The phase behavior of three N-alkyl-substituted perylene diimide derivatives is examined by differential scanning calorimetry and polarized optical microscopy. The occurrence of multiple phase transitions indicates several crystalline and several liquid crystalline phases. X-ray diffraction measurements show that the liquid crystalline phases display high structural ordering in all three dimensions:  smectic layers are formed, and within these smectic layers an additional ordering in columns is observed. Molecular modeling confirms this result and substantiates smectic ordering with interdigitating alkyl chains that determine the distance between the smectic layers. The ordering in columns is favored by π−π interactions between the cofacially oriented perylene molecules and by the elliptic shape of the molecule. Finally, intermolecular dipole−dipole interactions between the carbonyl groups of the imide moieties cause the perylene molecules to orient on average with a slight rotation between neighboring mo...

Liquid Crystalline Perylene diimides : Architecture and Charge Carrier Mobilities

Owing to their atomic layer thickness, strong light–material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

2D Materials for Optical Modulation: Challenges and Opportunities.

Recent advances in the synthesis and modification of carbon-based 2D materials for application in energy conversion and storage

Cluster-surface interaction: From soft landing to implantation

Laser-assisted synthesis, reduction and micro-patterning of graphene: Recent progress and applications

We have studied single-shot femtosecond laser ablation of graphite by combining a variety of experimental techniques including Raman spectroscopy, atomic force microscopy as well as time of flight spectrometry. The comprehensive analysis reveals insights into the ablation process by exploring the surface structure, the fluence dependence, and the structural dynamics of the detachment. The results show formation and detachment of charged carbon products (such as graphene nanoflakes) from the surface. Time-resolved measurements of ion yields and velocities reveal strong quenching and revival of Coulomb explosion as a function of delay time in the range of 100--200 fs, suggesting a displacive motion between the topmost surface layers which regulates the optical properties of the system.

Ultrafast laser ablation of graphite

The authors report single-shot femtosecond laser ablation experiments on the highly oriented pyrolitic graphite surface. The yield of positively charged carbon clusters has been measured by time-of-flight mass spectrometry. The reconstructed velocity distribution of atomic carbon ions indicates that the observed Coulomb explosion is most pronounced for fluences slightly above the damage threshold. Atomic force microscopy studies of the damage spots indicate nanoscopic removal of intact monolayers. The results imply a significant degree of charge localization on the surface graphite layers.

Nanoscopic Coulomb explosion in ultrafast graphite ablation

N,N‘-Dimethylperylenetetracarboxylic acid diimide (Me-PTCDI) thin films were prepared by vapor deposition technique. The photogeneration and radiative singlet and nonradiative triplet exciton recom...

Optical and magnetooptical measurements of n,n'-dimethylperylene-3,4,9,10-tetracarboxylic acid diimide thin films

The dynamics of ultrafast laser ablation of graphite have been investigated by femtosecond time resolved autocorrelation measurements at $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The results indicate that multiply charged atomic ions and ionized carbon clusters are repelled by a transient electric field developed on the surface during the laser pulse, the process known as impulsive Coulomb explosion. From time resolved measurements, we estimate that the decay time constant of the surface field is $125\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$.

Emission of ions and charged clusters due to impulsive Coulomb explosion in ultrafast laser ablation of graphite

Excitation and recombination dynamics of the photoexcited charge carriers in porous silicon membranes were studied using a femtosecond pump-probe technique. Near-infrared pulses (800 nm, 60 fs) were used for the pump while, for the probe, we employed different wavelengths in the range between 3.4 and $5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{m}$ covering the medium wavelength infrared range. The data acquired in these experiments consist of simultaneous measurements of the transmittance and reflectance as a function of the delay time between the pump and probe for different pump fluences and probe wavelengths. To evaluate the results, we developed an optical model based on the two-dimensional Maxwell-Garnett formula, incorporating the free-carrier Drude contribution and nonuniformity of the excitation by the Wentzel-Kramers-Brillouin model. This model allowed the retrieval of information about the carrier density as a function of the pump fluence, time, and wavelength. The carrier density data were analyzed to reveal that the recombination dynamics is governed by Shockley-Read-Hall and Auger processes, whereas the diffusion has an insignificant contribution. We show that, in porous silicon samples, the Auger recombination process is greatly enhanced at the wavelength corresponding to the infrared-active vibrational modes of the molecular impurities on the surface of the pores. This observation of surface-vibration-assisted Auger recombination is not only for porous silicon in particular, but for low-dimension and bulk semiconductors in general. We estimate the time constants of Shockley-Read-Hall and Auger processes, and demonstrate their wavelength dependence for the excited carrier density in the range of ${10}^{18}--{10}^{19}\phantom{\rule{0.28em}{0ex}}1/{\text{cm}}^{3}$. We demonstrate that both processes are enhanced by up to three orders of magnitude with respect to the bulk counterpart. In addition, we provide a plethora of the physical parameters evaluated from the experimental data, such as the dielectric function and its dependence on the injection level of the free carriers, charge-carrier scattering time related high-frequency conductivity, and the free-carrier absorption at the midwave infrared range.

A. Kaplan

Papers

Ultrafast laser ablation of graphite

Nanoscopic Coulomb explosion in ultrafast graphite ablation

Optical and magnetooptical measurements of n,n'-dimethylperylene-3,4,9,10-tetracarboxylic acid diimide thin films

Emission of ions and charged clusters due to impulsive Coulomb explosion in ultrafast laser ablation of graphite

Carrier dynamics and surface vibration-assisted Auger recombination in porous silicon