Monolayer graphene was first obtained as a transferable material in 2004 and has stimulated intense activity among physicists, chemists and material scientists. Much research has been focused on developing routes for obtaining large sheets of monolayer or bilayer graphene. This has been recently achieved by chemical vapour deposition (CVD) of CH(4) or C(2)H(2) gases on copper or nickel substrates. But CVD is limited to the use of gaseous raw materials, making it difficult to apply the technology to a wider variety of potential feedstocks. Here we demonstrate that large area, high-quality graphene with controllable thickness can be grown from different solid carbon sources-such as polymer films or small molecules-deposited on a metal catalyst substrate at temperatures as low as 800 °C. Both pristine graphene and doped graphene were grown with this one-step process using the same experimental set-up.

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. Doping with electron-donor nitrogen heteroatoms can modulate the electronic properties of graphene to produce an n-type semiconductor. Here we demonstrate the growth of monolayer nitrogen-doped graphene in centimeter-scale sheets using a chemical vapor deposition process with pyridine as the sole source of both carbon and nitrogen. High-resolution transmission microscopy and Raman mapping characterizations indicate that the nitrogen-doped graphene sheets are uniformly monolayered. The existence of nitrogen-atom substitution in the graphene planes was confirmed by X-ray photoelectron spectroscopy. Electrical measurements show that the nitrogen-doped graphene exhibits an n-type behavior, different from pristine graphene. The preparation of large-area nitrogen-doped graphene provides a viable route to modify the properties of monolayer graphene and promote its applications in electronic devices.

fluorescence, fluorescence resonance energy transfer (FRET), fluorescence lifetime measurement, and multiphoton microscopy. Herein, we try to illustrate the main limitations and future directions of NPs in these areas of study.

Because of its excellent dielectric properties, silicon oxide (SiO(x)) has long been used and considered as a passive, insulating component in the construction of electronic devices. In contrast, here we demonstrate resistive switches and memories that use SiO(x) as the sole active material and can be implemented in entirely metal-free embodiments. Through cross-sectional transmission electron microscopy, we determine that the switching takes place through the voltage-driven formation and modification of silicon (Si) nanocrystals (NCs) embedded in the SiO(x) matrix, with SiO(x) itself also serving as the source of the formation of this Si pathway. The small sizes of the Si NCs (d ∼ 5 nm) suggest that scaling to ultrasmall domains could be feasible. Meanwhile, the switch also shows robust nonvolatile properties, high ON/OFF ratios (>10(5)), fast switching (sub-100-ns), and good endurance (10(4) write-erase cycles). These properties in a SiO(x)-based material composition showcase its potentials in constructing memory or logic devices that are fully CMOS compatible.

Here we demonstrate a general transfer-free method to directly grow large areas of uniform bilayer graphene on insulating substrates (SiO(2), h-BN, Si(3)N(4), and Al(2)O(3)) from solid carbon sources such as films of poly(2-phenylpropyl)methysiloxane, poly(methyl methacrylate), polystyrene, and poly(acrylonitrile-co-butadiene-co-styrene), the latter leading to N-doped bilayer graphene due to its inherent nitrogen content. Alternatively, the carbon feeds can be prepared from a self-assembled monolayer of butyltriethoxysilane atop a SiO(2) layer. The carbon feedstocks were deposited on the insulating substrates and then caped with a layer of nickel. At 1000 °C, under low pressure and a reducing atmosphere, the carbon source was transformed into a bilayer graphene film on the insulating substrates. The Ni layer was removed by dissolution, affording the bilayer graphene directly on the insulator with no traces of polymer left from a transfer step. The bilayer nature of as-grown samples was demonstrated by I(G)/I(2D) Raman mapping, the statistics of the full-width at half-maximum of the Raman 2D peak, the selected area electron diffraction patterns over a large area, and randomly imaged graphene edges by high-resolution transmission electron microscopy.

The nature of the conducting filaments in many resistive switching systems has been elusive. Through in situ transmission electron microscopy, we image the real-time formation and evolution of the filament in a silicon oxide resistive switch. The electroforming process is revealed to involve the local enrichment of silicon from the silicon oxide matrix. Semi-metallic silicon nanocrystals with structural variations from the conventional diamond cubic form of silicon are observed, which likely accounts for the conduction in the filament. The growth and shrinkage of the silicon nanocrystals in response to different electrical stimuli show energetically viable transition processes in the silicon forms, offering evidence for the switching mechanism. The study here also provides insights into the electrical breakdown process in silicon oxide layers, which are ubiquitous in a host of electronic devices.

We have fabricated a centimeter-size single-layer graphene device, with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10-10000 cm -1 ), we measured the dynamic conductivity change induced by electrical gating and thermal annealing.Both methods were able to effectively tune the Fermi energy, E F , which in turn modified the Drude-like intraband absorption in the terahertz as well as the '2E F onset' for interband absorption in the midinfrared. These results not only provide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics. KEYWORDS: graphene, Fermi level, terahertz dynamics, infrared spectroscopyThe AC dynamics of Dirac fermions in graphene have attracted much recent attention. The influence of linear dispersions, two-dimensionality, electron-electron interactions, and disorder on the dynamic conductivity, σ(ω), has been theoretically investigated, 1-11 whereas unique terahertz (THz) and mid-infrared (MIR) properties have been identified for novel optoelectronic applications. 12-17 For example, it has been predicted that the response of Dirac fermions to an applied AC electric field of frequency ω would automatically contain all odd harmonics of (2n+1)ω, where n is an integer, implying extremely high nonlinearity. 13,14 Furthermore, creation of electrons and holes through interband optical pumping is expected to lead to population inversion near the Dirac point, resulting in negative σ(ω), or gain, in the THz to MIR range. 12,17 While initial experimental investigations on graphene have concentrated on DC characteristics, these recent theoretical studies have instigated a flurry of new experimental activities to uncover unusual AC properties. A number of experiments have already confirmed the so-called universal optical conductivity σ 0 = e 2 /4 ! (e: electronic charge and ! : reduced Planck constant) for interband transitions in a wide spectral range. [18][19][20][21] On the other hand, experimental studies of the intraband conductivity have been very limited, [21][22][23][24] Here, we describe our THz and MIR spectroscopy study of large-area (centimeter scale), single-layer graphene with an electrically tunable Fermi level. In a field-effect transistor configuration consisting of graphene on a SiO 2 /p-Si substrate, the transmitted intensity of THz and MIR electromagnetic waves was observed to change with the gate voltage. The Drude-like intraband conductivities and the '2E F onset' of the interband transitions, monitored through time-domain THz spectroscopy (TDTS) and Fourier-transform IR (FTIR) spectroscopy, respectively, were both modulated by the gate voltage. By analyzing the spectral shape of the induced changes with appropriate models, we were able to determine ...

Transparent electronic memory would be useful in integrated transparent electronics. However, achieving such transparency produces limits in material composition, and hence, hinders processing and device performance. Here we present a route to fabricate highly transparent memory using sio x as the active material and indium tin oxide or graphene as the electrodes. The two-terminal, nonvolatile resistive memory can also be configured in crossbar arrays on glass or flexible transparent platforms. The filamentary conduction in silicon channels generated in situ in the sio x maintains the current level as the device size decreases, underscoring their potential for high-density memory applications, and as they are two-terminal based, transitions to three-dimensional memory packages are conceivable. As glass is becoming one of the mainstays of building construction materials, and conductive displays are essential in modern handheld devices, to have increased functionality in form-fitting packages is advantageous.