438 research outputs found
Evolution of Raman G and G'(2D) Modes in Folded Graphene Layers
Bernal- and non-Bernal-stacked graphene layers have been systematically
studied by Raman imaging and spectroscopy. Two dominant Raman modes, G and G'
(or 2D) of folded graphene layers exhibit three types of spectral features when
interlayer lattice mismatches, defined by a rotational angle varies. Among
these folded graphene layers, the most interesting one is the folded graphene
layers that present an extremely strong G mode enhanced by a twist-induced Van
Hove singularity. The evolution of Raman G and G' modes of such folded graphene
layers are probed by changing the excitation photon energies. For the first
time, doublet splitting of the G' mode in folded double-layer (1 + 1) and of
the G mode in folded tetra-layer (2 + 2) graphene are clearly observed and
discussed. The G' mode splitting in folded double-layer graphene is attributed
to the coexistence of inner and outer scattering processes and the trigonal
warping effect as well as further downwards bending of the inner dispersion
branch at visible excitation energy. While the two peaks of the G mode in
folded tetra-layer graphene are assigned to Raman-active mode (E2g) and lattice
mismatch activated infrared-active mode (E1u), which is further verified by the
temperature-dependent Raman measurements. Our study provides a summary and
thorough understanding of Raman spectra of Bernal- and non-Bernal-stacked
graphene layers and further demonstrates the versatility of Raman spectroscopy
for exploiting electronic band structures of graphene layers.Comment: 29 pages, 10 figure
A Case Study of Upper-Room UVGI in Densely-Occupied Elementary Classrooms by Real-Time Fluorescent Bioaerosol Measurements
Recently, the requirement to continuously collect bioaerosol samples using shorter response times has called for the use of real-time detection. The decreased cost of this technology makes it available for a wider application than military use, and makes it accessible to pharmaceutical and academic research. In this case study, real-time bioaerosol monitors (RBMs) were applied in elementary school classrooms—a densely occupied environment—along with upper-room ultraviolet germicidal irradiation (UVGI) devices. The classrooms were separated into a UVGI group and a non-UVGI control group. Fluorescent bioaerosol counts (FBCs) were monitored on 20 visiting days over a four-month period. The classroom with upper-room UVGI showed significantly lower concentrations of fine size (\u3c3 μm) and total FBCs than the control classroom during 13 of the 20 visiting days. The results of the study indicate that the upper-room UVGI could be effective in reducing FBCs in the school environment, and RBMs may be applicable in reflecting the transient conditions of the classrooms due to the dynamic activity levels of the students and teachers
Stacking sequence determines Raman intensities of observed interlayer shear modes in 2D layered materials - A general bond polarizability model
2D layered materials have recently attracted tremendous interest due to their
fascinating properties and potential applications. The interlayer interactions
are much weaker than the intralayer bonds, allowing the as-synthesized
materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading
to different physical properties. Here, we show that regardless of the space
group of the 2D material, the Raman frequencies of the interlayer shear modes
observed under the typical configuration blue shift for AB stacked materials,
and red shift for ABC stacked materials, as the number of layers increases. Our
predictions are made using an intuitive bond polarizability model which shows
that stacking sequence plays a key role in determining which interlayer shear
modes lead to the largest change in polarizability (Raman intensity); the modes
with the largest Raman intensity determining the frequency trends. We present
direct evidence for these conclusions by studying the Raman modes in few layer
graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles
calculations and Raman spectroscopy. This study sheds light on the influence of
stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D
layered materials in general, and leads to a practical way of identifying the
stacking sequence in these materials.Comment: 30 pages, 8 figure
A Case Study of Upper-Room UVGI in Densely-Occupied Elementary Classrooms by Real-Time Fluorescent Bioaerosol Measurements
Recently, the requirement to continuously collect bioaerosol samples using shorter response times has called for the use of real-time detection. The decreased cost of this technology makes it available for a wider application than military use, and makes it accessible to pharmaceutical and academic research. In this case study, real-time bioaerosol monitors (RBMs) were applied in elementary school classrooms—a densely occupied environment—along with upper-room ultraviolet germicidal irradiation (UVGI) devices. The classrooms were separated into a UVGI group and a non-UVGI control group. Fluorescent bioaerosol counts (FBCs) were monitored on 20 visiting days over a four-month period. The classroom with upper-room UVGI showed significantly lower concentrations of fine size (\u3c3 μm) and total FBCs than the control classroom during 13 of the 20 visiting days. The results of the study indicate that the upper-room UVGI could be effective in reducing FBCs in the school environment, and RBMs may be applicable in reflecting the transient conditions of the classrooms due to the dynamic activity levels of the students and teachers
Magnetic Oscillation of Optical Phonon in ABA- and ABC-Stacked Trilayer Graphene
We present a comparative measurement of the G-peak oscillations of phonon
frequency, Raman intensity and linewidth in the Magneto-Raman scattering of
optical E2g phonons in mechanically exfoliated ABA- and ABC-stacked trilayer
graphene (TLG). Whereas in ABA-stacked TLG, we observe magnetophonon
oscillations consistent with single-bilayer chiral band doublets, the features
are flat for ABC-stacked TLG up to magnetic fields of 9 T. This suppression can
be attributed to the enhancement of band chirality that compactifies the
spectrum of Landau levels and modifies the magnetophonon resonance properties.
The drastically different coupling behaviour between the electronic excitations
and the E2g phonons in ABA- and ABC-stacked TLG reflects their different
electronic band structures and the electronic Landau level transitions and thus
can be another way to determine the stacking orders and to probe the
stacking-order-dependent electronic structures. In addition, the sensitivity of
the magneto-Raman scattering to the particular stacking order in few layers
graphene highlights the important role of interlayer coupling in modifying the
optical response properties in van der Waals layered materials.Comment: 25 pages, 6 figure
Methylated DNMT1 and E2F1 Are Targeted for Proteolysis by L3MBTL3 and CRL4DCAF5 Ubiquitin Ligase
Many non-histone proteins are lysine methylated and a novel function of this modification is to trigger the proteolysis of methylated proteins. Here, we report that the methylated lysine 142 of DNMT1, a major DNA methyltransferase that preserves epigenetic inheritance of DNA methylation patterns during DNA replication, is demethylated by LSD1. A novel methyl-binding protein, L3MBTL3, binds the K142-methylated DNMT1 and recruits a novel CRL4DCAF5 ubiquitin ligase to degrade DNMT1. Both LSD1 and PHF20L1 act primarily in S phase to prevent DNMT1 degradation by L3MBTL3-CRL4DCAF5. Mouse L3MBTL3/MBT-1 deletion causes accumulation of DNMT1 protein, increased genomic DNA methylation, and late embryonic lethality. DNMT1 contains a consensus methylation motif shared by many non-histone proteins including E2F1, a key transcription factor for S phase. We show that the methylation-dependent E2F1 degradation is also controlled by L3MBTL3-CRL4DCAF5. Our studies elucidate for the first time a novel mechanism by which the stability of many methylated non-histone proteins are regulated
Complex magnetic and spatial symmetry breaking from correlations in kagome flat bands
We present the mean-field phase diagram of electrons in a kagome flat band
with repulsive interactions. In addition to flat-band ferromagnetism, the
Hartree-Fock analysis yields cascades of unconventional magnetic orders driven
by onsite repulsion as filling changes. These include a series of
antiferromagnetic (AFM) spin-charge stripe orders, as well as an evolution from
AFM to intriguing noncoplanar spin orders with tetrahedral
structures. We also map out the phase diagram under extended repulsion at half
and empty fillings of the flat band. To examine the possibilities beyond
mean-field level, we conduct a projective symmetry group analysis and identify
the feasible spin liquids and the magnetic orders derivable from
them. The theoretical phase diagrams are compared with recent experiments on
FeSn and FeGe, enabling determination of the most likely magnetic instabilities
in these and similar kagome flat-band materials.Comment: 7+18 pages, 4+10 figure
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