Prospects of Colloidal Copper Chalcogenide Nanocrystals

Abstract

Over the past few years, colloidal copper chalcogenide nanocrystals (NCs) have emerged as promising alternatives to conventional Cd and Pb chalcogenide NCs. Owing to their wide size, shape, and composition tunability, Cu chalcogenide NCs hold great promise for several applications, such as photovoltaics, lighting and displays, and biomedical imaging. They also offer characteristics that are unparalleled by Cd and Pb chalcogenide NCs, such as plasmonic properties. Moreover, colloidal Cu chalcogenide NCs have low toxicity, potentially lower costs, and excellent colloidal stability. This makes them attractive materials for the large-scale deployment of inexpensive, sustainable, and environmentally benign solution-processed devices. Nevertheless, the synthesis of colloidal Cu chalcogenide NCs, especially that of ternary and quaternary compositions, has yet to reach the same level of mastery as that available for the prototypical Cd chalcogenide based NCs. This review provides a concise overview of this rapidly advancing field, sketching the state of the art and highlighting the key challenges. We discuss recent developments in the synthesis of size-, shape-, and composition-controlled NCs of Cu chalcogenides, with emphasis in strategies to circumvent the limitations arising from the need to precisely balance the reactivities of multiple precursors in synthesizing ternary and quaternary compositions. In this respect, we show that topotactic cation-exchange reactions are a promising alternative route to complex multinary Cu chalcogenide NCs and hetero-NCs, which are not attainable by conventional routes. The properties and potential applications of Cu chalcogenide NCs and hetero-NCs are also addressed.

Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS2 Quantum Dots

“…40,41,51,52,55−58,61 However, it is as yet unclear whether the wide tunability of the optical properties of CIS NCs (and other ternary copper chalcogenides) can be attributed solely to quantum confinement effects, in striking contrast with the II–VI, IV–VI, and III–V binary analogues. 40 The foremost difference between ternary copper (and silver) chalcogenides (I–III–VI 2 ) and binary II–VI and IV–VI compounds is that the former can easily tolerate large deviations from stoichiometry, both in the I/III ratio and VI/(I + III) ratio, resulting in a rich defect chemistry.…”

“…40,41,51,52,55−58,61 However, it is as yet unclear whether the wide tunability of the optical properties of CIS NCs (and other ternary copper chalcogenides) can be attributed solely to quantum confinement effects, in striking contrast with the II–VI, IV–VI, and III–V binary analogues. 40 The foremost difference between ternary copper (and silver) chalcogenides (I–III–VI 2 ) and binary II–VI and IV–VI compounds is that the former can easily tolerate large deviations from stoichiometry, both in the I/III ratio and VI/(I + III) ratio, resulting in a rich defect chemistry. 62−65 This has a dramatic impact on the optical properties of ternary copper chalcogenides, because the upper valence band in these materials is composed primarily of Cu 3d orbitals hybridized with the p orbitals of the chalcogen, while the conduction band consists of Cu 4s orbitals with some mixing of p character from the group VI element, which makes their band gap composition-dependent.…”

“…The nearly stoichiometric character of the wz CIS QDs studied in the present work is also evidenced by the absence of localized surface plasmon resonance bands, which are typically associated with Cu-rich CIS NCs. 40 …”

“…Moreover, the PL quantum yields of bare CIS QDs are generally low (≤10%), 40,41,54,57,58,61 and therefore the observed PL spectra may not necessarily reflect the size, shape, and composition polydispersity of the ensemble. The inadequacy of the PL peak positions for quantitative studies of the size-dependence of the band gap is further aggravated by the fact that the PL in CIS NCs (and other ternary Cu chalcogenides) is characterized by broad bandwidths [full width at half-maximum (fwhm), ∼200–400 meV] and large global Stokes shifts (Δ ST ≈ 300–500 meV).…”

Functionalisation of Colloidal Transition Metal Sulphides Nanocrystals: A Fascinating and Challenging Playground for the Chemist

“…Copper (II) sulphide (CuS) [10,12,32,404] has attracted considerable interest for its potential use as a material for transparent conductive films [12] able to transmit only the visible part of the solar spectrum, as a catalyst [405] and as a filler to enhance conductivity or wear resistance of polymeric materials. Doping with CuS was also reported to raise the critical temperature of some superconductors.…”