1,721,991 research outputs found
Electromechanically tunable frequency-agile metamaterial bandpass filters for terahertz waves
No full textTunable transmission filters are essential for various key terahertz applications including miniaturized spectrometers, hyperspectral imagers and channel selectors in high-speed wireless communication systems. However, tunable terahertz transmission filters have remained elusive, so far. Here, we report an electromechanically reconfigurable microcantilever integrated complementary metamaterial to function as a frequency-agile bandpass transmission filter at terahertz spectral region. A large tunable range of 70% at central frequency of 0.455 THz is designed and the experimental results show switching of terahertz transmission frequency range of 54% at central frequency of 0.5 THz. The proposed electromechanically reconfigurable metadevice provides large tunable range, uses electrical control, and is fabricated using CMOS compatible materials and process, thus making it highly attractive for the realization of plethora of miniaturized high-performance terahertz components and devices
INFLUENCE OF SUBSTRATE ON PHYSICAL PROPERTIES OF TRANSITION METAL COMPLEXES THIN FILMS
Get PDFIn order to exploit the potential of transition metal complexes in novel optoelectronic devices, it is crucial to gain a thorough understanding of their interfacial electronic properties with conducting substrates and establish interconnection between structural and physical properties of transition metal complexes thin films on technologically important substrates. The thesis aims to establish the interconnection between structural and physical properties of tetrapyrrole complexes by underscoring the significance of intermolecular interactions, structural deformations, and molecular order, providing useful insights for the creation and control of functional molecular films, crucial for the technological applications of organic semiconductors. A multi-technique characterization approach has been adopted approach based on electronic and absorption spectroscopies and structural diffraction techniques. Inverse photoemission spectroscopy has been utilized to highlight the influence of intermolecular interactions on unoccupied density of states as empty orbitals are more sensitive to the intermolecular interaction due to their higher delocalization around the macrocycle.
Photoemission and X-ray absorption spectroscopy have been utilized to demonstrate the ability of graphene to control the interaction of FeTPP-Cl adsorbed onto Gr/Ni(111) and Gr/Pt(111) substrates. It has been demonstrated that interfacial charge transfer can be significantly influenced by the introduction of graphene buffer layer between transition metal complexes adsorbed on metallic substrates. Inverse photoemission spectroscopy has been utilized for metal phthalocyanine and metal octaethylporphyrin thin films on metallic substrates to investigate the subtle changes induced by central metal atom, intermolecular interactions and peripheral ethyl groups on unoccupied density of states.
FePc and CoPc thin films deposited on technologically important substrates such as SiOx/Si , ITO and Au reveals that the vibrational properties have strong dependence on molecular stacking adopted by molecules which in turn depends on substrate roughness, intermolecular interactions and molecule-substrate interactions. We have employed vacuum deposited Zinc Octaethyl porphyrin (ZnOEP) thin films with a different degree of long-range order as model systems An asymmetrical stretching of skeletal carbon atoms of the porphyrin conformer has been observed and attributed to the ordered molecular stacking and intermolecular interactions. X-ray absorption near edge structure (XANES) establishes a symmetry reduction in the molecular conformer involving skeletal carbon atoms of the porphyrin ring for the ordered films highlighting the consequences of increased pi stacking of ZnOEP molecules adopting triclinic structure. The observed asymmetrical stretching of pi conjugation network of porphyrin structure can have significant implications on the charge transport and light harvesting ability of porphyrin thin films on substrates.In order to exploit the potential of transition metal complexes in novel optoelectronic devices, it is crucial to gain thorough understanding of their interfacial electronic properties with conducting substrates and establish interconnection between structural and physical properties of transition metal complexes thin films on technologically important substrates. The thesis aims to establish the interconnection between structural and physical properties of tetrapyrrole complexes by underscoring the significance of intermolecular interactions, structural deformations and molecular order, providing useful insights for the creation and control of functional molecular films, crucial for the technological applications of organic semiconductors. A multi-technique characterization approach has been adopted approach based on electronic and absorption spectroscopies and structural diffraction techniques. Inverse photoemission spectroscopy has been utilized to highlight the influence of intermolecular interactions on unoccupied density of states as empty orbitals are more sensitive to the intermolecular interaction due to their higher delocalization around the macrocycle.
Photoemission and X-ray absorption spectroscopy have been utilized to demonstrate the ability of graphene to control the interaction of FeTPP-Cl adsorbed onto Gr/Ni(111) and Gr/Pt(111) substrates. It has been demonstrated that interfacial charge transfer can be significantly influenced by the introduction of graphene buffer layer between transition metal complexes adsorbed on metallic substrates. Inverse photoemission spectroscopy has been utilized for metal phthalocyanine and metal octaethylporphyrin thin films on metallic substrates to investigate the subtle changes induced by central metal atom, intermolecular interactions and peripheral ethyl groups on unoccupied density of states.
FePc and CoPc thin films deposited on technologically important substrates such as SiOx/Si , ITO and Au reveals that the vibrational properties have strong dependence on molecular stacking adopted by molecules which in turn depends on substrate roughness, intermolecular interactions and molecule-substrate interactions. We have employed vacuum deposited Zinc Octaethyl porphyrin (ZnOEP) thin films with a different degree of long-range order as model systems An asymmetrical stretching of skeletal carbon atoms of the porphyrin conformer has been observed and attributed to the ordered molecular stacking and intermolecular interactions. X-ray absorption near edge structure (XANES) establishes a symmetry reduction in the molecular conformer involving skeletal carbon atoms of the porphyrin ring for the ordered films highlighting the consequences of increased pi stacking of ZnOEP molecules adopting triclinic structure. The observed asymmetrical stretching of pi conjugation network of porphyrin structure can have significant implications on the charge transport and light harvesting ability of porphyrin thin films on substrates
Black Phosphorus: Pristine and doped surface investigations using Scanning Tunneling Microscopy
No full textBlack Phosphorus (bP) is the most stable allotrope of phosphorus, first synthesized in
1914 by Bridgman. It was investigated along with other layered materials like GaS, GaSe,
GaTe, graphite, boron nitride and transition metal dichalcogenides for nearly a century.
One important characteristic of these layered materials is that they are composed of twodimensional (2D) sheets of covalently bonded atoms that are kept together by van der
Waals forces. Therefore, they were perceived as interesting materials with an ambition
to achieve thinner and thinner materials, up to a single monolayer, called 2D materials.
In 2004, Andre Geim and Konstantin Novoselov demonstrated the first successful preparation of one-atomic-thin carbon films, called graphene. They used the technique of
scotch tape exfoliation and studied the marvelous properties of graphene, which got
them the Nobel Prize in physics in 2010. With the discovery of interesting properties of
graphene, a search for other 2D materials started. A range of such materials have been
realized since then, like hexagonal boron nitride (h-BN), silicene, germanene, stanene,
and transition metal dichalcogenides (TMDCs). Black Phosphorus (bP) is an important
part of this class of 2D materials, from which phosphorene is exfoliated.
Phosphorene, first exfoliated in 2014, has emerged as an important material. With its
band gap tunable with thickness (from 2.0 eV for the single layer to 0.3 eV for the
bulk), it occupies a special position between zero band gap graphene and high band
gap TMDCs. Anisotropy is another important aspect discovered in its properties like
effective mass, mobility, thermal conductivity, and plasmon resonance. This opens a
gate for potential applications in electronics, photonics, thermoelectrics, and for gas
sensing devices.
Surface studies of bP are quite limited so far. There have been some works reported
showing bP atomic resolution and tunneling spectra on bP surfaces. However, most of
them have been performed on cleaved bulk bP crystals. From a 2D application point of
view, however, it is important to study thin exfoliated bP flakes. The majority of work
reported until now on thin bP flakes concerns electrical transport or optical experiments,
performed on flakes encapsulated in a protective layer. The reason for this is to protect
the material from oxidation upon exposure to air, since bP is known to be highly reactive.
One STM study on a few nanometer-thin bP flake shows surface atomic resolution, but
mainly focuses on the spectroscopic properties of bP. Since surface plays an important
role in nanomaterials due to the high surface-to-volume ratio, it is very important to
understand the surface behavior of such materials.
Here, we have prepared samples on which surface studies on exfoliated bP flakes could be
done. We have prepared samples by exfoliating bP in a glove bag continuously flushed with nitrogen, which provides an inert atmosphere and protects the reactive bP surface
from oxidation. We have used graphene on silicon carbide (SiC) as a substrate, which
provides large atomically flat terraces of SiC and a conducting graphene sheet on top
– fulfilling major requirements for scanning tunneling microscopy (STM) experiments.
We mount the sample inside the glove bag and transfer it under inert atmosphere, which
helps in preserving the highly reactive bP surface from oxidation. This is confirmed by
flat, clean, and oxide-free bP surfaces as seen in STM.
We initially started with the investigation of surface behavior with temperature. We
found that 200 ◦C to 300 ◦C is a proper temperature range for cleaning the surface. We
saw defects on the clean bP surface, reported earlier to be the reason for the intrinsic
p-type doping of bP. At 375 ◦C to 400 ◦C, eye-shaped craters started to develop on the
surface due to phosphorus desorption. Meanwhile, we solved an existing debate in the
literature regarding the orientation of the long axis of these elongated craters: along
the crystallographic armchair direction due to an atomic phosphorus desorption mechanism, or along the crystallographic zigzag direction due to a molecular P2 desorption
mechanism, both investigated by electron microscopy and diffraction studies. Armed
with the power of atomic resolution imaging enabled by the STM technique, we were
able to resolve smaller craters, which are the seeds of the larger craters reported in
previous studies. With a statistical analysis, we confirmed the specific directionality of
the orientation of these anisotropic craters. Furthermore, with the help of atomic resolution provided by STM, we could directly compare the crater alignment with respect
to atomic arrangement, and found that the long axis of the craters is aligned with the
zigzag direction – thus solving the existing debate in the literature.
bP is intrinsically p-doped. Some works were performed to obtain an n-type behavior
by doping, as it would allow p-type and n-type behavior in the same material, very
useful for basic diode applications. In one paper focusing on devices, copper adatom
doping has been shown to yield n-type behavior, using transport measurements. Here,
we study copper growth morphology on bP. We observed the preference of copper atoms
to occupy atomic vacancies of the bP surface. We also observed an alignment of copper
islands along the crystallographic armchair direction of bP, and a step decoration of
copper islands at bP step edges. With scanning tunneling spectroscopy, we studied the
transfer doping of bP by copper at the local atomic level and found a shift of the Fermi
level in bP from p-type behavior to n-type behavior. We also observed an increase in
the band gap value measured on doped bP, consistent with DFT calculations
Replication Data for: Color‐Sensitive Ultrafast Optical Modulation and Switching of Terahertz Plasmonic Devices
No full text2D micro‐nanostructured metal films with hole arrays show promising features such as the extraordinary transmission of light. Such systems are interesting in the field of subwavelength photonics and nonlinear optics due to their high field confinement in addition to their inherent spectral scalability and frequency selective response. Several active schemes to control the extraordinary transmission are recently demonstrated. However, these dynamic devices do not reveal any obvious color‐dependent modulation of the resonant transmission behavior. Here, color‐sensitive ultrafast modulation of extraordinary resonant transmission of terahertz (THz) waves through 2D metallic hole arrays is demonstrated. Pumping the silicon beneath the metallic array with light of different colors and identical fluences exhibit significantly different ultrafast switching dynamics and modulation. The color‐dependent sensitivity and control of THz waves at an ultrafast timescale provide an extra degree of freedom that opens up new opportunities for future applications in active subwavelength optics, optoelectronics, and all‐optical switching of THz photonic devices
News, noise, and Indian business cycle
No full textNew Keynesian dynamic stochastic general equilibrium models with various specifications of technology, markup, and interest rate shocks are estimated with Indian data using Kalman filter based pure and Bayesian likelihood estimation. Preference and interest rate shocks are found to be important for output determination, whereas markup and interest rate shocks are important for inflation. News, as contained in stock market variables and arising from anticipated interest rates, affects growth of gross domestic product. Interest rate shock is anticipated at horizon of one quarter and out of total variance explained by interest rate shock, one third is due to the anticipated shock. Anticipated interest rate shock diminishes the share of preference shock in output determination. Although markup shock has a large share, its persistence is low. There is evidence that permanent component of technology is not well anticipated. Once we incorporate this, technology shocks affect output more, although they still remain much below US levels. Implications for policy include forward guidance on interest rates, less reaction to short-term supply shocks, and allowing technology shocks to play out
Oil price dynamics in times of uncertainty: revisiting the role of demand and supply shocks
No full textDrivers of real oil prices have been explored extensively in the literature with little consensus. Using a new identification scheme based on forecast error variance, we identify oil-specific demand, demand, and oil supply shocks that maximize the sum of forecast error variance of three variables explained by their respective shocks. The estimation, with the sample period until 2007, suggests that the three identified shocks have similar effects as in the early literature, with oil-specific demand shocks playing a prominent role. However, in the post-crisis period supply shocks have emerged as a source of short-run increases in oil prices, and demand shocks do not have a long-run effect on prices, unlike in the pre-crisis period. By further including risk in the model, we show that the importance of supply shock in driving oil prices in the short run is not driven by global risk. These estimates overwhelmingly suggest zero short-run supply elasticity - a matter of debate in the recent literature. Aside from oil-specific demand shocks, six episodes (including COVID-19) and a time-varying VAR with stochastic volatility identified based on forecast error variance suggest that other shocks, in particular supply shocks, have also played a significant role in driving oil prices in different episodes which cannot be ignored while evaluating the oil price dynamics.</p
Usage of formal financial services in India:: Demand barriers or supply constraints?
No full textEstimating the role of both demand-side and supply-side factors in financial inclusion and its distribution is important for policy making. However, existing literature has primarily focused on supply-side factors. In this context, this paper estimates relative importance of removing demand-side barriers and eliminating supply constraints to enhance financial inclusion in India. It also measures the extent of concentration of usage of formal financial services among richer households. Results suggest that, while availability of banking services has a significant positive effect on usage of formal financial services, its contribution in inducing households to use formal financial services is very small compared to the contribution of factors, such as education, income, employment status, gender and social norms, that influence the demand for formal financial services. It highlights the importance of placing greater emphasis on addressing demand-side barriers, rather than on improving physical availability of banking services, to promote financial inclusion in India
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