88 research outputs found
Novel magnetic properties of graphene: Presence of both ferromagnetic and antiferromagnetic features and other aspects
Investigations of the magnetic properties of graphenes prepared by different
methods reveal that dominant ferromagnetic interactions coexist along with
antiferromagnetic interactions in all the samples. Thus, all the graphene
samples exhibit room-temperature magnetic hysteresis. The magnetic properties
depend on the number of layers and the sample area, small values of both
favoring larger magnetization. Molecular charge-transfer affects the magnetic
properties of graphene, interaction with a donor molecule such as
tetrathiafulvalene having greater effect than an electron-withdrawing molecule
such as tetracyanoethyleneComment: 16 pges, 5 figure
Quenching of fluorescence of aromatic molecules by graphene due to electron transfer
Investigations on the fluorescence quenching of graphene have been carried
out with two organic donor molecules, pyrene butanaoic acid succinimidyl ester
(PyBS, I) and oligo(p-phenylenevinylene) methyl ester (OPV-ester, II).
Absorption and photoluminescence spectra of I and II recorded in mixture with
increasing the concentrations of graphene showed no change in the former, but
remarkable quenching of fluorescence. The property of graphene to quench
fluorescence of these aromatic molecules is shown to be associated with
photo-induced electron transfer, on the basis of fluorescence decay and
time-resolved transient absorption spectroscopic measurements.Comment: 18 pages, 6 figure
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
The two-dimensional layer of molybdenum disulfide (MoS2) has recently
attracted much interest due to its direct-gap property and potential
applications in optoelectronics and energy harvesting. However, the synthetic
approach to obtain high quality and large-area MoS2 atomic thin layers is still
rare. Here we report that the high temperature annealing of a thermally
decomposed ammonium thiomolybdate layer in the presence of sulfur can produce
large-area MoS2 thin layers with superior electrical performance on insulating
substrates. Spectroscopic and microscopic results reveal that the synthesized
MoS2 sheets are highly crystalline. The electron mobility of the bottom-gate
transistor devices made of the synthesized MoS2 layer is comparable with those
of the micromechanically exfoliated thin sheets from MoS2 crystals. This
synthetic approach is simple, scalable and applicable to other transition metal
dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to
arbitrary substrates, providing great opportunities to make layered composites
by stacking various atomically thin layers.Comment: manuscript submitted on 11-Dec-2011, revision submitted on
16-Feb-201
Spin-half paramagnetism in graphene induced by point defects
Using magnetization measurements, we show that point defects in graphene -
fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments
with spin 1/2. Both types of defects lead to notable paramagnetism but no
magnetic ordering could be detected down to liquid helium temperatures. The
induced paramagnetism dominates graphene's low-temperature magnetic properties
despite the fact that maximum response we could achieve was limited to one
moment per approximately 1000 carbon atoms. This limitation is explained by
clustering of adatoms and, for the case of vacancies, by losing graphene's
structural stability.Comment: 14 pages, 14 figure
Synthesis, properties and potential applications of two-dimensional transition metal dichalcogenides
Strategies for the Synthesis of Graphene, Graphene Nanoribbons, Nanoscrolls and Related Materials
Single-layer graphene (SLG), the 3.4 Ă
thick two-dimensional sheet of sp2 carbon atoms, was first prepared in 2004 by mechanical exfoliation of graphite crystals using the scotch tape technique. Since then, SLG has been prepared by other physical methods such as laser
irradiation or ultrasonication of graphite in liquid media. Chemical methods of synthesis of SLG are more commonly used; the most popular involves preparation of single-layer graphene oxide followed by reduction with a stable reagent, often assisted by microwave heating. This method yields
single-layer reduced graphene oxide. Other methods for preparing SLG include chemical vapour deposition over surfaces of transition metals such as Ni and Cu. Large-area SLG has also been prepared by epitaxial growth over SiC. Few-layer graphene (FLG) is prepared by several methods; arc discharge
of graphite in hydrogen atmosphere being the most convenient. Several other methods for preparing FLG include exfoliation of graphite oxide by rapid heating, ultrasonication or laser irradiation of graphite in liquid media, reduction of few-layer graphene oxide, alkali metal intercalation
followed by exfoliation. Graphene nanoribbons, which are rectangular strips of graphene, are best prepared by the unzipping of carbon nanotubes by chemical oxidation or laser irradiation. Many graphene analogues of inorganic materials such as MoS2, MoSe2 and BN have been
prepared by mechanical exfoliation, ultrasonication and by chemical methods involving high-temperature or hydrothermal reactions and intercalation of alkali metals followed by exfoliation. Scrolls of graphene are prepared by potassium intercalation in graphite or by microwave irradiation of
graphite immersed in liquid nitrogen
Unusual magnetic properties of graphene and related materials
High-temperature ferromagnetism in graphene and other graphite-derived materials reported by several workers has attracted considerable interest. Magnetism in graphene and graphene nanoribbons is ascribed to defects and edge states, the latter being an essential feature of these materials. Room-temperature ferromagnetism in graphene is affected by the adsorption of molecules, especially hydrogen. Inorganic graphene analogues formed by layered materials such as BN and MoS2 also show such ferromagnetic behaviour. Magnetoresistance observed in graphene and graphene nanoribbons is of significance because of the potential applications
Synthesis, characterization and spectroscopy of composites of graphene with CdSe and CdS nanoparticles
1239-1247Nanocomposites of few-layer graphene with
nanoparticles of CdSe and CdS have been synthesized by two different methods,
one involving ultrasonication of a mixture of graphene and the chalcogenide
nanoparticles, and another involving assembly at the organic-aqueous interface.
The nanocomposites have been examined by electron microscopy, electronic
absorption and photoluminescence spectroscopies as well as Raman spectroscopy.
Electron microscopy reveals that the nanoparticles are dispersed on the
graphene surface. Raman spectra show the presence of definitive electronic
interaction between the nanoparticles and graphene depending on the capping
agent. Photoluminescence spectra are markedly influenced by the interaction of
the nanoparticles with the graphene surface, depending on the capping agent
Employing synergistic interactions between few-layer WS<sub>2</sub> and reduced graphene oxide to improve lithium storage, cyclability and rate capability of Li-ion batteries
The aim of the contribution is to introduce a high performance anode alternative to graphite for lithium-ion batteries (LiBs). A simple process was employed to synthesize uniform graphene-like few-layer tungsten sulfide (WS2) supported on reduced graphene oxide (RGO) through a hydrothermal synthesis route. The WS2âRGO (80:20 and 70:30) composites exhibited good enhanced electrochemical performance and excellent rate capability performance when used as anode materials for lithium-ion batteries. The specific capacity of the WS2âRGO composite delivered a capacity of 400â450 mAh gâ1 after 50 cycles when cycled at a current density of 100 mA gâ1. At 4000 mA gâ1, the composites showed a stable capacity of approximately 180â240 mAh gâ1, respectively. The noteworthy electrochemical performance of the composite is not additive, rather it is synergistic in the sense that the electrochemical performance is much superior compared to both WS2 and RGO. As the observed lithiation/delithiation for WS2âRGO is at a voltage ≈1.0 V (≈0.1 V for graphite, Li+/Li), the lithium-ion battery with WS2âRGO is expected to possess high interface stability, safety and management of electrical energy is expected to be more efficient and economic
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