Molecular orientation dependence of hole-injection barrier in pentacene thin film on the Au surface in organic thin film transistor

Ihm, Kyuwook; Kim, Bongsoo; Kang, Tai‐Hee; Kim, Ki-Jeong; Joo, Minho; Kim, Tae Hyeong; Yoon, Sang Soo; Chung, Sukmin

doi:10.1063/1.2227712

Cited by 69 publications

(27 citation statements)

References 12 publications

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“…While variations of the ionization potential (IP; the molecular equivalent of the work function) depending on the molecular orientation on a substrate have been reported before [10][11][12][13][14][15][16] , the prevalent interpretation in terms of variable photo-hole screening could never be satisfactorily quantified. Here, we propose a qualitatively different and novel explanation for the intriguing observation that one and the same molecule can have different -still welldefined -IPs if part of an ordered supramolecular structure.…”

Section: S Duhm Et Al Nature Materials Acceptedmentioning

confidence: 99%

“…To confirm the validity of our model, we performed DFT calculations also for lying and standing layers of pentacene, where the difference between the two respective IPs was experimentally determined to be ca. 0.5 eV and no satisfactory explanation could be found 11,13,14 . In good agreement, our calculations yield a difference of 0.6 eV.…”

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confidence: 96%

“…36 We thus conclude that the 0.6 eV [0.4 eV] difference in the IP between standing and lying DH6T [6T] can not be explained in terms of photo-hole screening effects alone, and that another mechanism must be involved. 8 S. Duhm et al, Nature Materials, accepted In order to understand the remarkable finding of one molecule having a different IP depending on its orientation in an ordered supramolecular structure [10][11][12][13][14][15][16] 28 , we performed plane-wave based DFT calculations employing periodic boundary conditions and the repeated-slab approach on single layers of standing and lying DH6T and 6T molecules based on available structural data (see Methodology section for details) 13, 29,[37][38][39] . The occupied density-of-states (DOS) for a lying (L) and standing (S) layer of DH6T is shown in Figure 1b.…”

Section: S Duhm Et Al Nature Materials Acceptedmentioning

confidence: 99%

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Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

et al. 2008

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S. Duhm et al., Nature Materials, acceptedWhile an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical π-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modeling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic/organic heterojunctions, hole-or electron-blocking layers, and reduced barriers for charge-carrier injection in organic electronic devices. S. Duhm et al., Nature Materials, acceptedIt is well established that the work function (Φ) of metals depends on the crystal face 1-3 .Φ is defined as the energy difference between the Fermi level (E F ) and the electrostatic potential above the sample, the vacuum level (V vac ). For, e.g., copper, Φs of the (100), (110), and (111) surfaces are spread over a range of 0.5 eV 1,2 . As E F is constant, this observation has been explained by the difference in the intrinsic "surface dipole": Differences in the geometric and, consequently, electronic structure cause a different amount of the electronic cloud to spill out of the bulk into the vacuum 3, 4 . The resulting dipole raises V vac to a larger or smaller extent and thus impacts Φ 4, 5 . Note that this effect can only be observed for laterally extended surfaces, as the spatial region above the sample where V vac is raised reaches farther away from the surface with increasing sample size (i.e., area of the exposed surface) 6, 7 . Small metal clusters with multiple facets of different crystal orientations have only one well-defined work function 8, 9 .For van der Waals (i.e., non-covalent) crystals of non-dipolar molecules, surface dipoles and work-function anisotropy have not yet been explored 6 . While variations of the ionization potential (IP; the molecular equivalent of the work function) depending on the molecular orientation on a substrate have been reported before [10][11][12][13][14][15][16] , the prevalent interpretation in terms of variable photo-hole screening could never be satisfactorily quantified. Here, we propose a qualitatively different and novel explanation for the intriguing observation that one and the same molecule can have different -still welldefined -IPs if part of an ordered supramolecular structure. S. Duhm et al., Nature Materials, acceptedWe performed X-ray photoelectron spectroscopy (XPS) and u...

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Section: S Duhm Et Al Nature Materials Acceptedmentioning

confidence: 99%

mentioning

confidence: 96%

Section: S Duhm Et Al Nature Materials Acceptedmentioning

confidence: 99%

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Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

et al. 2008

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“…3 Several phenomenological findings showed destructive behaviors of adsorbed Au atoms such as heat transfer or the precipitation of big Au grains into the pentacene crystal. [4][5][6][7][8][9][10][11] However, the detailed information on the microscopic mechanism of the collapsing crystal by introduced Au atoms is lacking.…”

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confidence: 99%

“…Au was thermally evaporated from a home-made effusion cell onto the pentacene crystal, which was grown by a vapor deposition on the SiO x / Si surface. [11][12][13] The effusion cell was surrounded by a water cooled shroud with 20 mm diameter hole. Fluxes of thermally evaporated pentacene molecules and Au atoms were controlled to within 0.2 and 0.05 Å / sec using a thickness monitor.…”

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Early stages of collapsing pentacene crystal by Au

Ihm

Chung

Kang

et al. 2008

Applied Physics Letters

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The characteristic feature of metal contacts with gold on organics is deterioration of the organic crystals during the contact formation. The unveiled key challenge is to probe dynamic details of the microscopic evolution of the organic crystal when the atomic Au is introduced. Here, we report how the collapse of the pentacene crystal is initiated even by a few Au atoms. Our results indicate that the gentle decoupling of intra and intermolecular π-π interactions causes the localization of the lowest unoccupied molecular orbital as well as the removal of cohesive forces between molecules, leading to the subsequent crystal collapse.

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Dielectric Interface Engineering

Nathan

et al. 2011

Organic Thin Film Transistor Integration

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Molecular orientation dependence of hole-injection barrier in pentacene thin film on the Au surface in organic thin film transistor

Cited by 69 publications

References 12 publications

Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

Early stages of collapsing pentacene crystal by Au

Dielectric Interface Engineering

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