Graphene is an important material with potential application in spintronics. Edge-modified zigzag graphene nanoribbons (ZGNR) are investigated with density functional theory. The modifications are realized by saturating the dangling edge bonds by different terminal groups, such as H, NH2, NO2, and CH3. Such modification has a significant impact on the ZGNR electronic structure. Half-metallicity is observed when ZGNR is terminated by NO2 groups at one edge and by CH3 groups on the other side. Free energy analysis suggests that edge-modification is a practical way in experiment to realize half-metallicity.
Chemical vapor deposition (CVD) is an important method to synthesis grapheme on a substract. Recently, Cu becomes the most popular CVD substrate for graphene growth. Here, we combine electronic structure calculation, molecular dynamics simulation, and thermodynamics analysis to study the graphene growth process on Cu surface. As a fundamentally important but previously overlooked fact, we find that carbon atoms are thermodynamically unfavorable on Cu surface under typical experimental conditions. The active species for graphene growth should thus mainly be CH x instead of atomic carbon. Based on this new picture, the nucleation behavior can be understood, which explains many experimental observations and also provides us a guide to improve graphene sample quality.
Our first principles calculations reveal that oxidative cut of graphene is realized by forming epoxy and then carbonyl pairs. Direct forming carbonyl pair to tear graphene up from an edge position is not favorable in energy. This atomic picture is valuable for developing effective graphene manipulation means. The proposed epoxy pairs may be related to some long puzzling experimental observations on graphene oxide.Due to its novel physical properties and great potential in various applications, graphene has attracted an intense research interest recently. i A big challenge in graphene research is the massive production of high quality samples. The existing physical approaches ii prohibit producing and processing graphene on large scales. In this context, the versatile chemistry of carbon may offer a promising alternative for cost-effective mass production of graphene, as demonstrated by its graphene-oxide (GO) synthesis route. Upon oxidation, graphite readily exfoliates as single sheets in water, forming GO. The π conjugation in graphene can then be largely restored by reducing GO. iii,iv Oxidation now becomes an important chemical means to manipulate graphitic materials.It has been observed that, during oxidation process, graphitic structures automatically break down into smaller parts. iv,v For electronics applications, it is very desirable to cut graphene with designed shape and size. Therefore, an atomistic understanding of the mechanism for such oxidative breakup of graphene sheets is especially valuable. Based on first-principles calculations, an unzipping mechanism has been proposed, vi where the epoxy groups formed during oxidation were suggested to have a preference of aligning in a line. The aligned epoxy groups then induce a rupture of the underlying C-C bonds (Figure 1a). However, it is still not clear how the graphene sheets can eventually break up, since even after the rupture of the C-C bonds the graphene sheet remains bridged by O atoms. Actually, a recent study shows that the mechanical strength of the graphene sheet is not strongly affected by the presence of epoxy chain and an epoxy line defect only weakens the fracture stress of the sheet by approximately 16%. vii This result indicates that, although the epoxy chain breaks the underlying C-C bonds, it does not really cause a breakup of the material by itself. The chemistry of the whole breakup process is still not clear.Previous experimental study on GO has suggested the existence of carbonyl groups, viii and a very recent two dimensional NMR experiment shows that the carbonyl groups are spatially separated from the majority sp 2 , C-OH, and epoxide carbons. ix This result indicates that carbonyl groups mainly distribute at the GO edge, and may thus be closely related to the oxidative break process. In this communication, based on density functional theory (DFT), we reveal how the oxygen attacks can break up atomic structure of graphene. Both the middle-site-initiated and the edge-site-initiated processes are studied, and the former based on i...
Nearly free electron (NFE) states are widely existed on atomically smooth surfaces in two-dimensional materials. Since they are mainly distributed in free space, these states can in principle provide ideal electron transport channels without nuclear scattering. Unfortunately, NFE states are typically unoccupied, and electron doping is required to shift them toward the Fermi level and, thus, to be involved in electron transport. Instead of occupying these NFE states, it is more desirable to have intrinsic nucleus-free two-dimensional electron gas in free space (2DEG-FS) at the Fermi level without relying on doping. Inspired by a recently identified electride material, we suggest that Ca2N monolayer should possess such a 2DEG-FS state, which is then confirmed by our first-principles calculations. Phonon dispersion in Ca2N monolayer shows no imagery frequency indicating that the monolayer structure is stable. A mechanical analysis demonstrates that Ca2N bulk exfoliation is feasible to produce a freestanding monolayer. However, in real applications, the strong chemical activity of 2DEG-FS may become a practical issue. It is found that some ambient molecules can dissociatively adsorb on the Ca2N monolayer, accompanying with a significant charge transfer from the 2DEG-FS state to adsorbates. To protect the 2DEG-FS state from molecule adsorption, we predict that graphane can be used as an effective encapsulating material. A well-encapsulated intrinsic 2DEG-FS state is expected to play an important role in low-dimensional electronics by realizing nuclear scattering free transport.
Clean hydrogen production is highly desirable for future energy needs, making the understanding of molecular-level phenomena underlying photocatalytic hydrogen production both fundamentally and practically important. Water splitting on pure TiO 2 is inefficient, however, adding sacrificial methanol could significantly enhance the photocatalyzed H 2 production. Therefore, understanding the photochemistry of methanol on TiO 2 at the molecular level could provide important insights to its photocatalytic activity. Here, we report the first clear evidence of photocatalyzed splitting of methanol on TiO 2 derived from time-dependent two-photon photoemission (TD-2PPE) results in combination with scanning tunneling microscopy (STM). STM tip induced molecular manipulation before and after UV light irradiation clearly reveals photocatalytic bond cleavage, which occurs only at Ti 4+ surface sites. TD-2PPE reveals that the kinetics of methanol photodissociation is clearly not of single exponential, an important characteristic of this intrinsically heterogeneous photoreaction.
At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field decreases with the increase of the ribbon width to induce the half-metallicity. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remains half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may lead to some important applications in spinstronics.
Although there have been many reports on the preparation and applications of various polymer nanofibers with the electrospinning technique, the understanding of synthetic parameters in electrospinning remains limited. In this article, we investigate experimentally the influence of solvents on the morphology of the poly(vinyl pyrrolidone) (PVP) micro/nanofibers prepared by electrospinning PVP solution in different solvents, including ethanol, dichloromethane (MC) and N,N‐dimethylformamide (DMF). Using 4 wt % PVP solutions, the PVP fibers prepared from MC and DMF solvents had a shape like a bead‐on‐a‐string. In contrast, smooth PVP nanofibers were obtained with ethanol as a solvent although the size distribution of the fibers was somewhat broadened. In an effort to prepare PVP nanofibers with small diameters and narrow size distributions, we developed a strategy of using mixed solvents. The experimental results showed that when the ratio of DMF to ethanol was 50:50 (w/w), regular cylindrical PVP nanofibers with a diameter of 20 nm were successfully prepared. The formation of these thinnest nanofibers could be attributed to the combined effects of ethanol and DMF solvents that optimize the solution viscosity and charge density of the polymer jet. In addition, an interesting helical‐shaped fiber was obtained from 20 wt % PVP solution in a 50:50 (w/w) mixed ethanol/DMF solvent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3721–3726, 2004
Electrical control of spin polarization is very desirable in spintronics, since electric fields can be easily applied locally, in contrast to magnetic fields. Here, we propose a new concept of bipolar magnetic semiconductors (BMS) in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. BMS is thus expected to have potential for various applications. Our band structure and spin-polarized electronic transport calculations on semi-hydrogenated single-walled carbon nanotubes confirm the existence of BMS materials and demonstrate the electrical control of spin-polarization in them.
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