The broad direction of our research is to develop novel semiconductors with desired electronic structures that can give rise to interesting photophysical and optoelectronic properties. We study these properties in solutions, thin films (employing active device geometry), and also at the level of a single nanocrystal. 

 

 

 

 

 

 

 

 

 

 

 

 

 

Our work is interdisciplinary in nature involving materials chemistry, spectroscopy, and some amount of condensed matter physics. We strongly believe that the development of optoelectronic materials has at least four equally important components, namely, synthesis, properties, computational studies, and device fabrication. In our lab, we do synthesis, properties and, preliminary device fabrication. But often we do a collaboration with other groups to have the best expertise in all the four aspects.

 

Basic thermodynamics dictates that Gibbs' free energy (G = H - TS) of a crystal decreases at T > 0 K with the presence of defects because of the increase in configuration entropy. Therefore, semiconductor crystals always have defects, which typically trap charge carriers reducing the efficiency of the optoelectronic process. To minimize the defect density, very stringent synthesis conditions are required, making only a few semiconductors like Si, Ge, GaAs, and CdTe as successful candidates for optoelectronic applications. The cost of processing is very high because of the requirement of stringent preparation conditions. Our current and future research is aimed to minimize the detrimental effects of defects on optoelectronic properties, while the material is prepared by employing easy methodologies (ideally solution processed).    

    

Four major research topics of our group are discussed below. 

 

(i) Colloidal CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals:

Defect density in colloidal nanocrystals is high because of the high surface to volume ratio. So, our target is not to prepare defect-free materials, instead, our strategy of success is to look for defect-tolerant materials. Defects might be there, but the energy of the defects is tuned in such a way that defect states do not trap charge carriers. In other words, defects are forgiving.

Our contributions:

• Establishing the fact that surface defects in CsPbX3 nanocrystals do not trap charge carriers. This is an extraordinary property of CsPbX3 nanocrystals compared to all previous semiconductor nanocrystals

• Elucidating the band-edge energies, extinction coefficients of optical absorption, and crystal structure of CsPbX3 nanocrystals

• Elucidating surface chemistry of CsPbX3 nanocrystals and then optimizing it for fabrication of photodetector and LED

 

 

 

 

 

 

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(ii) Pb-free Metal Halide Perovskite for Optoelectronics:

Our research in this direction is not limited to overcome the toxicity and stability issues of Pb-halide perovskites. Our research is focused on discovering potential Pb-free systems that can show defect-tolerant electronic and optical properties.

Our contributions:

• First report of synthesis and optical properties of colloidal Cs3Sb2I9, Cs3Bi2I9, and  TlX (X = Br and I) nanocrystals

• Mn- and Yb-doping in Cs2AgInCl6 double perovskite showing downconversion emission

• Till date, no Pb-free system showed defect tolerance similar to Pb-halide perovskite

 

 

 

 

 

 

 

 

 

 

 

 

(iii) Plasmonics and Magnetically Doped Colloidal Metal Oxide Nanocrystals

We dope magnetic ions such as Mn2+ or Fe3+ or Cr3+ in colloidal transparent conducting oxide ITO (Sn4+- doped In2O3) nanocrystals. Sn4+ doping provides delocalized conduction-band-electron (CBe), which in turn gives rise to localized surface plasmon resonance (LSPR) band in the infrared region. The idea is to use magnetic and optical signals to study the interaction between delocalized CBe and localized d-electrons of magnetic dopants.

Our achievements:

• Synthesis of Fe-Sn, Mn-Sn, and Cr-Sn codoped In2O3 nanocrystals

• Finding signatures of ferromagnetic interactions between remotely located magnetic ions through the delocalize CBe

• The record best LSPR figure of merit (quality factor) in Cr-Sn codoped In2O3 nanocrystals

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(iv) Surface Engineering of Nanocrystals for Better Charge Transport

Target here is to achieve solution processed and flexible films for active semiconductor layer as well as electrodes for optoelectronic applications. The idea is to design surface chemistry in a way that the barrier for charge injection (or extraction) to (from) nanocrystal is minimized.   

• Colloidal synthesis of organic-free metal chalcogenide nanocrystals for better charge transport in films

• Oriented attachment of PbS and PbSe nanocrystals at room temperature forming solution processed and flexible films with good electronic conductivity (~100 S/cm)

• Electronically coupled TiN nanocrystal with N-doped few-layer graphene as counter electrode in solar cells.