During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

We describe the LDA bandstructure of YBa_2Cu_3O_7 in the 2 eV range from the Fermi energy using orbital projections and compare with YBa_2Cu_4O_8. Then, the high-energy and chain-related degrees of freedom are integrated out and we arrive at two, nearest-neighbor, orthogonal, two-center, 8-band Hamiltonians, the even and odd bands of the bi-layer. Of the 8 orbitals, Cu{x2-y2}, O2x, O3y, and Cus have \sigma character and Cu{xz}, Cu{yz} O2z, and O3z have \pi character. The roles of the Cu_s orbital, which has some Cu{3z2-1} character, and the four \pi orbitals are as follows: Cu_s provides 2nd- and 3rd-nearest-neighbor (t' and t') intra-plane hopping, as well as hopping between planes (t_{perp}). The \pi -orbitals are responsible for bifurcation of the saddle-points for dimpled planes. The 4-\sigma-band Hamiltonian is generic for flat CuO_2 planes and we use it for analytical studies. The reduction of the \sigma-Hamiltonian to 3- and 1-band Hamiltonians is explicitly discussed and we point out that, in addition to the hoppings commonly included in many-body calculations, the 3-band Hamiltonian should include hopping between all 2nd-nearest-neighbor oxygens and that the 1-band Hamiltonian should include 3rd-nearest-neighbor hoppings. We calculate the single-particle hopping between the planes of a bi-layer. We show that the inclusion of t' is crucial for understanding ARPES for the anti-ferromagnetic insulator Sr_2CuO_2Cl_2. Finally, we estimate the value of the inter-plane exchange constant for an un-doped bi-layer in mean-field theory using different single-particle Hamiltonians.

LDA energy bands, low-energy Hamiltonians, t', t'', t_{perp}(k), and J_{perp}

LDA energy bands, low-energy hamiltonians, t′, t″, t⊥ (k), and J⊥

Nanoparticles of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline (PDDP) ranging from tens to hundreds of nanometers were prepared by using the reprecipitation method. Their excitonic transitions responsible for absorption and emission, as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that PDDP nanoparticles possess a special size dependence in their optical properties. We identified an extended charge-transfer (CT) state stemming from PDDP molecules closely stacking in nanoparticles and observed its shift to the high-energy side with decreasing nanoparticle size due to exciton confinement. At the same time, the molecular π−π* absorption of the nanoparticles was also blue-shifted, accompanied by an almost unchanging n−π* absorption as a result of the reduced overlap of the pyrazoline ring π orbital and decreased intermolecular interactions. Moreover, S1 and CT states were in equilibrium in the nanoparticles, and the pro...

Size Effects on the Optical Properties of Organic Nanoparticles

The influence of the organic layer thickness on short-circuit photocurrent spectra and efficiency is investigated in heterojunction photovoltaic cells with the electron donor materials poly(p-phenylenevinylene) (PPV) and Cu-phthalocyanine (CuPc), respectively, together with C60 as electron acceptor material. The main process of photocurrent generation after light absorption, exciton generation, and exciton diffusion in the bulk of the absorbing material is given by the exciton dissociation at the donor–acceptor interface. We determined a strong dependence of the optimum layer thickness of the absorbing material on the exciton diffusion length by systematically varying the layer thickness of the electron donor material. Additionally, a significant photocurrent contribution occurred due to light absorption and exciton generation in the C60 layer with a subsequent hole transfer to PPV, respectively, CuPc at the dissociation interface. Using a simple rate equation for the exciton density we estimated the exci...

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Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells

Optical spectra for ${\mathrm{C}}_{60}$ films, obtained by elliposmetry, are reported for the energy range from 1.9 to 9.5 eV. Our results are compared with related results from other optical techniques as well as photoemission results. Strong similarity to absorption measurements of dissolved ${\mathrm{C}}_{60}$ results from the strong molecular character of the solid films. We fit our spectra with Lorentzian line shapes to determine energy positions and shapes of the transitions. We observe the onset of optical absorption near 1.9 eV. We also report initial results from alkali-metal doping and a useful method for incorporating K into the bulk of film samples.

Optical transitions of C 60 films in the visible and ultraviolet from spectroscopic ellipsometry

The dielectric tensor elements of oriented YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} samples, as obtained by spectroscopic ellipsometry, are presented for several oxygen stoichiometries in the photon energy range between 1.66 and 5.5 eV. For {delta}=0 and 1, we extend this range up to 9.5 eV using a recently developed synchrotron radiation ellipsometer. We compare the experimental results with computed values for the dielectric function obtained from a linear-muffin-tin-orbitals band-structure calculation. We find good general agreement for both the orientational and compositional dependence. In particular, both experiment and theory show {ital c} polarization for a peak near 2.7 eV for {delta}{lt}0.5, while a transition in that spectral region is {ital a} polarized for YBa{sub 2}Cu{sub 3}O{sub 6}. In spite of the difficulty in predicting insulating behavior of the low-oxygen material, these results show that the local-density-approximation picture is successful in describing much of the electronic structure determining higher-energy excitations. In addition, the measured dependence of the dielectric tensor on the oxygen stoiochiometry provides a reference for optical determination of the oxygen content and orientation in applications, such as thin-film or single-crystal growth.

Anisotropy and oxygen-stoichiometry dependence of the dielectric tensor of YBa2Cu3O7- delta (0 <= delta <= 1).

The design, construction, and performance of an automatic photometric ellipsometer with rotating analyzer for the VUV region (5–30 eV) is described. The use of a synchrotron radiation source and triple‐reflection polarizers makes this new spectral region accessible to spectroscopic ellipsometry. New dielectric function data are presented for InP(110), YBa2Cu3O7, and epitaxial CaF2/Si(111).

Spectroscopic ellipsometry with synchrotron radiation

The dielectric function of cubic CdS films grown epitaxially on InP(110) has been determined by spectroscopic ellipsometry in the photon energy range 4.5-18 eV. Structures in the imaging part of e below 9.5 eV are assigned to transitions at critical points of the Brillouin zone and are compared with of measurements on hexagonal CdS performed in the same energy range. Structures in e 2 above 12 eV are assigned to transitions from cadmium 4d and sulfur 3s states into the conduction band

Dielectric function of cubic and hexagonal CdS in the vacuum ultraviolet region.

We have investigated the dependence of the visible-ultraviolet dielectric function of Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7} on the Pr content {ital x} using rotating analyzer ellipsometry. Based on recently published band-structure calculations, we believe that the valency of Pr can be deduced from the optical properties in this spectral region. Interband transitions should shift to higher energies for Pr{sup 4+} and to lower energies for Pr{sup 3+} when compared to the optical spectra of YBa{sub 2}Cu{sub 3}O{sub 7}. In the case of tetravalent Pr ions, transitions into unoccupied Pr 5{ital d} levels should be observable. Since the experimental energies for interband transitions shift down with increasing {ital x} and no additional structure can be seen, we conclude that Pr is trivalent in Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}.

D. Fuchs

Papers

Optical transitions of C 60 films in the visible and ultraviolet from spectroscopic ellipsometry

Anisotropy and oxygen-stoichiometry dependence of the dielectric tensor of YBa2Cu3O7- delta (0 <= delta <= 1).

Spectroscopic ellipsometry with synchrotron radiation

Dielectric function of cubic and hexagonal CdS in the vacuum ultraviolet region.

Optical investigation of the valency of Pr in Y 1-x Pr x Ba 2 Cu 3 O 7-δ