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Electronic Materials: The New Physical Insights
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PEROVSKITE NANOCRYSTALS
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DOPING IN SEMICONDUCTOR NANOCRYSTALS
Doping in semiconductor nanocrystals including chalcogenide nanocrystals is extensively studied. The doping process itself as well as dopant-induced changes in optical, electrical, and magnetic properties have been investigated. However, these are limited to selective hosts and dopants. This needs to be further explored as doping might result in Cd- and Pb-free materials with comparable optical properties. These are the nextgeneration nanocrystals for solid-state lighting and hence offer more physical insights for obtaining intense and stable emission. Recently, doping of Mn in CsPbCl3 perovskite nanocrystals was reported where the exciton energy of the host nanocrystals was transferred to Mn d-states, leading to spinpolarized Mn d-d emission. However, unlike chalcogenide doping, insights into the doping process in these ionic solids are not yet understood. The widely established growth doping cannot be established here as the entire process ceases instantaneously. Hence, the mechanism of doping impurity ions in these perovskites needs to be explored. Apart from Mn, other transition metals were successfully doped in chalcogenide nanocrystals and their optical properties were also studied, but these doping strategies are limited to perovskite nanocrystals. One more emerging functional property of these doped nanocrystals is based on localized surface plasmon resonance (LSPR). When an aliovalent metal cation is doped in a nanocrystal, it results in free charge carriers in the crystal, which induce LSPR, and this is typically tuned in the NIR spectral window (J. Phys. Chem. Lett. 2014, 5, 976). While several oxides, doped oxides, and chalcogenides have already shown tunable plasmon absorption, still greener material with LSPR behavior is needed. Such materials having absorption with a higher extinction coefficient are more important, and their implementation in solar light harvesting also needs more attention.
CHALCOPYRITE AND KESTERITE TYPES OF NANOCRYSTALS
Among other electronic materials, the chalcopyrites (CuFeS2) and kesterites (Cu2(ZnFe)SnS4) families of nanocrystals are known as the most important green materials that can be used for both light-emitting and photovoltaic devices. Chalcopyrites are group I−III−VI semiconductors, where CuInS2 and CuGaS2 are the most widely studied. On Zn or Cd incorporation, these provide tunable emissions in the entire visible window and also are efficient materials for charge-carrier transport in light to electricity conversion. While the chemical and physical processes in their synthesis are widely studied, the origin of the emission and control of the interband state for the optical tunability is still not completely understood (J. Phys. Chem. C 2016, 120, 5207). Similarly, the nature of defect states that help in enhancing charge-transport efficiency needs deeper investigation. Although the activity of these materials is not comparable to the commercialized multicrystalline silicon and other well-reported systems (GaAs, CdTe, and lead perovskites), the low cost and lower toxicity makes them ideal photovoltaic materials for future applications. In addition, the physical insights into coupling these materials with various other semiconductors and even noble metals are still not widely explored like the group II−VI chalcogenide family of nanocrystals. Hence even though these nanostructures are largely investigated, several opportunities are still available and need to be explored. © 2017 American Chemical Society
TETRAHEDRITE TYPE OF NANOCRYSTALS
Tetrahedrites (Cu12−xICuxIISb4S13) are another member of the multinary family of nanocrystals containing Sb, which have been less explored compared with chalcopyrite and kesterite nanocrystals. The band positions of these materials allow these to be ideal for harvesting visible light, and the multinary nature with defects should help charge transport for enhancing photovoltaic performance. However, synthesis of these materials and study of their electronic properties are still unexplored and might be investigated. There can also be replacement of CuI and CuII with several mono- and bivalent cations, respectively, leading to different substituted tetrahedrites.
In the current research on electronic materials, one of the hot areas that has newly emerged is the functional properties of perovskite nanocrystals. Even though the possible scopes of these materials were predicted a long time ago, these started their journey in the experimental lab very recently. Both inorganic and organic−inorganic hybrid lead perovskites have been studied, but the physical insights associated with their optical and photovoltaic properties still remain widely unexplored (J. Phys. Chem. C 2017, 121, 1941). The beauty of these materials is their fabulously tunable optical properties that are contrastingly affected by interface, solvent, crystal phase, orientation, and also their dimensions. Moreover, their stability and the influence of surface modifications that retain their functional properties still pose big questions. Hence this requires in-depth understanding of the photophysical process during surface modification and solvent dispersion. In addition, there is a window of opportunity for coupling of perovskites to other semiconductors or metals to generate functional materials that go beyond lead-based perovskites. Hence even though the physical processes in their synthesis and their physical properties are to some extent understood, these materials are still at an embryonic state in comparison with the widely studied chalcogenide semiconductor quantum dots.
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Published: September 7, 2017 18973
DOI: 10.1021/acs.jpcc.7b08003 J. Phys. Chem. C 2017, 121, 18973−18974
The Journal of Physical Chemistry C
Viewpoint
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PHYSICAL PROCESSES IN ELECTRONIC MATERIALS SYNTHESIS The root of all of these exciting functional properties is related to synthesis, which controls the dimension and composition of the materials. Again, this highlights that understanding the physical processes involved in the synthesis of nanomaterials is crucial. Typically, this involves determining parameters controlling the kinetic and thermal factors in nucleation and growth processes in the formation of a particular family of nanocrystals. Although in a few cases, particularly for metal chalcogenides, these have been established, more generally this is unexplored. Hence physical insights into the formation of functional electronic materials and mostly the multinary family of nanocrystals, their heterostructures with other metal or semiconductors, etc. are still open and need further investigation. The control of kinetic and thermodynamic parameters regulating the dispersity, dimensions, and shape of these electronic materials needs to be achieved for understanding the origin of the involved functional properties.
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ELECTRONIC MATERIALS FOR ENERGY APPLICATIONS In addition to optoelectronic and photovoltaic applications, these electronic materials also have tremendous opportunities to be used as efficient photocatalysts for water splitting. Even though a large number of semiconductors and metal−semiconductor nanoheterostructures have been explored as photocatalysts for hydrogen evolution from water (J. Phys. Chem. Lett. 2010, 1, 1051) and carbon dioxide reduction, their numbers are still limited. Hence the development of more efficient materials and particularly appropriate combinations of materials having ideal band alignments is required for facilitating the transport of photogenerated charge carriers. Even non-Cd-based catalysts that are relevant to hydrogen energy conversion efficiency might be further explored. In summary, there are important opportunities for research leading to new physical insights into the synthesis and properties of many families of electronic materials. Only a few possibilities have been mentioned in this Viewpoint; certainly many more are to be found. I hope these will help our readers and authors to understand what we mean by research leading to new physical insights in physical chemistry. Narayan Pradhan Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India
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AUTHOR INFORMATION
ORCID
Narayan Pradhan: 0000-0003-4646-8488 Notes
The author declares no competing financial interest.
18974
DOI: 10.1021/acs.jpcc.7b08003 J. Phys. Chem. C 2017, 121, 18973−18974
