Considering typical spectra of a broad range of carbonaceous materials from gas-shale to nanotubes, various ways by which defects show up in Raman spectra are exampled and discussed. The position, resonance behavior, and linewidth of both the D and G bands are compared, even if in some cases obtaining accurate information on the materials from the fitting parameters is a difficult task. As a matter of fact, even if a full picture is unreachable, defining parameter trends is one acceptable option. Two ways to determine the linewidth, either graphically and or by fitting are proposed in order to be able to compare literature data. The relationship between the crystallite size obtained from the linewidth and from X-ray diffraction, which is complementary to the Tuinstra and Koenig law, is examined. We show that a single approach is not possible unless modeling is performed and therefore that analysis of Raman spectra should be adapted to the specificities of each sample series, i.e., a minimum of knowledge about the materials is always required.
Hotspot
engineering has the potential to transform the field of
surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive
and reproducible detection of analytes. However, the ability to controllably
generate SERS hotspots, with desired location and geometry, over large-area
substrates, has remained elusive. In this study, we sculpt artificial
edges in monolayer molybdenum disulfide (MoS2) by low-power
focused laser-cutting. We find that when gold nanoparticles (AuNPs)
are deposited on MoS2 by drop-casting, the AuNPs tend to
accumulate predominantly along the artificial edges. First-principles
density functional theory (DFT) calculations indicate strong binding
of AuNPs with the artificial edges due to dangling bonds that are
ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation
of AuNPs along the artificial edges intensifies plasmonic effects
in these regions, creating hotspots exclusively along the artificial
edges. DFT further indicates that adsorption of AuNPs along the artificial
edges prompts a transition from semiconducting to metallic behavior,
which can further intensify the plasmonic effect along the artificial
edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated)
edges of the MoS2 sheet. To demonstrate the practical utility
of this concept, we use our substrate to detect Rhodamine B (RhB)
with a large SERS enhancement (∼104) at the hotspots
for RhB concentrations as low as ∼10–10 M.
The single-step laser-etching process reported here can be used to
controllably generate arrays of SERS hotspots. As such, this concept
offers several advantages over previously reported SERS substrates
that rely on electrochemical deposition, e-beam lithography, nanoimprinting,
or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety
of 2D material platforms.
The century-old controversy over two contradicting theories on radiation pressure of light proposed by Abraham and Minkowski can come to an end if there is a direct method to measure the surface deformation of the target material due to momentum transfer of photons. Here we have investigated the effect of radiation pressure on the surface morphology of Graphene Oxide (GO) film, experienced due to low power focused laser irradiation. In-depth investigation has been carried out to probe the bending of the GO surface due to radiation pressure by Atomic Force Microscopy (AFM) and subsequently the uniaxial strain induced on the GO film has been probed by Raman Spectroscopy. Our results show GO film experience an inward pressure due to laser radiation resulting in inward bending of the surface, which is consistent with the Abraham theory. The bending diameter and depth of the irradiated spot show linear dependence with the laser power while an abrupt change in depth and diameter of the irradiated spot is observed at the breaking point. Such abrupt change in depth is attributed to the thinning of the GO film by laser irradiation.
This study argues that India's urban growth is more sluggish than most observers believe and that the developed states and large cities are receiving most of that growth, while backward areas and smaller towns are tending to stagnate. Although adult male migration is considered as an effective mechanism for improving economic well-being and escaping poverty, their net migration into urban areas has not gone up over the past five decades. A slow and topheavy pattern of Indian urbanisation is contributing to persistent inequalities. More importantly, the recent moves for empowerment of local bodies through a new system of urban governance have also contributed to increased spatial inequality. Obvious responses would be more inclusive population policies in the successful and large urban centres or, alternatively, more support for economic growth in smaller urban centres. The first is likely to face political obstacles, as urban elites and the middle class do not want to accommodate a large influx of low-income migrants. The second is also likely to face economic obstacles, as it can be difficult to find good public investment opportunities in smaller urban centres. However, without urban growth, the pursuit of both economic growth and equality will eventually be compromised.
The experimental
identification of structural transitions in layered
black phosphorus (BP) under mechanical stress is essential to extend
its application in microelectromechanical (MEMS) devices under harsh
conditions. High-pressure Raman spectroscopic analysis of BP flakes
suggests a transition pressure at ∼4.2 GPa, where the BP’s
crystal structure progressively transforms from an orthorhombic to
a rhombohedral symmetry (blue phosphorus, bP). The phase transition
has been identified by observing a transition from blueshift to redshift
of the in-plane characteristic Raman modes (B2g and Ag
2) with increasing
pressure. Recovery of the vibrational frequencies for all three characteristic
Raman modes confirms the reversibility of the structural phase transition.
First-principles calculations provide insight into the behavior of
the Raman modes of BP under high pressure and reveal the mechanism
responsible for the partial phase transition from BP to bP, corresponding
to a metastable equilibrium state where both phases coexist.
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