2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

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 have reviewed humidity sensors based on various materials for both relative and absolute humidity, including ceramic, semiconducting, and polymer materials. In the majority of publications, there are few papers dealing with absolute humidity sensors, which have extensive applications in industry. We reviewed extensively absolute humidity sensors in this article, which is unique comparing with other reviews of humidity sensors. The electrical properties of humidity sensors such as sensitivity, response time, and stability have been described in details for various materials and a considerable part of the review is focused on the sensing mechanisms. In addition, preparation and characterization of sensing materials are also described. For absolute humidity sensors, mirrorbased dew-point sensors and solid-state Al2O3 moisture sensors have been described. As the major problem in Al2O3 moisture sensors, long-term instability, has been solved, � -Al2O3 moisture sensors may have promising future in industry.

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Humidity Sensors: A Review of Materials and Mechanisms

We provide an overview of the IMPACT molecular mechanics program with an emphasis on recent developments and a description of its current functionality. With respect to core molecular mechanics technologies we include a status report for the fixed charge and polarizable force fields that can be used with the program and illustrate how the force fields, when used together with new atom typing and parameter assignment modules, have greatly expanded the coverage of organic compounds and medicinally relevant ligands. As we discuss in this review, explicit solvent simulations have been used to guide our design of implicit solvent models based on the generalized Born framework and a novel nonpolar estimator that have recently been incorporated into the program. With IMPACT it is possible to use several different advanced conformational sampling algorithms based on combining features of molecular dynamics and Monte Carlo simulations. The program includes two specialized molecular mechanics modules: Glide, a high-throughput docking program, and QSite, a mixed quantum mechanics/molecular mechanics module. These modules employ the IMPACT infrastructure as a starting point for the construction of the protein model and assignment of molecular mechanics parameters, but have then been developed to meet specialized objectives with respect to sampling and the energy function.

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Integrated Modeling Program, Applied Chemical Theory (IMPACT).

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

Nonlinear optical phenomena, C.N. Ironside and R.W. Munn nonlinear optical devices C.N. Ironside and R.W. Munn optical nonlinear effects in semiconductors, C.N. Ironside and R.W. Munn nonlinear glasses, J.S. Aitchison ferroelectric crystals, M. Ebrahimzadeh and A.I. Ferguson Molecular crystals, R.T. Bailey, et al polyners, G.H. Cross Langmuir-Blodgett films, P. Hodge and N. McKeown liquid crystals, H.J. Coles.

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Principles and applications of nonlinear optical materials

An improved general theory of electronic transport in molecular crystals with local linear electron–phonon coupling is presented. It is valid for arbitrary electronic and phonon bandwidths and for arbitrary electron–phonon coupling strength, yielding small‐polaron theory for narrow electronic bands and strong coupling, and semiconductor theory for wide electronic bands and weak coupling. Detailed results are derived for electronic excitations fully clothed with phonons and having a bandwidth no larger than the phonon frequency; the electronic and phonon densities of states are taken as Gaussian for simplicity. The dependence of the diffusion coefficient on temperature and on the other parameters is analyzed thoroughly. The calculated behavior provides a rational interpretation of observed trends in the magnitude and temperature dependence of charge‐carrier drift mobilities in molecular crystals.

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General theory of electronic transport in molecular crystals. I. Local linear electron–phonon coupling

A generalized transformation is applied to a model Hamiltonian incorporating both local and nonlocal exciton–phonon coupling. For the application of the usual transport theory, the difference between the transformed coupling and its thermal average over the free phonon ensemble must be small. This is achieved by a temperature‐dependent set of transformation coefficients defined by a transcendental equation which is soluble numerically after some approximation. Nonlocal coupling increases both the exciton binding energy and the usual band narrowing, but it also changes the band shape in a way which may introduce new minima and band broadening. The exciton velocities are then no longer related to the exciton hopping and scattering rates in the same way as in local coupling.

Theory of electronic transport in molecular crystals. II. Zeroth order states incorporating nonlocal linear electron–phonon coupling

The importance of dipolar interactions is highlighted in a wide range of molecular crystal properties. Molecules respond via an effective polarizability (appropriate to the crystal phase) to a local field arising from the applied field and the fields of the surrounding induced dipoles, determined self-consistently. An exact treatment is possible for perfect crystals and crystals with localized imperfections, yielding the inverse dielectric function characterizing the crystal response. From this, results follow for the crystal linear and nonlinear dielectric response, for Coulomb exciton energies, and for the intensities of lattice vibrational spectra. A direct extension yields the polarization energy associated with the induced dipoles. This provides insight into the energetics of excess charge carriers in perfect and imperfect crystals and of charge-transfer excitons, into electronic Stark spectra, and into aspects of lattice dynamics and phase transitions.

Electric dipole interactions in molecular crystals

Electronic transport in molecular crystals is studied for simultaneous local and nonlocal linear electron–phonon coupling using a generalized polaron Hamiltonian derived previously. Nonlocal coupling increases the scattering, giving lower band contributions and higher hopping contributions. It also gives a phonon‐assisted term which dominates at high temperature, leading eventually to a constant diffusion coefficient whose magnitude depends on the ratio of the nonlocal to local coupling and is independent of transfer integral. Incorporating nonlocal coupling thus mainly increases the magnitude of the difffusion coefficient and decreases its temperature dependence.

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R. W. Munn

Papers

Principles and applications of nonlinear optical materials

General theory of electronic transport in molecular crystals. I. Local linear electron–phonon coupling

Theory of electronic transport in molecular crystals. II. Zeroth order states incorporating nonlocal linear electron–phonon coupling

Electric dipole interactions in molecular crystals

Theory of electronic transport in molecular crystals. III. Diffusion coefficient incorporating nonlocal linear electron–phonon coupling