Optical absorbance of oriented thin films

Niko, A.; Meghdadi, F.; Draxl, Claudia; Vogl, P.; Leising, G.

doi:10.1016/0379-6779(95)03447-r

Cited by 84 publications

(48 citation statements)

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“…The bandlike features have been detected experimentally for short oligomers only [8,9]. The majority of attempts follow the so-called oligomer approach, in which the characteristics of short oligomers of increasing chain length are extrapolated to infinite polymers [10,11]. There are still significant discrepancies among different theories, as well as between theories and experimental results, and many fundamental properties of CP including bandwidth, band gap, and the influence of various structural defects, etc., are under debate [7].…”

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confidence: 99%

Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

Wang

Lin

2011

Phys. Rev. Lett.

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We study single conjugated polyphenylene oligomers consisting of 3n (2 n 12) phenyl units by means of cryogenic scanning tunneling microscopy and spectroscopy. The spatially resolved local densities of states reveal a progressive development of a continuous conduction band out of discrete molecular orbitals as the length of the oligomers increases. The experimental results are satisfactorily described by tight-binding calculations which gave a conduction band bandwidth of 4:5 AE 0:2 eV and a band gap of 3:1 AE 0:2 eV for an infinitely long polymer. We observed two types of defects, known as conformational torsional angle misfit and metasite kink. Tight-binding as well as density-functional theory model calculations confirm that both types of defects effectively destroy the delocalization. DOI: 10.1103/PhysRevLett.106.206803 PACS numbers: 73.22.Àf, 73.61.Ph, 68.37.Ef, 82.35.Cd One-dimensional (1D) conjugated polymers (CP) are both interesting scientifically and important for technological applications [1]. Theoretical descriptions of CP follow one of two routes: a delocalized band theory of solid-state physics or a localized molecular orbital approach of quantum chemistry [2][3][4]. In spite of their different perspectives, both approaches predict that the molecular orbitals of short oligomers develop progressively into a broad band in the long-length limit [5][6][7]. The bandlike features have been detected experimentally for short oligomers only [8,9]. The majority of attempts follow the so-called oligomer approach, in which the characteristics of short oligomers of increasing chain length are extrapolated to infinite polymers [10,11]. There are still significant discrepancies among different theories, as well as between theories and experimental results, and many fundamental properties of CP including bandwidth, band gap, and the influence of various structural defects, etc., are under debate [7].In particular, precise experimental determination of band structure, i.e., the energy-momentum dispersion, of CP or long oligomers has had limited successes largely due to the difficulty of growing high-purity single crystalline samples [12]. Above all, the existence of defects presents a severe challenge since defects in a 1D system can strongly alter its electronic structure [13]. Thus to be able to measure the electronic structure of a single CP whose molecular structure, including the details of defects, can be resolved simultaneously is highly desirable. Recently, Repp et al. demonstrated that scanning tunneling microscopy (STM) can provide an unprecedented view of the electronic structures of individual conjugated oligomers [14]. In this Letter we report the conductance band structures of single poly(para-phenylene) (PPP) oligomers determined by cryogenic (5 K) STM. We examine the energy-momentum dispersion of the PPP oligomers of up to 16 nm in length (36 phenyl units) and derive the conduction band bandwidth, energy gap, and the effective mass of charge carriers. Furthermore, we demonstrate and discuss how...

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confidence: 99%

Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

Wang

Lin

2011

Phys. Rev. Lett.

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“…[ 1 ] In particular, they were successfully used as active layers in organic thin fi lm transistors [ 2 ] or in organic light-emitting devices. [ 3 , 4 ] The optical characteristics of the single molecules are highly anisotropic [ 5 ] and allow for the emission of linearly polarized light. [ 6 ] Since, in the fi nal device, a large ensemble of molecules is present, its performance depends strongly on the degree of ordering in the active layer.…”

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confidence: 99%

Uniform π‐System Alignment in Thin Films of Template‐Grown Dicarbonitrile‐Oligophenyls

Klappenberger

Kühne

Marschall

et al. 2011

Adv Funct Materials

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The ordering and conformational properties of dicarbonitrile‐para‐oligophenyls are studied with complementary methods, namely X‐ray structure analysis, low‐temperature scanning tunneling microscopy, and near‐edge X‐ray absorption fine‐structure spectroscopy. The packing of the functionalized variants differs from their technologically interesting para‐oligophenyl counterparts, both in the bulk crystal phase and in thin films grown by organic molecular beam epitaxy (OMBE) under ultra‐high vacuum conditions on the Ag(111) surface. In the crystal phase, the conformation depends on the number n of phenyl rings, exhibiting an intriguing screw‐like structure in the case of n = 4 at room temperature as well as at 180 K. For OMBE‐grown thin films, the whole series acquires the same type of conformation, characterized by alternately twisted phenyl rings, similar to the pure oligophenyl species. However, for all tested molecules, the orientation of the molecular reference plane is uniform within the entire film and coincides with the surface plane. This contrasts with the herringbone ordering adopted by the phenyl backbones without the carbonitrile groups. Our results demonstrate how the functionalization of moieties with extended conjugated electron systems can help to improve the structural homogeneity in technologically relevant organic thin films.

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“…The morphology and the optical properties of the resulting nanostructures are determined by the functional groups attached to the molecular building block. Most of the nanostructures show strong, polarised blue luminescence after excitation with unpolarised UV-light under normal incidence, which indicates lying molecules on the surface as light emitters [160,161] ordered with a high degree of crystallinity. The plane of polarisation is always approximately perpendicular to muscovite 1 1 0 , i.e.…”

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confidence: 99%

Organic Molecular Nanotechnology

Schiek

Balzer

Al‐Shamery

et al. 2008

Small

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Zur Homepage der Dissertation AbstractFormation of needle-like crystals from inorganic as well as from organic semiconductors such as para-phenylenes, α-thiophenes and phenyl-thiophene co-oligomers on dielectric surfaces has attracted increasing interest within the last years. The para-hexaphenylene has proven to be a versatile building block for generating long, parallely aligned organic nanofibers by a self-assembly process on a muscovite mica substrate via sublimation in high vacuum. These nanofibers possess outstanding optical properties like strong, polarised blue fluorescence in high quantum yields after unpolarised excitation with UV light, waveguiding and lasing features. Furthermore, they are chemically and thermically stable and can be easily detached from the growth substrate to serve as key elements in next generation optoelectronic and nanophotonic devices. Owing to their typical height and width on a nanometer scale and length up to one millimeter, the fibers bridge the gap between microscopic and macroscopic dimensions. The morphology of the nanofibers (length, height, width, aspect ratio, bending into rings) is susceptable to process parameters and pretreatment of the substrate. But chemically functionalisation of the molecular building block as key step of this thesis widens the scope of possibilities: Nanoaggregates with tailored morphology and optical properties are generated from designed functionalised molecular building blocks. Functional groups have been implemented at the 1,4´´´-para-positions of p-quaterphenylenes using a non-trivial Suzuki cross-coupling strategy. Symmetrically as well as non-symmetrically functionalised oligomers have been achieved in excellent yields and high purity. Additionally, the synthesis of phenyl-thiophene cooligomers and their acceptability for the nanofiber growth on muscovite mica via organic molecule deposition in high vacuum are presented. The fluorescence peak emission frequency of the nanostructures from the substituted p-quaterphenylenes shifts within the blue depending on the functionalisation. While dimensions are still influenced by the growth parameters, the overall shape (i.e. cross-section) of the aggregates alters significantly with the functional group. Even new properties like nonlinear optical activity are accessible. Due to intrinsic nonzero hyperpolarisability of push-pull functionalised oligomers, the respective nanofibers act as frequency doublers and emit strong second harmonic light after excitation with near infrared femtosecond laser pulses. Note that this design approach is possible while conserving the concept of highly crystalline nanofibers. Thus, a new route of bottom-up nanotechnology is presented starting from organic molecular synthesis towards generating of tailormade organic nanofibers opening new prospects for future nanotechnology. 3 ZusammenfassungDie Bildung von nadelförmigen Kristallen aus anorganischen und organischen Halbleitermolekülen wie zum Beispiel para-Phenylenen, α-Thiophenen und Phenylthiophen Co-oligomeren fa...

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Optical absorbance of oriented thin films

Cited by 84 publications

References 9 publications

Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

Uniform π‐System Alignment in Thin Films of Template‐Grown Dicarbonitrile‐Oligophenyls

Organic Molecular Nanotechnology

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