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.

Carbon nanotubes: opportunities and challenges

Experimental studies of the superconductive properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why ${\mathrm{A}}_{3}$${\mathrm{C}}_{60}$ are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential ${\mathrm{\ensuremath{\mu}}}^{\mathrm{*}}$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity, are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.

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Superconductivity in fullerides

Organic semiconductor heterojunction (HJ) energy level offsets are modeled using a combination of Marcus theory for electron transfer, and generalized Shockley theory of the dark current density vs voltage $(J\text{\ensuremath{-}}V)$ characteristics. This model is used to fit the $J\text{\ensuremath{-}}V$ characteristics of several donor-acceptor combinations commonly used in thin film organic photovoltaic cells. In combination with measurements of the energetics of donor-acceptor junctions, the model predicts tradeoffs between the junction open-circuit voltage $({V}_{\mathrm{OC}})$ and short-circuit current density $({J}_{\mathrm{SC}})$. The ${V}_{\mathrm{OC}}$ is found to increase with light intensity and inversely with temperature for 14 donor-acceptor HJ materials pairs. In particular, we find that ${V}_{\mathrm{OC}}$ reaches a maximum at low temperature $(\ensuremath{\sim}175\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ for many of the heterojunctions studied. The maximum value of ${V}_{\mathrm{OC}}$ is a function of the difference between the donor ionization potential and acceptor electron affinity, minus the binding energy of the dissociated, geminate electron-hole pair: a general relationship that has implications on the charge transfer mechanism at organic heterojunctions. The fundamental understanding provided by this model leads us to infer that the maximum power conversion efficiency of double heterostructure organic photovoltaic cells can be as high as 12%. When combined with mixed layers to increase photocurrent and stacked cells to increase ${V}_{\mathrm{OC}}$, efficiencies approaching 16% are within reach.

Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells

Metal coating on suspended carbon nanotubes and its implication to metal–tube interaction

Synchrotron-radiation and x-ray photoemission studies of the valence states of condensed phase-pure ${\mathrm{C}}_{60}$ showed seventeen distinct molecular fetaures extending \ensuremath{\sim}23 eV below the highest occupied molecular states with intensity variations due to matrix-element effects involving both cluster and free-electron-like final states. Pseudopotential calculations established the orign of these features, and comparison with experiment was excellent. The sharp C 1s main line indicated a single species, and the nine satellite structures were due to shakeup and plasmon features. The 1.9-eV feature reflected transitions to the lowest unoccupied molecular level of the excited state.

Electronic structure of solid C60: Experiment and theory.

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C28. Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C28 in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C28 with large highest occupied molecular orbital—lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C28 cage to make, for example, C28H4, or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C28. An example of this second, inside route to C28 stabilization is reported here: the laser and carbon-arc production of U@C28.

https://science.sciencemag.org/content/sci/257/5077/1661.full.pdf

Uranium Stabilization of C28: A Tetravalent Fullerene

Electronic-structure studies of ${\mathrm{C}}_{60}$ condensed on metal surfaces show that the energy levels derived from the fullerene align with the substrate Fermi level, not the vacuum level. For thick layers grown on metals at 300 K, the binding energy of the C 1s main line was 284.7 eV and the center of the band derived from the highest occupied molecular orbital was 2.25 eV below the Fermi level. For monolayer amounts of ${\mathrm{C}}_{60}$ adsorbed on Au and Cr, however, the C 1s line was broadened asymmetrically and shifted to lower binding energy, the shakeup features were less distinct, and a band derived from the lowest unoccupied molecular orbital (LUMO) was shifted toward the Fermi level. These monolayer effects demonstrate partial occupancy of a LUMO-derived state, dipole formation, and changes in screening that are associated with LUMO occupancy. Results for ${\mathrm{C}}_{60}$ monolayers on n-type GaAs(110) show transfer of \ensuremath{\le}0.02 electron per fullerene, as gauged by substrate band bending. For ${\mathrm{C}}_{60}$ on p-type GaAs, however, the bands remained flat because electron redistribution was not possible, and the ${\mathrm{C}}_{60}$-derived energy levels were aligned to the substrate vacuum level.

C60 bonding and energy-level alignment on metal and semiconductor surfaces.

XPS probes of carbon-caged metals

Photoemission and inverse photoemission studies of thin films of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ reveal the distribution of occupied and empty electronic states of these molecular solids. X-ray photoemission results also show the C 1s main line and features related to \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ shakeups, electron energy losses, and plasmons. Potassium doping produces changes that can be related to the occupation of states derived from the lowest unoccupied molecular orbitals of the fullerenes and band-structure effects. Important differences are observed upon K doping of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, particularly in states near the Fermi level, and these would be reflected in the electron-phonon coupling, superconductivity, and the phase diagram. Resistivity measurements for ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a resistivity minimum for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and a dependence on stoichiometry that is indicative of dispersed conducting micrograins in an insulating medium. Oxygen-exposure studies demonstrate that ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films are unstable.

T. R. Ohno

Papers

Electronic structure of solid C60: Experiment and theory.

Uranium Stabilization of C28: A Tetravalent Fullerene

C60 bonding and energy-level alignment on metal and semiconductor surfaces.

XPS probes of carbon-caged metals

C60 and C70 fullerenes and potassium fullerides.