiodicity have been fitted to a layer thickness of 12 nm. Such pronouncedoscillations only occur if a well-defined, flat, and homogeneous pentacenelayer exists. It can be assigned to the first layers of the pentaceneadsorbate located directly at the substrate surface.35,36 The second set of features present is Bragg peaks at 0.407 and 0.814Å–1, corresponding to d-spacingsof 15.44 and 7.72 Å, respectively. Comparison with literaturedata shows that these are the 001 and 002 peaks of the pentacene thin-filmphase. With a low nominal thickness of 20 nm, these peaks exhibita small shift to lower qz values, which is due to a layer-thickness-dependent interferencebetween the beam diffracted by the pentacene crystallites and theoptical reflectivity from the substrate.29 As the pattern shows peaks belonging to the 00L series, this further means that the crystallites are formed by upright-standingmolecules, with the 001 plane parallel to the substrate surface.


Self-LimitedGrowth in Pentacene Thin Films
SpecularX-ray reflectivity data of pentacene films of various nominal thicknesses;curves are vertically shifted for clarity. The dashed lines show thetheoretical Bragg peak positions of the pentacene thin-film phase(TF) and Campbell phase (C).12,15
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5384045&req=5

fig1: SpecularX-ray reflectivity data of pentacene films of various nominal thicknesses;curves are vertically shifted for clarity. The dashed lines show thetheoretical Bragg peak positions of the pentacene thin-film phase(TF) and Campbell phase (C).12,15
Mentions: Specular X-ray diffraction patterns from samples of different nominalpentacene thicknesses are depicted in Figure 1. The curves for all samples show an increasein intensity at low angles, which represents the area of total externalreflection and reducing footprint as the scattering angle (vector)increases. At about 0.03 Å–1 the intensitystarts to decrease, typical for X-rays starting to penetrate the substratesurface. In the curve of the 20 nm sample, there are two distinctfeatures superimposed. First, Kiessig fringes34 of two different periodicities occur, meaning that there are atleast two types of layers in this system. Higher periodic Kiessigfringes (hardly noticeable in this image representation) correspondto the 150 nm silicon dioxide layer of the substrate. Fringes of lowerperiodicity have been fitted to a layer thickness of 12 nm. Such pronouncedoscillations only occur if a well-defined, flat, and homogeneous pentacenelayer exists. It can be assigned to the first layers of the pentaceneadsorbate located directly at the substrate surface.35,36 The second set of features present is Bragg peaks at 0.407 and 0.814Å–1, corresponding to d-spacingsof 15.44 and 7.72 Å, respectively. Comparison with literaturedata shows that these are the 001 and 002 peaks of the pentacene thin-filmphase. With a low nominal thickness of 20 nm, these peaks exhibita small shift to lower qz values, which is due to a layer-thickness-dependent interferencebetween the beam diffracted by the pentacene crystallites and theoptical reflectivity from the substrate.29 As the pattern shows peaks belonging to the 00L series, this further means that the crystallites are formed by upright-standingmolecules, with the 001 plane parallel to the substrate surface.

View Article: PubMed Central - PubMed

ABSTRACT

Pentaceneis one of the most studied organic semiconducting materials. Whilemany aspects of the film formation have already been identified invery thin films, this study provides new insight into the transitionfrom the metastable thin-film phase to bulk phase polymorphs. Thisstudy focuses on the growth behavior of pentacene within thin filmsas a function of film thickness ranging from 20 to 300 nm. By employingvarious X-ray diffraction methods, combined with supporting atomicforce microscopy investigations, one crystalline orientation for thethin-film phase is observed, while three differently tilted bulk phaseorientations are found. First, bulk phase crystallites grow with their00L planes parallel to the substrate surface; second,however, crystallites tilted by 0.75° with respect to the substrateare found, which clearly dominate the former in ratio; third, a differentbulk phase polymorph with crystallites tilted by 21° is found.The transition from the thin-film phase to the bulk phase is rationalizedby the nucleation of the latter at crystal facets of the thin-film-phasecrystallites. This leads to a self-limiting growth of the thin-filmphase and explains the thickness-dependent phase behavior observedin pentacene thin films, showing that a large amount of material ispresent in the bulk phase much earlier during the film growth thanpreviously thought.

No MeSH data available.