[en] We obtain an effective parametrization of the bulk electronic structure of InP within the tight-binding scheme. Using these parameters, we calculate the electronic structure of InP clusters with the size ranging up to 7.5 nm. The calculated variations in the electronic structure as a function of the cluster size is found to be in excellent agreement with experimental results over the entire range of sizes, establishing the effectiveness and transferability of the obtained parameter strengths

[en] Metal halide perovskite crystal structures have emerged as a class of optoelectronic materials, which combine the ease of solution processability with excellent optical absorption and emission qualities. However, the most promising perovskite structures rely on lead as a cationic species, thereby hindering commercial application. The replacement of lead with non-toxic alternatives such as tin has been studied in bulk but not in nanocrystals. In this work, we synthesize Sn and Pb based alloy perovskite nanocrystals by direct synthesis method, taking mixture of Pb and Sn precursors in the desired ratio leading to quantum dots (QDs) of CsPb_1−xSn_xBr_yI_3−y with successful Sn incorporation into the host lattice. As colloidal stability of these QDs is a crucial factor for device applications, we have studied the stability of the QDs under different conditions for these Sn based QDs and have found them to degrade faster upon using anti-solvents during washing process. In order to stabilize them, we have devised a purification method that is also discussed. Further, even though the optical and crystal structure stability in some of the inorganic perovskites leaves much room for improvement, so far there have been no studies on the structure property correlation. Here we study their structural purity and their optical stability after understanding the structure property correlation in CsPbI₃ and CsPbBr₃ perovskite structures. The stability of Sn doped perovskites obtained from a logical understanding of structure property correlation is found to be extremely stable across the series of compounds for upto three months. (paper)

[en] Recently the metal halide perovskite nanoparticles have attracted a lot of attention for various opto-electronic applications like micro/nano lasers, light emitting diodes, photodetectors along with solar cells due to their low fabrication costs as well as excellent electronic and optical properties. However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high band gap materials. In a bid towards minimizing energy loss, in this talk I introduce unique pathways to harvest additional photons via enhanced absorption and additional excitonic recombination pathways that are so far not extensively explored through doping and heterostructure formation. (author)

[en] Calculations of Slater-Koster (SK) parameters appearing in the tight-binding method using sp³d⁵ basis sets for both the cationic and anionic species are presented for ZnS and CdS. We have adjusted these parameters to match the band structures obtained from the full potential linear augmented plane wave method. This operation has been carried out for a variety of structures namely zinc blende, wurtzite, rocksalt, CsCl and for a wide range of near-neighbor distances. The SK parameters have slightly different values for the same near-neighbor distance in different structures. Therefore, a least-squares fitting has been performed separately for each parameter as a function of only the near-neighbor distance to guarantee the transferability of these parameters to different structural environments. The fitted parameters are then used to calculate the electronic structure of small-sized clusters of ZnS and CdS in given geometries and the results are compared with ab initio results. A fairly good agreement found in the one-electron energy spectrum and total energy confirms transferability of the parameters to different length scales. A detailed account of the calculation procedure and calibration results is given in the present paper. These parameters can be used to study the electronic structure of large-sized clusters where first-principles methods are computationally demanding. It may be mentioned that the SK parameters do not satisfy the R^-(l+l'+1) Harrison scaling law for larger values of the near-neighbor distance R.

[en] Absolute energies of band edges have proven to be very important for various applications like hydrogen generation, solar water splitting and solar cell optimization. Energy differences as small as 50–100 meV have been shown to largely affect device efficiencies. Device operational temperature can vary largely and temperature dependence of band gap is well known in bulk semiconductor literature. However, there are only a few studies on variation of band gap in quantum dots and none of them characterize the relative energy variation of band edges in spite of their importance in various applications. This is mainly due to the absence of an internal standard that can be used to study the variation of band edges. Here, in this paper, we introduce a technique wherein we utilize Cu dopant emission as an internal probe. Using this technique, we report the variation of band gap, conduction band and valence band edges of CdS and CdSe quantum dots as a function of temperature and size. We found that band gap variation is similar to that of bulk but with a higher average phonon energy. The band edge variation is characterized by a dominant conduction band shift for larger sizes with decreasing temperature while the smaller size QDs show the variation in both conduction band and valence band. Further, we have also utilized this method to study the binding energy of the trap states as a function of temperature using Cu photoluminescence quantum yield and average lifetime of Cu photoluminescence. (paper)

[en] Transition metal doping of semiconductor nanocrystals (NCs) can generate new optical, magnetic, properties through dopant-host interaction. Although Mn²⁺ doping in semiconducting NCs has been studied for decades, Mn doped perovskite NCs have opened up new avenues for optoelectronic applications due to signature Mn d-d emission. However, Mn doping in bromide-based perovskite NCs have not shown this signature peak sowing doubts about the efficient doping in these systems. Here, we demonstrate that the chemical bonding and local environment of Mn obtained using electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) is similar to that of chloride-based perovskites. However, the differences in optical properties between the chloride and bromide-based perovskites NCs arises due to fundamental difference in mechanism in perovskite NCs compared to the II-VI semiconductor quantum dots. This provides some insight into this problem from a fundamental perspective leading to more efficient synthesis techniques for applications.

[en] Efficient and environmentally benign visible light responsive materials have been sought after owing to their interesting applications such as visible light photocatalysis, visible light water splitting and visible light sensing. In this research study, the effect of co-doping on the absorption and electrical properties of ZnS quantum dots is studied. Upon co-doping of Fe and Cu into ZnS quantum dots, a new absorption band in the visible region is observed. Furthermore, these quantum dots show photoresponse in the visible region unlike their undoped counterparts that is only effective in the UV region, suggesting their utility in light sensing applications.