Graphene metrology and devices

Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (Γ2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and Γ2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (θ), the values of the A2D/AG ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A2D/AG ratio decreases (respectively, increases) and, conversely, Γ2D increases (respectively, decreases) as a function of N for small (respectively, large) θ. An intermediate behavior has also been found and is interpreted as the presence of both small and large θ within the studied area. These results confirm that neither A2D/AG nor Γ2D are definitive criteria to identify single-layer graphene, or to count N in FLG.

Since the invention of the transmission electron microscope (TEM) in 1931 by Max Knoll and Ernst Ruska, modern TEMs underwent remarkable developments in its overall stability and electron optics. In particular, correction of higher-order lens aberrations has facilitated structural characterization of single molecules at atomic resolution. This improvement in resolution has greatly assisted the design and development of new nano-objects in the field of materials and biological sciences. However, taking full advantage of these developments requires high chemical purity in the sample preparation technique and an electron transparent sample support that not only ensures the reliability in structural characterization, but also boosts the signal-to-noise ratio of TEM images. Moreover, the electron beam in TEMs, in addition to image formation, can be used for structural transformation due to the strong interaction of electrons with the investigated material. In this thesis, the structure of mass-selected molecular species, deposited on graphene via electro-spray ion beam deposition (ES-IBD), is investigated by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) at acceleration voltages between 60 and 80 kV. Due to its impressive electronic and mechanical properties, single- layer graphene (SLG) is used as TEM substrate throughout this work. The transfer methodology of the SLG, obtained by mechanical exfoliation or chemical vapor deposition (CVD), to the TEM grids is presented, followed by thorough structural characterization to analyze the morphology and crystallinity of the transferred SLG using HRTEM. First, beam-sensitive organic molecules such as cytochrome-class hemeprotein (cytochrome-c), chaperonin complex protein molecule (GroEL), copper-pthalocyanine (CuPc) and com- plex molecular magnetic systems in the form of single molecule magnets (SMMs) were investigated. The results showed that despite the presence of the highly conductive substrate and the use of low electron acceleration voltages, small organic molecules did not survive. However, large globular proteins showed some level of electron resistance before suffering radiation damage. Next, the molecular structure of single phosphotungstic acid (PTA) anion molecules deposited by either ES-IBD or the drop-cast method was analyzed and compared. The results showed homogeneous morphology of the samples prepared by ES-IBD. Further- more, by varying coverage and landing energy, it was possible to modify the required morphology. This allowed better control of deposition as compared to the drop cast ap- proach. A main advantage of ES-IBD is the chemical purity during deposition. This is not guaranteed in the drop-cast technique which is performed under ambient conditions. The main part of this thesis deals with the electron-beam-induced transformation of alkali-iodide molecules (CsI, RbI, KI and NaI). The deposited alkali iodide nanoparticles were intentionally exposed to the electron beam to disintegrate them into single ions. It is found that during electron exposure these ions form regular clusters consisting of 4 to 7 atoms, or 2D-crystals. Notably, in the case of CsI, 2D-crystals form only on BLG, whereas on SLG only atomic clusters were formed. HRTEM in combination with electron energy-loss spectroscopy (EELS) is employed to identify the atomic species of the 2D crystals on the atomic scale. In the case of NaI, 2D-crystals form on SLG. Their crystal structure corresponds to the CsCl-type, which has not been reported before. The etching behavior of graphene in the presence of alkali-iodide species is observed in HRTEM experiments for all the deposited species (CsI, RbI, KI, and NaI). By mea- suring the increase in the perimeter of etched holes in SLG, the rate of etching has been calculated. We found that etching was faster in the presence of both alkali and halide ions, as compared to the presence of only one ionic species. First-principles DFT simulation studies were performed to understand the interaction of alkali-iodide dimers on SLG. These calculations yielded quantitative estimates on the charge transfer between the dimer species and the underlying SLG. It was found that adsorption of alkali-iodide on SLG reduces the bonding electron density between the carbon atoms, thus reducing the C-C binding energy. This effect makes the SLG susceptible to damage when exposed to 80-keV electrons, which establishes the catalytic behavior of alkali-iodide species as etchants of SLG.

Graphene, a single atomic layer of graphite, consists of carbon atoms arranged in a honeycomb lattice. Since its experimental isolation on insulating SiO2 (silica) substrates in 2004,1 it has quickly taken the scientific and technological communities by storm. It has been dubbed as a ‘carbon flatland,’ ‘carbonwonderland,’ and a new ‘wonder material’ because of its extraordinary material properties. For instance, it is the ultimate 2D system: it is softer than silk yet stronger than steel; it conducts heat better than copper; and it conducts electricity better than silicon. It is also transparent and elastic, yet as a capping layer it is impermeable to even the smallest molecules. These and other material properties continue to astound. The 2010 Nobel Prize in physics was awarded to Andre Geim andKonstantinNovoselov for their groundbreaking experiments on graphene. The initial flurry of activity following discovery mainly focuses on single-layer graphene. Its few-layer cousins were largely ignored, since they are assumed to be similar to graphite, a well-studied material. More recently, however, researchers have realized that few-layer graphene (FLG) hosts a wealth of fascinating physics and potential applications.2–5 For instance, band structures are more amenable to band-gap engineering and application in digital electronics, while allowing stronger electron interactions that lead to novel correlated phenomena. Equally fascinating is the dependence of their properties on the manner in which successive graphene layers are stacked.6–9 Most FLG and graphite are Bernal-stacked. In other words, one corner of the hexagons of the top sheet is located above the center of the hexagons of the bottom sheet. Because the adjacent atoms in each graphene sheet are considered crystallographically inequivalent, there are up to 2n 2 distinct stacking sequences in n-layer graphene, with different band structures. Stacking order thus provides an important yet rarely explored degree of freedom for tuning the electronic properties of FLG. Figure 1. (a) Schematics of ABA (Bernal)and ABC (rhombohedral)stacked three-layer graphene (TLG), respectively. (b) Resistance (R) vs. charge density n (in log-linear scales) for suspended ABA (red curve) and ABC (blue curve) TLG devices. Upper inset: Scanning electron microscopy image of a suspended TLG graphene device. Scale bar: 2 m.

Graphene modification with gold nanoparticles using the gas aggregation technique

Raman-assessed structural evolution of as-deposited few-layer graphene by He/H2 arc discharge during rapid-cooling thinning treatment

The dielectrophoretic integration of single- and few-layered graphenes from three distinct graphene suspensions is presented, enabling the parallel assembly of individual two-dimensional nanostructures at predefined locations. The first suspension is an aqueous solution of graphene oxide, the second is ultrasonically exfoliated pristine graphene in N-methyl-pyrrolidone (NMP), and the third is exfoliated graphene in surfactant-stabilized 1 wt % aqueous SDBS solutions. The most crucial aspect for the successful thin flake deposition is the solution quality of the exfoliated graphene. After dielectrophoresis, single-layer graphene oxide is placed between the electrodes, which, while initially insulating, recovers its electrical conductivity following thermal reduction. From the chemically unmodified graphene-NMP solutions, the directed assembly of electrically active few-layer graphene flakes is realized, with flake thicknesses in the range 8–30 nm. Liquid phase exfoliation in water-surfactant solutions yields significantly thicker flake dimensions from 50 to several 100 nm due to the higher enthalpy of mixing in the dispersion. To achieve single-layer pristine graphene dielectrophoretic deposition, higher solution qualities must be available, consisting largely of single-layer graphene sheets. The reported research provides an important framework for parallel fabrication approaches of graphene-based devices.

Dielectric and absorption properties of as-grown Bilayer and Single layer graphene at the microwave frequencies

Recent studies on twisted bilayer and multi-layer graphene have opened new avenues of research in condensed matter physics. These artificially made superlattices showed very intriguing electrical properties, including superconductivity at certain commensurate rotation angles between the stacked layers. Even though electrical properties are studied to some extent, the effect of rotation angle on the thermal properties has not received adequate attention. In particular, the thermal expansion of these systems is not well understood. In this direction, we have studied thermal expansion coefficient of few-layer graphene samples with rotational misorientation. These samples were synthesized using chemical vapor deposition technique from a solid carbon precursor. The Raman spectra of these samples indicated the presence of rotational stacking faults, which was confirmed by selected area electron diffraction measurements. Finally, the in-plane thermal expansion coefficient of this system was obtained using temperature dependent Raman spectroscopy for T = 4.5 to 300K. Thermal expansion coefficient was determined to be negative in this temperature range and the corresponding room temperature value was obtained to be ≈ -3.47 × 10-6 K-1. This value is comparable to that of few- layer graphene. We suggest that the thermal expansion of multilayer graphene with rotational stacking faults follows the thermal expansion of the individual few-layer crystallites rather than showing bulk graphitic behavior.

One of the fundamental issues in friction is understanding the atomic details of how two materials slide against each other and start to wear. Whether this involves single-atom processes or the collective motion of atoms has been open to debate for some time. Here we report direct observations of this via in situ studies within a transmission electron microscope. We observed for both graphite and molybdenum disulfide that single atomic layers are transferred from the material to a sliding tip to form a transfer layer, and subsequent sliding takes place by sliding of single layers of graphite or molybdenum disulfide against each other. Despite the similarity of the end result, how the single layers are formed is quite different; with graphite, it involves buckling/wrinkling ~3 nm ahead of the tip, whereas with molybdenum disulfide it is via direct transfer of single sheets. Graphite is more like plastic wrap, molybdenum disulfide more like a pack of cards. This difference is attributed to the large difference in the bending modulus and strength of monolayers in the two cases. In both cases, collective processes are taking place.

Strong interaction of graphene with light accounts for one of its most remarkable properties: the ability to absorb 2.3% of the incident light's energy within a single atomic layer. Free carrier injection via field-effect gating can dramatically vary the optical properties of graphene, thereby enabling fast graphene-based modulators of the light intensity. However, the very thinness of graphene makes it difficult to modulate the other fundamental property of the light wave: its optical phase. Here we demonstrate that considerable phase control can be achieved by integrating a single-layer graphene (SLG) with a resonant plasmonic metasurface that contains nanoscale gaps. By concentrating the light intensity inside of the nanogaps, the metasurface dramatically increases the coupling of light to the SLG and enables control of the phase of the reflected mid-infrared light by as much as 55° via field-effect gating. We experimentally demonstrate graphene-based phase modulators that maintain the amplitude of the reflected light essentially constant over most of the phase tuning range. Rapid nonmechanical phase modulation enables a new experimental technique, graphene-based laser interferometry, which we use to demonstrate motion detection with nanoscale precision. We also demonstrate that by the judicious choice of a strongly anisotropic metasurface the graphene-controlled phase shift of light can be rendered polarization-dependent. Using the experimentally measured phases for the two orthogonal polarizations, we demonstrate that the polarization state of the reflected light can be by modulated by carrier injection into the SLG. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.

Low Energy Electron Microscopy (LEEM) is a child of surface science and was motivated by the desire to image surfaces with Low Energy Electron Diffraction (LEED) beams in a manner similar to that used in transmission electron microscopy of crystalline specimens which produce transmission diffraction beams. In surface science it is usual to monitor surface changes with surface-sensitive probes and, therefore, LEEM instruments were designed from the very beginning to allow a large variety of in-situ experiments [1]. This chapter will not describe the basics, possibilities and limitations of LEEM and its extensions which are amply discussed in recent reviews [2–4], It will rather focus on the possibilities of LEEM for in-situ studies as illustrated by work of the author and his collaborators.

Pentacene on graphene: Differences between single layer and bilayer

Fast STEM image simulation in low-energy transmission electron microscopy by the accurate Chen-van-Dyck multislice method.

This work demonstrated the effects of 1.2 GeV high-energy electron beam irradiation on a few-layers of graphene (FLG) and multi-layer graphene (MLG) films grown via an in-house hot wire chemical vapour deposition (HWCVD) system. The FLG and MLG films were grown on highly doped n-type c-Si (100) substrates which were pre-treated using argon plasma (50 W) for 1 min and 10 min, respectively. The as-prepared samples were then irradiated using a 1.2 GeV high-energy electron beam with a dosage of 1.2 × 109 e-/cm2 at atmospheric and room temperature ambient conditions. The effects of the irradiation-mediated defects on the carbon lattice structure of both graphene samples were validated from the decreased sp2 C=C carbon content, and the increase in the adventitious carbon contamination C-O-C content. Raman results showed an elevation of the ID/IG ratio and blue-shift of the 2D and G band peaks for both the irradiated samples, which validated the mediated defects due to the dislocation of carbon atoms in the graphene sheets. The blue-shifted of 2D and G peaks were much more significant in the MLG than FLG which may indicate a better self-reconstructing property for the MLG atomic network, compared to the FLG. The stability of the films against high-energy electron beam irradiation was validated by their conductivity and surface topography. In conclusion, HWCVD grown graphene nanoplatelet films have high potential for graphene-based high-energy charged particle detectors.

High values of Young’s modulus, excellent fracture strength, extremely high carrier mobility and optical transmittance make graphene films a promising and emerging candidate for next generation transparent and conductive electrodes for future optoelectronic devices. In this brief review, the research progress of graphene-based transparent and conductive electrode will be presented, with an emphasis on the preparation of single or few layers of graphene and their films as well as the latest developments of their applications in rapidly evolving optoelectronic devices, such as displays, solar cells and memory devices. Keywords: Graphene, graphene oxide, optoelectronic devices, transparent conductive films.