In this work we present a low cost and scalable technique, via ambient pressure chemical vapor deposition (CVD) on polycrystalline Ni films, to fabricate large area (∼cm2) films of single- to few-layer graphene and to transfer the films to nonspecific substrates. These films consist of regions of 1 to ∼12 graphene layers. Single- or bilayer regions can be up to 20 μm in lateral size. The films are continuous over the entire area and can be patterned lithographically or by prepatterning the underlying Ni film. The transparency, conductivity, and ambipolar transfer characteristics of the films suggest their potential as another materials candidate for electronics and opto-electronic applications.

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Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition

Raman spectroscopy in graphene

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

Production, properties and potential of graphene

1. Introduction to Positron Annihilation.- 1.1 Positron Method.- 1.2 Annihilation of Free Positrons.- 1.3 Experimental Techniques.- 1.3.1 Lifetime Measurements.- 1.3.2 Angular Correlation Measurements.- 1.3.3 Line-Shape Measurements.- 1.3.4 Correlation Between Lifetime and Momentum.- 1.4 Positroniurn Formation and Annihilation.- 1.5 Topics of Positron Studies.- 1.5.1 Metals.- 1.5.2 Metal Defects.- 1.5.3 Ionic Crystals.- 1.5.4 Slow Positrons and Positronium.- 1.5.5 Gases and Low-Temperature Phenomena.- 1.5.6 Molecular Sol ids.- 1.5.7 Positronium Chemistry.- 1.6 Summary.- References.- 2. Electron Momentum Densities in Metals and Alloys.- 2.1 Theory.- 2.1.1 Momentum Density.- 2.1.2 Many-Body Effects.- 2.1.3 Positron Thermalization, Effective Mass and Other Thermal Effects.- a) Thermalization.- b) Effective Mass.- c) Other Thermal Effects.- 2.2 Wave Functions.- 2.2.1 Positron Wave Function.- 2.2.2 Electron Band Structure and Wave Functions.- a) OPW Method.- b) APW Method.- c) KKR and Related Methods.- d) Other Methods.- 2.2.3 Symmetry Properties of Aj(?,k).- a) Radial Behavior.- b) Directional Symmetry.- 2.3 Experimental Techniques.- 2.3.1 2? Angular Correlation Measurements.- 2.3.2 Rotating Specimen Method.- 2.3.3 Doppler Broadening.- 2.3.4 Specimen Preparation.- 2.3.5 Corrections.- a) "Beam Profile" Correction.- b) Diffraction Effect.- c) Angular Resolution and Positron Thermal Motion.- d) Finite Slit Length.- 2.3.6 Analysis.- 2.4 Momentum Density Work in Metals.- 2.4.1 Alkali Metals.- 2.4.2 Other Simple Metals.- 2.4.3 Oriented Graphite, Diamond, Silicon, and Germanium.- 2.4.4 Noble Metals.- 2.4.5 Transition Metals and Rare Earths.- 2.5 Disordered Alloys and Ordered Metallic Compounds.- 2.5.1 Disordered Alloys.- 2.5.2 Metallic Compounds.- 2.6 Conclusion.- References.- 3. Positron Studies of Lattice Defects in Metals.- 3.1 Annihilation Parameters for Defect Studies.- 3.1.1 The Defect Trapping Phenomenon and Its Effect.- 3.1.2 Positron States and Lifetime Spectra.- 3.1.3 Momentum Density Parameters.- 3.2 Monovacancies in Equilibrium.- 3.2.1 The Naive Approach to Temperature Effects.- 3.2.2 Prevacancy Effects.- 3.2.3 Other Complications.- 3.2.4 Vacancy Formation Enthalpy Measurements.- 3.2.5 Characteristic or Threshold Temperatures.- 3.2.6 Pressure Experiments.- 3.3 Nonequilibrium Studies.- 3.3.1 The "Many Defects" Problem.- 3.3.2 Deformation, Quenching, and Irradiation Experiments.- 3.3.3 Annealing Studies.- 3.3.4 Positron Studies of Voids.- 3.4 Defect Studies in Alloys.- 3.4.1 Defect vs Impurity Problems.- 3.4.2 Vacancy Studies.- 3.4.3 Phase Transitions and Boundary Effects.- 3.5 Liquid and Amorphous Metals.- References.- 4. Positrons in Imperfect Solids: Theory.- 4.1 Positron Distribution, Mobility, and Trapping.- 4.1.1 Positron Implantation, Slowing Down, and Thermalization.- 4.1.2 Mobility and Diffusion.- 4.1.3 Positron Distribution in Solids.- 4.1.4 Annihilation Characteristics and Electron-Positron Correlation in Pure Metals.- 4.1.5 Effect of Temperature on Annihilation Characteristics.- 4.1.6 Trapping at Defects.- 4.1.7 Self-trapping.- 4.2 Defects in Metals.- 4.2.1 Electronic Structure of Defects.- 4.2.2 Positron-Defect Interaction.- 4.2.3 Annihilation Characteristics.- 4.2.4 Applications.- a) Vacancies.- b) Dislocations.- c) Impurities and Alloys.- d) Vacancy Clusters.- e) Surfaces.- 4.3 Nonmetals.- 4.4 Conclusions.- References.- 5. Positrons in Ionic Solids..- 5.1 Experimental Methods.- 5.1.1 Standard Experimental Techniques.- 5.1.2 Special Experimental Techniques.- a) Two-Parameter Age-Momentum Measurements.- b) Magnetic Quenching Measurements.- 5.1.3 Experimental Difficulties.- a) Analysis of Multicomponent Lifetime Spectra.- b) Analysis of Multicomponent Momentum Distributions.- c) Source and Surface Contributions to Lifetime Spectra.- d) Radiation Damage Due to the Positron Source.- 5.2 Annihilation Characteristics in Alkali Halides.- 5.2.1 Room Temperature Measurements on Crystals with Low Defect Concentration.- a) Lifetime Spectra.- b) Angular Correlation Curves.- c) Doppler-Broadened Annihilation Line Shape.- d) Three-Quantum Annhi1ation.- 5.2.2 Temperature Effects.- 5.2.3 Annihilation in Crystals with High Defect Concentration.- a) Thermal Defect Generation.- b) Thermal Quenching.- c) Additive Coloration.- d) F ? F-Conversion.- e) Aggregation of F Centers.- f) Doping with Divalent Impurities.- g) Defect Creation by Ionizing Radiation.- h) Plastic Deformation.- i) Mixed Crystals.- 5.2.4 Magnetic Field Effects.- a) Crystals with Low Defect Concentration.- b) Additively Colored Crystals.- c) Doped Crystals.- 5.3 Positron States in Alkali Halides.- 5.3.1 Intrinsic States.- a) Nearly Free Positrons in a Perfect Lattice.- b) Quasi-Positronium in Perfect Crystals.- 5.3.2 Annihilation Centers.- a) CA Center.- b) aA+ Center.- c) CA- and cA-(Ca2+) Centers.- d) aA Center.- e) a2A+ and a3A+ Centers.- 5.3.3 Kinetics of State Formation.- a) Slowing Down.- b) Quasi-Positronium Formation in Perfect Crystals.- c) Formation of A Centers.- 5.4 Annihilation in Other Ionic Compounds.- 5.4.1 Hydrides of Alkali and Alkaline-Earth Metals.- 5.4.2 Copper, Silver, Gold, Thallium Halides.- 5.4.3 Alkaline-Earth Halides.- 5.4.4 Alkaline-Earth Oxides.- References.- Additional References with Titles.

Positrons in solids

Variable-energy (0--25 keV) positron stopping and annihilation behavior is studied in a multilayer structure, which has subsequent (\ensuremath{\sim}3000-A\r{}-thick) ZnS and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ layers on a glass substrate. Direct information on positron slowing-down properties is obtained. The positron implantation profile is shown to possess the shape of a derivative of a Gaussian function, in contrast to the more commonly used exponential profile. The mean positron penetration depth varies with incident positron energy E (in keV) as z\ifmmode\bar\else\textasciimacron\fi{}=[4.0(3) \ensuremath{\mu}g/${\mathrm{cm}}^{2}$](E${)}^{1.62(5)}$. The feasibility of the present technique for depth profiling of heterogeneous samples is considered. The accuracy of determining the positions of the interfaces is typically less than 100 A\r{} in the present system. The mobility of positrons in ZnS and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ layers is observed to be very low. This corresponds to positron trapping into structural defects with a relatively high concentration.

Profiling multilayer structures with monoenergetic positrons.

We have performed positron annihilation experiments on CdTe films grown by vacuum evaporation at 220 °C on both plain glass and indium‐tin‐oxide‐coated glass substrates. By checking the linearity of the valence annihilation parameter S versus the core annihilation parameter W we introduce a method to analyze the data which directly shows that the same vacancy defect can be present in all the films. By comparing the core annihilation parameter at the defect to that at the VCd vacancy we can identify this defect as the divacancy VCd‐VTe. Its concentration in the films decreases from about 1018 to less than 1016 cm−3 after annealing in air at 400 °C for about 30 min. Chlorine doping seems to stabilize the divacancies.

Positron trapping at divacancies in thin polycrystalline CdTe films deposited on glass

Iron oxide thin films were grown with gas-phase deposition on a glass substrate in order to study the effects of the deposition temperature and time on the film properties. Characterization of the samples was performed using x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy. It was observed that the film deposited at 350°C consisted of γ-Fe2O3 whereas films produced at temperatures between 400°C and 500°C could be identified as α-Fe2O3. Increasing the deposition temperature resulted in an increase of the grain size at temperatures between 350°C and 450°C. When the deposition time was decreased, a part of the iron ions were observed to be in the divalent state. Copyright © 2004 John Wiley & Sons, Ltd.

http://lib.tkk.fi/Diss/2007/isbn9789512288823/article4.pdf

Pekka J. Hautojärvi

Papers

Positrons in solids

Profiling multilayer structures with monoenergetic positrons.

Positron trapping at divacancies in thin polycrystalline CdTe films deposited on glass

Characterization of iron oxide thin films

Shallow positron traps in GaAs.