Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

/pdf/prospects-of-colloidal-nanocrystals-for-electronic-and-3dxpyhl3r7.pdf

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

-phenylenevinylene)s L4. Fluorene-Based Conjugated Polymers L4.1. Fluorene-Based Copolymers ContainingElectron-Rich MoietiesM4.2. Fluorene-Based Copolymers ContainingElectron-Deﬁcient MoietiesN4.3. Fluorene-Based Copolymers ContainingPhosphorescent ComplexesQ5. Carbazole-Based Conjugated Polymers R5.1. Poly(2,7-carbazole)-Based Polymers R5.2. Indolo[3,2-

https://ir.nctu.edu.tw:443/bitstream/11536/6508/1/000271856900020.pdf

Synthesis of Conjugated Polymers for Organic Solar Cell Applications

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.

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Charge Transport in Organic Semiconductors

Perylene tetracarboxylic diimide (PTCDI) derivatives stand out as one of the most investigated families of air-stable n-type organic semiconductors for organic thin-film transistors. Here, we use density functional theory to illustrate how it is possible to control the charge-transport parameters of PTCDIs as a function of the type, number, and positions of the substituents. Specifically, two strategies of functionalization related to core and end substitutions are investigated. While end-substituted PTCDIs present the same functional molecular backbone, their molecular packing in the crystal significantly varies; as a consequence, this series of derivatives constitutes an ideal test bed to evaluate the models that describe charge-transport in organic semiconductors. Our results indicate that large bandwidths along with small effective masses can be obtained with the insertion of appropriate substituents on the nitrogens, in particular halogenated aromatic groups.

Tuning the charge-transport parameters of perylene diimide single crystals via end and/or core functionalization: a density functional theory investigation.

The charge-transport parameters of the perfluoropentacene and perfluorotetracene crystals are studied with a joint experimental and theoretical approach that combines gas-phase ultraviolet photoelectron spectroscopy and density functional theory. To gain a better understanding of the role of perfluorination, the results for perfluoropentacene and perfluorotetracene are compared to those for their parent oligoacenes, that is, pentacene and tetracene. Perfluorination is calculated to increase the ionization potentials and electron affinities by approximately 1 eV, which is expected to reduce significantly the injection barrier for electrons in organic electronics devices. Perfluorination also leads to significant changes in the crystalline packing, which greatly affects the electronic properties of the crystals and their charge-transport characteristics. The calculations predict large conduction and valence bandwidths and low hole and electron effective masses in the perfluoroacene crystals, with the largest mobilities expected along the pi-stacks. Perfluorination impacts as well both local and nonlocal vibrational couplings, whose strengths increase by a factor of about 2 with respect to the parent compounds.

Impact of Perfluorination on the Charge-Transport Parameters of Oligoacene Crystals

We present a multimode vibrational analysis of the gas-phase ultraviolet photoelectron spectra of the first ionization in anthracene, tetracene, and pentacene, using electron-vibration constants computed at the density functional theory level. The first ionization of each molecule exhibits a high-frequency vibronic structure; it is shown that this regularly spaced feature is actually the consequence of the collective action of several vibrational modes rather than the result of the interaction with a single mode. We interpret this feature in terms of the missing mode effect. We also discuss the vibronic coupling constants and relaxation energies obtained from the fit of the photoelectron spectra with the linear vibronic model.

A multimode analysis of the gas-phase photoelectron spectra in oligoacenes

Petacene is one of the most promising organic semiconductors for thin-film transistors. Transport measurements in the past have established the presence of shallow traps but their origins have remained a mystery. Here we show that shallow traps in vapor-deposited crystalline pentacene thin films are due to local defects resulting from the sliding of pentacene molecules along their long molecular axis, while two-dimensional crystalline packing is maintained. Electronic structural calculation confirms that these sliding defects are shallow-charge traps with energies ⩽100meV above (below) the valence band maximum (conduction band minimum).

Demetrio A. da Silva Filho

Papers

Charge Transport in Organic Semiconductors

Tuning the charge-transport parameters of perylene diimide single crystals via end and/or core functionalization: a density functional theory investigation.

Impact of Perfluorination on the Charge-Transport Parameters of Oligoacene Crystals

A multimode analysis of the gas-phase photoelectron spectra in oligoacenes

Shallow trap states in pentacene thin films from molecular sliding