The nitrogen-vacancy (NV) colour centre in diamond is an important physical system for emergent quantum technologies, including quantum metrology, information processing and communications, as well as for various nanotechnologies, such as biological and sub-diffraction limit imaging, and for tests of entanglement in quantum mechanics. Given this array of existing and potential applications and the almost 50 years of NV research, one would expect that the physics of the centre is well understood, however, the study of the NV centre has proved challenging, with many early assertions now believed false and many remaining issues yet to be resolved. This review represents the first time that the key empirical and ab initio results have been extracted from the extensive NV literature and assembled into one consistent picture of the current understanding of the centre. As a result, the key unresolved issues concerning the NV centre are identified and the possible avenues for their resolution are examined.

The nitrogen-vacancy colour centre in diamond

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Discotic liquid crystals: a new generation of organic semiconductors

Pyrene-based materials for organic electronics.

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

An analytic model of the equilibrium hopping conductivity in a disordered organic semiconductor at large charge carrier densities is formulated. Calculated dependences of the equilibrium hopping mobility upon the carrier density are compared with recent experimental data obtained on doped poly(3-hexylthiophene) films. Doping is shown to create additional energy disorder due to potential fluctuations caused by the Coulomb field of randomly distributed dopant ions.

Charge carrier mobility in doped semiconducting polymers

An analytic expression for the effective transport energy in a positionally random and energetically disordered hopping system is obtained. It is shown that multiple carrier jumps within pairs of occasionally close localized states strongly affect the position of the effective transport level on the energy scale and lead to a noticeable difference between the effective transport energy and the energy of most probable jumps. In a hopping system with a Gaussian density-of-states energy distribution, the equilibrium carrier mobility is found to be an almost factorized function of temperature and concentration of localized states.

Effective transport energy versus the energy of most probable jumps in disordered hopping systems - art. no. 125125

Optical experiments, involving photoluminescence (PL), the PL excitation and quenching spectra, as well as transmission and its quenching, were used to analyse the photochromic behaviour of some vacancy-related complexes in diamond. The 2.156 eV, 1.945 eV and 1.68 eV optical centres in CVD diamond are attributed to the neutral nitrogen-vacancy, negative nitrogen-vacancy and neutral silicon-vacancy ([Si-V]0 ) centres, respectively. Oscillatory behaviour in the excitation spectrum of the 1.68 eV luminescence is observed and from the threshold of the oscillations a position of EC - 2.05 eV is suggested for the ground state of the [Si-V]0 centre.

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Photochromism of vacancy-related centres in diamond

Photoluminescence excitation (PLE) spectra, measured at varying temperatures and for different luminescent energies, are applied systematically to the study of defects in $\mathrm{I}a,$ $\mathrm{I}b,$ $\mathrm{II}b,$ and chemical-vapor-deposited (CVD) diamond. It is shown that the green luminescent band in CVD diamond films consists of two distinct components. The first one is attributed to recombination in the amorphous carbon phase. The second component increases with surface hydrogenation in CVD films, but an analogous band remains almost unchanged in Ib diamond. This band is ascribed to the donor-acceptor pair recombination. The photoionization threshold for transitions from the valence band to the donor level is determined as 3.25 eV. Based on PLE measurements the broad blue band is assigned to the vibronic band of a dislocation-related center with the ground state position in the range from ${E}_{V}$ to ${E}_{V}+0.37$ eV. A series of undocumented photoluminescence (PL) lines at 1.81, 1.84, 1.91, 2.02, 2.1, and 2.2 eV is characterized by PL and PLE techniques as belonging to radiative transitions from different excited states to the same ground state of a divacancy-related center. Oscillatory behavior in the PLE spectra from CVD diamond is used to deduce ${E}_{C}\ensuremath{-}2.24 \mathrm{eV}$ and ${E}_{C}\ensuremath{-}2.05 \mathrm{eV}$ as the optical ionization thresholds to the conduction band for the divacancy-related and the 1.68 eV centers, respectively.

Luminescence excitation spectra in diamond

An analytic model of the weak-field carrier transport in an energetically disordered and positionally random hopping system is formulated. Within the framework of this model, the carrier mobility can be calculated by either direct averaging of carrier hopping rates or by the use of the effective transport energy concept. It is shown that multiple carrier jumps within pairs of occasionally close hopping sites affect the position of the effective transport level on the energy scale. In good quantitative agreement with experimental data and results of Monte Carlo simulation, the temperature and concentration dependences of the mobility can be almost perfectly factorized, i.e. represented as a product of two functions one of which depends solely upon the temperature while the other governs the dependence upon the density of localized states. The model is also used for the calculation of trap-controlled hopping mobility and for the analysis of hopping transport at high charge-carrier densities.

http://iopscience.iop.org/article/10.1088/0953-8984/14/42/305/pdf

Guy Adriaenssens

Papers

Charge carrier mobility in doped semiconducting polymers

Effective transport energy versus the energy of most probable jumps in disordered hopping systems - art. no. 125125

Photochromism of vacancy-related centres in diamond

Luminescence excitation spectra in diamond

Weak-field carrier hopping in disordered organic semiconductors: the effects of deep traps and partly filled density-of-states distribution