Self-assembly of semiconducting polymer chains during crystallization from a liquid or melt dictates to a large degree the electronic properties of the resulting solid film. However, it is still unclear how charge transport pathways are created during crystallization. Here, we performed complementary in situ electrical measurements and synchrotron grazing incidence X-ray diffraction (GIXD), during slow cooling from the melt of highly regio-regular poly(3-hexylthiophene) (P3HT) films deposited on both graphene and on silicon. Two different charge transport mechanisms were identified, and were correlated to the difference in crystallites' orientations and overall amount of crystallites in the films on each surface as molecular self-assembly proceeded. On silicon, a weak charge transport was enabled as soon as the first edge-on lamellae formed, and further increased with the higher amount of crystallites (predominantly edge-on and randomly oriented lamellae) during cooling. On graphene however, the current remained low until a minimum amount of crystallites was reached, at which point interconnection of conducting units (face-on, randomly oriented lamellae and tie-chains) formed percolated conducting pathways across the film. This lead to a sudden rapid increase in current by ≈10 fold, and strongly enhanced charge transport, despite a much lower amount of crystallites than on silicon.

We investigate non-linear magneto-transport in a Hall bar device made from a strained InGaAs/InP quantum well: a material system with attractive spintronic properties. From extensive maps of the longitudinal differential resistance (r_xx) as a function of current and magnetic (B-) field phase diagrams are generated for quantum Hall breakdown in the strong quantum Hall regime reaching filling factor $\nu$=1. By careful illumination the electron sheet density (n) is incremented in small steps and this provides insight into how the transport characteristics evolve with n. We explore in depth the energetics of integer quantum Hall breakdown and provide a simple picture for the principal features in the r_xx maps. A simple tunneling model that captures a number of the characteristic features is introduced. Parameters such as critical Hall electric fields and the exchange-enhanced g-factors for odd-filling factors including nu=1 are extracted. A detailed examination is made of the B-field dependence of the critical current as determined by two different methods and compiled for different values of n. A simple rescaling procedure that allows the critical current data points obtained from r_xx maxima for even-filling to collapse on to a single curve is demonstrated. Exchange-enhanced g-factors for odd-filling are extracted from the compiled data and are compared to those determined by conventional thermal activation measurements. The exchange-enhanced g-factor is found to increase with decreasing n.

We investigate nonlinear magnetotransport in a Hall bar device made from a strained InGaAs/InP quantum well: a material system with attractive spintronic properties. From extensive maps of the longitudinal differential resistance (rxx) as a function of current and magnetic (B) field, phase diagrams are generated for quantum Hall breakdown in the strong quantum Hall regime reaching filling factor ν = 1. By careful illumination, the electron sheet density (n) is incremented in small steps and this provides insight into how the transport characteristics evolve with n. We explore in depth the energetics of integer quantum Hall breakdown and provide a simple picture for the principal features in the rxx maps. A simple tunneling model that captures a number of the characteristic features is introduced. We present results on the quantum Hall transport diamonds, a spin-flip resonance at high current near ν = 1, instabilities at large B field and current, and a zero-current anomaly. In addition, parameters such as critical Hall electric fields and the exchange-enhanced g factors for odd-filling factors including ν = 1 are extracted. A detailed examination is made of the B-field dependence of the critical current as determined by two different methods and compiled for different values of n. A simple rescaling procedure that allows the critical current data points obtained from rxx maxima for even filling to collapse onto a single curve is demonstrated. Exchange-enhanced g factors for odd filling are extracted from the compiled data and are compared to those determined by conventional thermal-activation measurements. The exchangeenhanced g factor is found to increase with decreasing n.

We developed a method of precise isotope labeling to visualize the continuous growth of graphene by chemical vapor deposition (CVD). This method allows us to see in real time the growth of graphene monocrystals at a resolution of a few seconds. This technique is used to extract the anisotropic growth rates, the formation of dendrites, and the dependence on adsorption area of methane on copper. We obtain a physical picture of the growth dynamics of graphene and its dependence on various parameters. Finally, our method is relevant to other CVD grown materials.

In situ X-ray diffraction analysis of P3HT films during cooling down on both Si and G.

We report on the effect of the annealing temperature on the crystallization and the electrical properties of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on single layer graphene. Electrical characterization showed that heating the P3HT film above the melting point (T-m) resulted in a higher vertical charge carrier mobility. Grazing incidence X-ray diffraction (GIXD) revealed that the film was actually less crystalline overall, but that it consisted of a much higher number of face-on crystallites. We moreover show that annealing above T-m removes the existing seeds still present in the film at lower temperatures and enhances face-on formation. These results provide a better understanding of the influence of the annealing temperature on polythiophene crystallization on graphene, and it shows that the annealing at higher temperature induces a more favorable crystalline orientation which enhances charge transport, despite the reduction in the overall crystallinity. These results should help in the design of more efficient graphene based organic electronic devices by controlling the crystalline morphology of the semiconducting film.

The crystallization and electrical characterization of the semiconducting polymer poly(3‐hexylthiophene) (P3HT) on a single layer graphene sheet is reported. Grazing incidence X‐ray diffraction revealed that P3HT crystallizes with a mixture of face‐on and edge‐on lamellar orientations on graphene compared to mainly edge‐on on a silicon substrate. Moreover, whereas ultrathin (10 nm) P3HT films form well oriented face‐on and edge‐on lamellae, thicker (50 nm) films form a mosaic of lamellae oriented at different angles from the graphene substrate. This mosaic of crystallites with π–π stacking oriented homogeneously at various angles inside the film favors the creation of a continuous pathway of interconnected crystallites, and results in a strong enhancement in vertical charge transport and charge carrier mobility in the thicker P3HT film. These results provide a better understanding of polythiophene crystallization on graphene, and should help the design of more efficient graphene based organic devices by control of the crystallinity of the semiconducting film.

The crystallinity and the electrical properties of thin films of the semiconducting polymer poly-3-hexylthiophene are investigated on a single layer of graphene by D. R. Barbero and team on page 664. Enhanced vertical charge transport and a much higher charge carrier mobility are measured in thicker films due to the face-on orientation induced by the graphene substrate and the formation of an interconnected path of crystallites.

We provide a comprehensive picture of magnetotransport in graphene monolayers in the limit of nonquantizing magnetic fields. We discuss the effects of two-carrier transport, weak localization, weak antilocalization, and strong localization for graphene devices of various mobilities, through theory, experiments, and numerical simulations. In particular, we observe a minimum in the weak localization and strong localization length reminiscent of the minimum in the conductivity, which allows us to make the connection between weak and strong localization. This provides a unified framework for both localizations, which explains the observed experimental features. We compare these results to numerical simulation and find a remarkable agreement between theory, experiment, and numerics. Various graphene devices were used in this study, including graphene on different substrates, such as glass and silicon, as well as low and high mobility devices.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D’ peak corresponding to a double phonon process involving both an interand intra-valley phonon. In order to both eliminate substrate effects and to probe large areas, we undertook to study Raman scattering for large scale chemical vapor deposition (CVD) grown graphene using two different isotopes (C12 and C13) so that we can effectively exclude and subtract the substrate contributions, since a heavier mass downshifts only the vibrational properties, while keeping all other properties the same.

Submillimeter dendritic-shaped crystals were synthesized by low pressure vapor chemical vapor deposition inside a copper enclosure. With their sixfold symmetry and fractal-like shape, the resulting crystals resemble snowflakes. The electronic properties of the devices were investigated down to sub-Kelvin temperatures, showing mobilities over 5000 cm(2)/Vs and the quantum Hall effect at 8T. The magnetoresistance also displayed a sharp peak at zero field which we attribute to weak localization.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double phonon process involving both an inter-and intra-valley phonon. In order to both eliminate substrate effects and to probe large areas, we undertook to study Raman scattering for large scale chemical vapor deposition (CVD) grown graphene using two different isotopes (C12 and C13) so that we can effectively exclude and subtract the substrate contributions, since a heavier mass downshifts only the vibrational properties, while keeping all other properties the same.

We compare the density dependence of the weak localization peak of graphene samples with the one of the computed localization length. The samples show a uniform density dependence of the relative magneto-resistance, which is similar to the computed relative localization length dependence. This analysis is performed for high mobility samples, such as large single crystal flakes as well as low mobility devices and polycrystalline large scale graphene. The magnetic field dependence of the relative localization length exhibits a striking universal behavior.

We look at the magnetic field induced weak localisation peak of graphene samples with different mobilities. At very low temperatures, low mobility samples exhibit a very broad peak as a function of the magnetic field, in contrast to higher mobility samples, where the weak localisation peak is very sharp. We analyze the experimental data in the context of the localisation length, which allows us to extract, both the localisation length and the phase coherence length of the samples, regardless of their mobilities. This analysis is made possible by the observation that the localisation length undergoes a generic weak localisation dependence with striking universal properties.

Highly dendritic graphene crystals up to 0.25 mm in diameter are synthesized by low pressure chemical vapor deposition inside a copper enclosure. With their six-fold symmetry and fractal-like shape, the crystals resemble snowflakes. The evolution of the dendritic growth features is investigated for different growth conditions, and surface diffusion is found to be the growth-limiting step responsible for the formation of dendrites. The electronic properties of the dendritic crystals are examined down to sub-Kelvin temperatures, showing a mobility of up to 6300 cm(2) V-1 s(-1) and quantum Hall oscillations are observed above 4 T. These results demonstrate the high quality of the transport properties despite their rough dendritic edges.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D’ peak corresponding to a double phonon process involving both an interand intra-valley phonon.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double phonon process involving both an inter- and intra-valley phonon.

This review examines the properties of graphene from an experimental perspective. The intent is to review the most important experimental results at a level of detail appropriate for new graduate students who are interested in a general overview of the fascinating properties of graphene. While some introductory theoretical concepts are provided, including a discussion of the electronic band structure and phonon dispersion, the main emphasis is on describing relevant experiments and important results as well as some of the novel applications of graphene. In particular, this review covers graphene synthesis and characterization, field-effect behavior, electronic transport properties, magnetotransport, integer and fractional quantum Hall effects, mechanical properties, transistors, optoelectronics, graphene-based sensors, and biosensors. This approach attempts to highlight both the means by which the current understanding of graphene has come about and some tools for future contributions.