Karsten Hannewald

Ab initio theory of charge-carrier conduction in ultrapure organic crystals

We present an ab initio description of charge-carrier mobilities in organic molecular crystals of high purity. Our approach is based on Holstein’s original concept of small-polaron bands but generalized with respect to the inclusion of nonlocal electron–phonon coupling. By means of an explicit expression for the mobilities as a function of temperature in combination with ab initio calculations of the material parameters, we demonstrate the predictive power of our theory by applying it to naphthalene. The results show a good qualitative agreement with experiment and provide insight into the difference between electron and hole mobilities as well as their peculiar algebraic and anisotropic temperature dependencies.

DFT studies using supercells and projector-augmented waves for structure, energetics, and dynamics of glycine, alanine, and cysteine.

A large variety of gas phase conformations of the amino acids glycine, alanine, and cysteine is studied by numerically efficient semi‐local gradient‐corrected density functional theory calculations using a projector‐augmented wave scheme and periodic boundary conditions. Equilibrium geometries, conformational energies, dipole moments, vibrational modes, and IR optical spectra are calculated from first principles. A comparison of our results with values obtained from quantum‐chemistry methods with localized basis sets and nonlocal exchange‐correlation functionals as well as with experimental data is made. For conformations containing strong intramolecular hydrogen bonds deviations in their energetic ordering occur, which are traced back to different treatments of spatial nonlocality in the exchange‐correlation functional. However, even for these structures, the comparison of calculated and measured vibrational frequencies shows satisfying agreement.

Charge transport in organic crystals: interplay of band transport, hopping and electron–phonon scattering

We present an ab initio description of charge transport in organic semiconductors based on a recently developed theory that goes beyond small-polaron and/or narrow-band models. The mobility expression is evaluated with parameters from density functional theory, and application to naphthalene crystals demonstrates substantial progress in the simulated temperature dependence and relative magnitudes for all transport directions. The scattering by phonons is described in a manner that goes beyond the Holstein model for small polarons and, consequently, significantly improves the temperature dependence and anisotropy of carrier mobility with respect to the previous narrow-band theory. The contributions of coherent and incoherent scattering processes are analyzed. Special emphasis is placed on the origin and understanding of the transition from band transport at low temperatures to hopping at high temperatures, both of which are fully included in the theory. Additionally, in contrast to earlier narrow-band theories, the unphysical divergence at zero temperatures is removed.

Guanine Crystals : A First Principles Study

We report first principles density functional theory studies on the basic ground state characteristics, dynamic properties, and the electronic structure of guanine crystals. The effect of water molecules within the crystal is studied in detail, and we discuss their influence on the structural, vibrational, and electronic properties. The geometries calculated for various crystal structures are compared with gas-phase calculations and available experimental data. Phonon frequencies and eigenvectors are predicted for intermolecular and intramolecular lattice vibrations. Vibrational and electronic density-of-states are presented and analyzed. The electronic band structure near the fundamental gap is calculated from the Kohn-Sham approach. We find that the former molecular HOMO states form a dispersive band in the pi-pi stacking direction upon condensation resulting in a large bandwidth of 0.83 eV. Consequences for the charge transport in layered van der Waals bonded organic molecular crystals are discussed.

Charge Transport in Guanine-Based Materials

Temperature-induced dynamic disorder is one of the great unknowns in charge transport through DNA and DNA-related systems. Using guanine crystals as a model system, we studied the temperature dependence and anisotropy of charge-carrier (hole) transport in guanine-based materials. Employing the Kubo formalism, we calculated the hole mobilities with ab initio DFT material parameters. Our findings are discussed in relation to transport pathways in DNA-based structures such as guanine quadruplexes and ribbons, which are considered to play a major role in DNA-based nanoelectronics. The mobility results are interpreted by means of a novel visualization method for transport channels that we derived from overlapping wave functions. An analysis of coherent and incoherent contributions to the mobility shows that, even in materials with high purity and long-range order such as crystals, only incoherent phonon-assisted hopping occurs at room temperature.

Ab initio description and visualization of charge transport in durene crystals

We study the charge transport through crystalline durene which exhibits large hole mobilities. By means of ab initio calculations, we determine the temperature-dependent polaron bandwidth and the mobility tensor of the charge carriers. The origin of the bandlike mobility curves is discussed, and the strong anisotropy of the mobility is analyzed in detail. We put a special focus on the relationship between crystal packing geometry, molecular wave function overlap, and charge transport properties. The results include a visualization of the transport channels in durene which can be regarded as a prototypical herringbone-stacked crystal.

Characteristics of small- and large-polaron motion in organic crystals.

Based on a generalized theory of charge transport in organic crystals we investigate the motion of polarons of arbitrary size. Within this theory, we analyze the influence of characteristic electronic, vibronic, and thermal energies and their role for the transport regimes. The polaron bandwidth is identified as the central temperature-dependent quantity. The size of the dressed charge carriers is used to analyze the correlation between the polaron size and transport mechanism. In addition, we propose that the band and hopping transport can be distinguished by investigating the mobility anisotropy. To do this only electronic structure parameters are necessary for comparison.

A note on temperature-dependent band narrowing in oligo-acene crystals

We present a theoretical description of polaron band narrowing in oligo-acene crystals due to electron–lattice interaction. The analysis is based on a model which takes both local and nonlocal contributions to the electron–phonon coupling into account. Different approximation schemes are discussed and compared. The theory is supplemented by quantitative ab initio calculations of the temperature dependence of polaron bandwidths in oligo-acene crystals which show the important role of in-plane nonlocal electron–phonon coupling.

Excitonic insulator through coherent pulse excitation

The excitonic insulator was predicted numerous times by theory, but the experimental observation is conspicuous by absence. We demonstrate that, even under ideal circumstances, an excitonic insulator cannot be created by means of coherent optical excitation. If dephasing is neglected, a non-stationary excitonic insulator may arise. If a dephasing is taken into account, a hypothetical excitonic insulator would disappear within the characteristic dephasing time. Moreover, we find that dephasing prevents the buildup of the excitonic insulator from the beginning.

Finite-size modelling of electrodes for quantum transport calculations using k-space ab initio techniques

Abstract We present an efficient and easy-to-use numerical treatment to check the necessary size of metallic leads with respect to their quantum-transport properties. The suggested method works entirely in k-space and avoids complicated projections of ab initio band structures onto real-space tight-binding-like Hamiltonians. As an illustrative example, we apply the method to various low-dimensional Au chains and discuss our DFT-based studies with respect to convergence speed and relevance of structural relaxation.