371 research outputs found
Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control
The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.close1
Preliminary Integrated Chronostratigraphy of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica
We use all available chronostratigraphic constraints â biostratigraphy, magnetostratigraphy,
radioisotopic dates, strontium-isotope stratigraphy, and correlation of compositional and physical properties
to well-dated global or regional records â to construct a preliminary age model for ANDRILL SMS Projectâs
AND-2A drillcore (77°45.488âS, 165°16.605âE, 383.57 m water depth). These diverse chronostratigraphic
constraints are consistent with each other and are distributed throughout the 1138.54 m-thick section,
resulting in a well-constrained age model. The sedimentary succession comprises a thick early and middle
Miocene section below 224.82 mbsf and a condensed middle/late Miocene to Recent section above
this. The youngest sediments are Brunhes age (<0.781 Ma), as confirmed by a radioisotopic age of
0.691±0.049 Ma at 10.23 mbsf and the occurrence of sediments that have normal magnetic polarity down
to ~31.1 mbsf, which is interpreted to be the Brunhes/Matuyama reversal (0.781 Ma). The upper section
is punctuated by disconformities resulting from both discontinuous deposition and periods of extensive
erosion typical of sedimentary environments at the margin of a dynamic ice sheet. Additional breaks in
the section may be due to the influence of tectonic processes. The age model incorporates several major
hiatuses but their precise depths are still somewhat uncertain, as there are a large number of erosional
surfaces identified within the stratigraphic section. One or more hiatuses, which represent a total 7 to 8
million years of time missing from the sedimentary record, occur between about 50 mbsf and the base of
Lithostratigraphic Unit (LSU) 3 at 122.86 mbsf. Similarly, between about 145 mbsf and the base of LSU
4 at 224.82 mbsf, one or more hiatuses occur on which another 2 to 3 million years of the sedimentary
record is missing. Support for the presence of these hiatuses comes from a diatom assemblage that
constrains the age of the core from 44 to 50 mbsf to 2.06-2.84 Ma, two radioisotopic dates (11.4 Ma)
and a Srâisotope date (11.7 Ma) that indicate the interval from 127 to 145 mbsf was deposited between
11.4 and 11.7 Ma, and three diatom occurrence datums from between 225.38 and 278.55 mbsf that
constrain the age of this upper part of Lithostratigraphic Unit (LSU) 5 to 14.29 - 15.89 Ma. Below the
boundary between LSU 5 and 6 sedimentation was relatively continuous and rapid and the age model is
well-constrained by 9 diatom datums, seven 40Ar-39Ar dates, one Sr-isotope date, and 19 magnetozones.
Even so, short hiatuses (less than a few hundred thousand years) undoubtedly occur but are beyond
the resolution of current chronostratigraphic age constraints. Diatom first and last occurrence datums
provide particularly good age control from the top of LSU 6 down to 771.5 mbsf (in LSU 10), where
the First Occurrence (FO) of Thalassiosira praefraga (18.85 Ma) is observed. The diatom datum ages
are supported by radioisotopic dates of 17.30±0.31 Ma at 640.14 mbsf (in LSU 9) and 18.15±0.35 and
17.93±0.40 Ma for samples from 709.15 and 709.18 mbsf (in LSU 10), respectively, and 18.71±0.33 Ma
for a sample from 831.67 mbsf (in LSU 11). The sediments from 783.69 mbsf to the base of the hole
comprise two thick normal polarity magnetozones that bound a thinner reversed polarity magnetozone
(958.59 - 985.64 mbsf). This polarity sequence most likely encompasses Chrons C5En, C5Er, and C6n
(18.056 - 19.772 Ma or slightly older given uncertainties in this section of the geomagnetic polarity
timescale), but could be also be Chrons C6n, C6r, and C6An.1n (18.748 - 20.213 Ma). Either polarity
sequence is compatible with the 40Arâ39Ar age of 20.01±0.35 Ma obtained from single-grain analyses of
alkali feldspar from a tephra sample from a depth of 1093.02 mbsf, although the younger interpretation
allows a better fit with chronostratigraphic data up-core. Given this age model, the mean sedimentation
rate is about 18 cm/k.y. from the top of LSU 6 to the base of the hole.Published221-2202.2. Laboratorio di paleomagnetismoN/A or not JCRreserve
Phonon driven transport in amorphous semiconductors: Transition probabilities
Inspired by Holstein's work on small polaron hopping, the evolution equations
of localized states and extended states in presence of atomic vibrations are
derived for an amorphous semiconductor. The transition probabilities are
obtained for four types of transitions: from one localized state to another
localized state, from a localized state to an extended state, from an extended
state to a localized state, and from one extended state to another extended
state. At a temperature not too low, any process involving localized state is
activated. The computed mobility of the transitions between localized states
agrees with the observed `hopping mobility'. We suggest that the observed
`drift mobility' originates from the transitions from localized states to
extended states. Analysis of the transition probability from an extended state
to a localized state suggests that there exists a short-lifetime belt of
extended states inside conduction band or valence band. It agrees with the fact
that photoluminescence lifetime decreases with frequency in a-Si/SiO
quantum well while photoluminescence lifetime is not sensitive to frequency in
c-Si/SiO structure.Comment: 41 pages, 3 figures, submitted to Phys. Rev.
Secondary crystalline phases identification in Cu2ZnSnSe4 thin films: contributions from Raman scattering and photoluminescence
In this work, we present the Raman peak
positions of the quaternary pure selenide compound
Cu2ZnSnSe4 (CZTSe) and related secondary phases that
were grown and studied under the same conditions. A vast
discussion about the position of the X-ray diffraction
(XRD) reflections of these compounds is presented. It is
known that by using XRD only, CZTSe can be identified
but nothing can be said about the presence of some secondary
phases. Thin films of CZTSe, Cu2SnSe3, ZnSe,
SnSe, SnSe2, MoSe2 and a-Se were grown, which allowed
their investigation by Raman spectroscopy (RS). Here we
present all the Raman spectra of these phases and discuss
the similarities with the spectra of CZTSe. The effective
analysis depth for the common back-scattering geometry
commonly used in RS measurements, as well as the laser penetration depth for photoluminescence (PL) were estimated
for different wavelength values. The observed
asymmetric PL band on a CZTSe film is compatible with
the presence of CZTSe single-phase and is discussed in the
scope of the fluctuating potentialsâ model. The estimated
bandgap energy is close to the values obtained from
absorption measurements. In general, the phase identification
of CZTSe benefits from the contributions of RS and
PL along with the XRD discussion.info:eu-repo/semantics/publishedVersio
Chemical and magnetic functionalization of graphene oxide as a route to enhance its biocompatibility
- âŠ