Papers by Dr. Dheerendra Singh Yadav
EEJP, 2023
Herein, we have formulated a simplistic semi-empirical model for Vicker's hardness of rare earth ... more Herein, we have formulated a simplistic semi-empirical model for Vicker's hardness of rare earth based pyrochlore compounds. We have considered the A B O structured 97 pyrochlore compounds for Vicker's hardness calculations. The plasmon energy (ħωp) depends on basic parameters of the material such as Ne-effective number of free electrons per unit volume participating in plasma oscillations, e-electronic charge and m-mass of an electron. The proposed model predicts that the experimental and theoretical values of Vicker's hardness increases as plasmon energy of pyrochlore increases. We have found that the calculated values are in better agreement with available experimental and theoretical data, which supports the validity of the model. This model supports the modeling of emerging functional pyrochlore compounds and helps to understand their mechanical properties for excellent thermal stability, superconductivities, batteries, ferroelectricity, water spitting, high ionic conductivity, good photoluminescence, inherent oxygen vacancies, exotic magnetism, and now-a-days most importantly in nuclear waste encapsulation and aerospace industry.
The Journal of Physical Chemistry B, 2016
Protein Configuration Landscape Fluctuations Revealed by Exciton Transition Polarizations in Single Light Harvesting Complexes
The Journal of Physical Chemistry B, 2016
Protein is a flexible material with broad distribution of conformations forming an energy landsca... more Protein is a flexible material with broad distribution of conformations forming an energy landscape of quasi-stationary states. Disentangling the system dynamics along this landscape is the key for understanding the functioning of the protein. Here we studied a photosynthetic antenna pigment-protein complex LH2 with single molecule 2-dimensional polarization imaging. Modelling based on the Redfield relaxation theory well describes the observed polarization properties of LH2 fluorescence and fluorescence excitation strongly suggesting that at 77 K the conformational subspace of the LH2 is limited to about 3 configurations with relatively frequent switching among each other. At room temperature the next level of fluctuations determines the conformational dynamics. The results support the multi-tier model of the energy landscape of proteins and demonstrate the potential of the method for the studies of structural dynamics in proteins.
The Journal of Physical Chemistry B, 2015
Dronpa is a GFP-related photochromic fluorescent protein used as probe in superresolution microsc... more Dronpa is a GFP-related photochromic fluorescent protein used as probe in superresolution microscopy. It is known that the photochromic reaction involves cis/trans isomerization of the chromophore and protonation/deprotonation of its phenol group, but the sequence in time of the two steps and their characteristic timescales are still the subject of much debate. We report here a comprehensive UV-visible transient absorption spectroscopy study of the photoactivation mechanism of Dronpa, covering all relevant timescales from ~100 fs to milliseconds. The Dronpa-2 variant was also studied and showed the same behavior. By carefully controlling the excitation energy to avoid multiphoton processes, we could measure both the spectrum and the anisotropy of the first photoactivation intermediate. We show that the observed few nanometer blue-shift of this intermediate is characteristic for a neutral cis chromophore, and that its anisotropy of ~0.2 is in good agreement with the reorientation of the transition dipole moment expected upon isomerization. These data constitute the first clear evidence that trans→cis isomerization of the chromophore precedes its deprotonation and occurs on the picosecond timescale, concomitantly to the excited-state decay. We found the deprotonation step to follow in ~10 μs and lead directly from the neutral cis intermediate to the final state.
Cyclodextrin Insulation Prevents Static Quenching of Conjugated Polymer Fluorescence at the Single Molecule Level
Small, 2013
Conjugated polymers (CPs) are promising materials for fluorescence imaging application. However, ... more Conjugated polymers (CPs) are promising materials for fluorescence imaging application. However, a significant problem in this field is the unexplained abnormally low fluorescence brightness (or number of fluorescence photons detected per one excitation photon) exhibited by most of CP single chains in solid polymer hosts. Here it is shown that this detrimental effect can be fully avoided for short chains of polyfluorene-bis-vinylphenylene (PFBV) embedded in a host polymer matrix of PMMA, if the conjugated backbone is insulated by cyclodextrin rings to form a polyrotaxane (PFBV-Rtx). Fluorescence kinetics and quantum yields are measured for the polymers in liquid solutions, pristine films, and solid PMMA blends. The fluorescence brightness of PFBV-Rtx single chains dispersed in a solid PMMA is very close to that expected for a chain with 100% fluorescence quantum yield, while the unprotected PFBV chains of the same length possess 4 times lower brightness. Despite this, the fluorescence decay kinetics are the same for both polymers, suggesting the presence of static or ultrafast fluorescence quenching in the unprotected polymer. About 80% of an unprotected PFBV chain is estimated to be completely quenched. The hypothesis is that the cyclodextrin rings prevent the quenching by working as 'bumpers' reducing the mechanical forces applied by the host polymer to the conjugated backbone and help retaining its conformational freedom. While providing a recipe for making CP fluorescence bright at the single-molecule level, these results identify a lack of fundamental understanding in the community of the influence of the environment on excited states in conjugated materials.
Evidence of excited state localization and static disorder in LH2 investigated by 2D-polarization single-molecule imaging at room temperature
Physical Chemistry Chemical Physics, 2013
Two-dimensional polarization fluorescence imaging of single light harvesting complexes 2 (LH2) of... more Two-dimensional polarization fluorescence imaging of single light harvesting complexes 2 (LH2) of Rps. acidophila was carried out to investigate the polarization properties of excitation and fluorescence emission simultaneously, at room temperature. In two separate experiments we excited LH2 with a spectrally narrow laser line matched to the absorption bands of the two chromophore rings, B800 and B850, thereby indirectly and directly triggering fluorescence of the B850 exciton state. A correlation analysis of the polarization modulation depths in excitation and emission for a large number of single complexes was performed. Our results show, in comparison to B800, that the B850 ring is a more isotropic absorber due to the excitonic nature of its excited states. At the same time, we observed a strong tendency for LH2 to emit with dipolar character, from which preferential localization of the emissive exciton, stable for minutes, is inferred. We argue that the observed effects can consistently be explained by static energetic disorder and/or deformation of the complex, with possible involvement of exciton self-trapping.
Bulk Modulus and Cohesive Energy of AII BVI (A = Mg, Ca, Sr, Ba and B = O, S, Se, Te) Solids
Journal of Pure Applied and Industrial Physics, 2018
Elastic Properties of Transition Metal Nitrides and Carbides
In this paper, we study the elastic properties (i.e. Bulk modulus B) of a series of early transit... more In this paper, we study the elastic properties (i.e. Bulk modulus B) of a series of early transition metal mono-nitrides and carbides crystallize in the NaCl-type face centered cubic (FCC) structure has been investigated using plasma oscillation theory of solids. The same approach has previously been applied to rock-salt structured rare-earth mono-chalcogenides (J. Alloys and Compd. 537 (2012) 250-254). We have presented an expression relating the bulk modulus B (in GPa) for early transition metal mono-nitrides (TMN) and carbides (TMC) with their plasmon energy ħωp (in eV). The bulk moduli of these compounds exhibit a linear relationship when plotted on a log-log scale against plasmon energy. We have applied the modified relation on rock-salt structured (TMN's & TMC's) solids. The trends are discussed and compared with available experimental data and theoretical results reported by earlier researchers.
Journal of Pure Applied and Industrial Physics
In this paper, we have investigated the electronic properties of rare earth mono-pnictides in roc... more In this paper, we have investigated the electronic properties of rare earth mono-pnictides in rock-salt structured using the plasma oscillations theory of solids. We have presented the expressions relating the homopolar gap (Eh), ionic gap (Ec), average energy gap (Eg), crystal ionicity (fi) and electronic polarizability (αe) for rare earth monopnictides with the plasmon energy (ћωp). The electronic properties such as homopolar gap, ionic gap, average energy gap, crystal ionicity and electronic polarizability of these compounds exhibit a linear relationship when plotted on a loglog scale against the plasmon energy (ћωp). A fairly good agreement has been found between observed and calculated values of Eh, Ec, Eg, fi, and αe for rare earth monopnictides.
Dielectric Properties of Complex Structured Solids
Journal of Pure Applied and Industrial Physics
Static and dynamical properties of II�VI and III�V group binary solids
Physica Scripta, 2012
In this paper, we extend to II?VI and III?V group binary solids of zinc blende (ZB) structure wit... more In this paper, we extend to II?VI and III?V group binary solids of zinc blende (ZB) structure with conduction d-electrons the calculation of static and dynamical properties such as bulk modulus (B) and cohesive energy or total energy (Ecoh) using the plasma oscillation theory of solids formalism already employed for ternary chalcopyrite semiconductors. The present method is not limited to tetrahedrally coordinated semiconductors and ternary chalcopyrites, but can be used for all semiconducting compounds. We have applied an extended formula on ZB structured binary semiconductors and found better agreement with the experimental data as compared to the values evaluated by previous researchers. The bulk modulus and cohesive energy of ZB-type structure compounds exhibit a linear relationship when plotted on a log?log scale against the plasmon energy ?p (in eV), but fall on a straight line. The results for bulk modulus differ from experimental values by the following amounts: ZnS 0.36%, ZnSe 10%, ZnTe 0.62%, CdS 1.8%, CdSe 7.4% and CdTe 1.6%, AlP 2.6%, AlAs 5.3%, AlSb 4.0%, GaP 0%, AlAs 0%, AlS 4.4%, InP 0%, InAs 0% and InSb 2.1%; and the results for cohesive energy differ from experimental values by the following amounts: ZnS 0.16%, ZnSe 0.73%, ZnTe 0.6%, CdS 7.6%, CdSe 3.5%, CdTe 2.5%, AlP 2.0%, AlAs 3.0%, AlSb 11.1%, GaP 14.6%, AlAs 17.0%, AlSb 8.7%, InP 4.3%, InAs 5.5% and InSb 0.6%.
Electronic, Optical and Mechanical Properties of a II B VI Semiconductors
The modified dielectric theory of solids is applied to investigate electronic, optical and mechan... more The modified dielectric theory of solids is applied to investigate electronic, optical and mechanical properties of A II B VI binary semiconductors ( ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgS, HgSe & HgTe ). The values of homopolar gaps (Eh), heteropolar gaps (Ec) and average energy gaps (Eg) were evaluated for these A II B VI groups of binary semiconductors with Zinc-blende (ZB) structure. The derived values of average energy gap (Eg) were found to be in excellent agreement with the values obtained from the Penn model except ZnO . The electronic polarizability was investigated using Chemla's relation and the values were found to be in a very good agreement with the results obtained from the Clausius–Mossotti relation. The crystal ionicity (fi) was evaluated and the obtained values were compared with the values obtained by different researchers. The evaluated values of crystal ionicity were used to calculate the electronic, optical, mechanical properties such as bulk modulus (B in GPa) cohesive energy or total energy (U in Ryd. electron) and microhardness (H in GPa) of these compound semiconductors. A good agreement has been found between calculated and experimental data.
Electronic properties of aluminum, gallium and indium pnictides
Physica Scripta, 2010
... gallium and indium pnictides Dheerendra Singh Yadav1 and SP Singh2 1 Department of Physics, C... more ... gallium and indium pnictides Dheerendra Singh Yadav1 and SP Singh2 1 Department of Physics, Ch Charan Singh PG College, Heonra, Etawah 206001, UP, India 2 Department of Physics, KK Post Graduate College, Etawah 206001, UP, India ...
Journal of Pure Applied and Industrial Physics
In this paper, we have investigated the electronic and mechanical properties of ternary I-III-VI2... more In this paper, we have investigated the electronic and mechanical properties of ternary I-III-VI2 chalcopyrite semiconductors using the modified dielectric theory of solids. The values of homopolar gap (Eh), heteropolar gap (Ec) and Penn gap (Ep) is evaluated for these compound semiconductors with chalcopyrite structure. The derived values of average energy gap or Penn gap (Ep) are found in good agreement with the values obtained from the Penn model. Phillips ionicity (fi) is evaluated and is used to determine the bulk modulus (B in GPa) and microhardness (H in GPa) for these materials. The results are in excellent agreement with the available experimental data as compared to the theoretical values reported by earlier researchers.
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Papers by Dr. Dheerendra Singh Yadav