CuInS₂‑Decorated Perovskite Nanoarchitecture: Halide-Driven Energy and Electron Transfer

Perovskite nanocrystals (NCs) are an emergent and game-changing entrant in semiconductor research, yet the research on the corresponding nanoheterostructures remains in its infancy. In this work, we fabricate a type II nanoarchitecture of CsPbX3 NCs (where X = Cl, Br, or I) and CuInS2 quantum dots to investigate the energy and charge transfer (ET and CT, respectively) processes. Optical measurements of CsPbX3/CuInS2 show efficient photoluminescence (PL) quenching when X = Br or I, while the PL quenching efficiency of the X = Cl compound is 2 orders of magnitude lower. We argue the drastic PL quenching in the X = I compound is solely due to the CT process, while for the X = Br compound, a predominantly ET process is active. In contrast to the driving force (−ΔG) for the CT process, we observe the reverse order of the electron transfer process, for which we propose the electron transfer occurs in the Marcus inverted region. Our halide-dependent controlled regulation of CT and ET processes in these nanoarchitectures may find promising optoelectronic applications

Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe Inverted Type I Core–Shell: A Structural Analysis through Carrier-Quenching Study

We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core–shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core–shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core–shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core–shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core–shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in both the core–shells have been demonstrated using electron (benzoquinone, BQ) and hole (pyridine, Py) quencher. The bleach recovery kinetics of CdS/CdSe core–shell recovers faster in the presence of both BQ and Py. However, for CdSe/CdS core–shell, the bleach recovers faster only in the presence of BQ while the bleach dynamics remain unaffected in the presence of Py. Our experimental observations suggest that in CdSe/CdS quasi-type II core–shell, photoexcited electrons are localized in CdS shell and holes are localized in CdSe core; however, in CdS/CdSe inverted core–shell both electrons and holes are localized in the CdSe shell

Slow Electron Cooling Dynamics Mediated by Electron–Hole Decoupling in Highly Luminescent CdS_<i>x</i>Se_1–<i>x</i> Alloy Quantum Dots

Spherical CdS_<i>x</i>Se_1–<i>x</i> alloy semiconductor QDs are receiving incredible research interest due to their composition-dependent optical tunability and charge carrier behavior. These highly luminescent alloy QDs can be used in several applications due to their very long excited state lifetime. Herein, we describe synthesis and characterization of highly luminescent CdSSe alloy QD using XRD, EDX, and HRTEM techniques. Steady state optical absorption and photoluminescence (PL) measurements show the nonlinear behavior of the alloy QDs with changing chalcogenide composition. Time-resolved photoluminescence (PL) measurements suggest CdS_0.7Se_0.3 alloy QD has much higher emission quantum yield (∼70%) and long radiative lifetime (24 ns) as compared to both CdS (ϕ_CdS = 24%, τ_CdS = 9.5 ns) and CdSe (ϕ_CdSe = 34%, τ_CdSe = 13.3 ns) QDs. Femtosecond transient absorption measurement has been carried out to unravel charge carrier dynamics in early and late time scale. Electron cooling time for CdS_0.7Se_0.3 alloy QD found to be extremely slow (τ_cooling = 8 ps) in contrast to both pure CdS (τ_cooling < 100 fs) as well as pure CdSe QD (τ_cooling = 600 fs) due to specially decoupled electron and hole in quasi type II core–shell structure. Charge recombination reaction found to be slowest in alloy QDs as compared to both CdS and CdSe QDs

Elucidating the Electronic Cross-Talk Dynamics across the Heterointerface of Janus CdSe/PbSe Nanocrystals

CdSe/PbSe Janus heteronanocrystals (HNCs) were synthesized in one pot, and the underlying reaction mechanism along with the epitaxy at the hexagonal CdSe–cubic PbSe heterojunction were investigated. During the initial stages of reaction, unusually large CdSe nanocrystals were formed due to rapid growth in the presence of Pb-oleate which cation exchanged asymmetrically to form the Janus structures. Distinct PbSe and CdSe domains were visualized after sufficient growth as seen from the high-resolution transmission electron microscopic images with a unique rock salt PbSe and wurtzite CdSe interface. The core Pb–Se bonds were differentiated from interfacial Pb–Se bonds through the X-ray photoelectron spectroscopy measurements. Transient absorption spectroscopy of the Janus NCs revealed intriguing spectroscopic signatures in both the spectral and time domains as manifested by the early population of higher excitonic states upon pulsed laser excitation along with broad TA spectra rich in higher excitonic states due to the intricate hybridization between the electronic states of two disparate materials. The TA measurements were well correlated with the formation of the Janus structure as new states emerged at the longer wavelength side in the TA spectra due to PbSe accompanied by a slow ∼5 ps additional electron cooling component arising due to hole localization in the PbSe domain

Slow Electron Cooling Dynamics Mediated by Electron–Hole Decoupling in Highly Luminescent CdS_<i>x</i>Se_1–<i>x</i> Alloy Quantum Dots

Spherical CdS_<i>x</i>Se_1–<i>x</i> alloy semiconductor QDs are receiving incredible research interest due to their composition-dependent optical tunability and charge carrier behavior. These highly luminescent alloy QDs can be used in several applications due to their very long excited state lifetime. Herein, we describe synthesis and characterization of highly luminescent CdSSe alloy QD using XRD, EDX, and HRTEM techniques. Steady state optical absorption and photoluminescence (PL) measurements show the nonlinear behavior of the alloy QDs with changing chalcogenide composition. Time-resolved photoluminescence (PL) measurements suggest CdS_0.7Se_0.3 alloy QD has much higher emission quantum yield (∼70%) and long radiative lifetime (24 ns) as compared to both CdS (ϕ_CdS = 24%, τ_CdS = 9.5 ns) and CdSe (ϕ_CdSe = 34%, τ_CdSe = 13.3 ns) QDs. Femtosecond transient absorption measurement has been carried out to unravel charge carrier dynamics in early and late time scale. Electron cooling time for CdS_0.7Se_0.3 alloy QD found to be extremely slow (τ_cooling = 8 ps) in contrast to both pure CdS (τ_cooling < 100 fs) as well as pure CdSe QD (τ_cooling = 600 fs) due to specially decoupled electron and hole in quasi type II core–shell structure. Charge recombination reaction found to be slowest in alloy QDs as compared to both CdS and CdSe QDs

Involvement of Sub-Bandgap States in Subpicosecond Exciton and Biexciton Dynamics of Ternary AgInS₂ Nanocrystals

We have synthesized three Ag_<i>x</i>InS₂ (AIS) ternary nanocrystals (NCs), where <i>x</i> varies from 0.25 to 1, and reported their biexcitonic feature which depends on the stoichiometry ratio of Ag/In. The broadening of absorption band and dual photoluminescence in different AIS NCs indicates the existence of Ag-related sub-bandgap (S-states) and antisite states. Ultrafast charge carrier dynamics in AIS NCs that involve multiple states like higher excited state, band-edge, Ag-related sub-bandgap, and antisite states have been carried out by employing femtosecond transient absorption spectroscopy, which strongly depends on Ag/In ratio. The probe-induced biexcitonic feature that originated from antisite state has been observed in these AIS NCs even at low pump fluency (<<i>N</i>> = ∼0.2). The enhancement of binding energy of biexciton and slow down of electron cooling dynamics has been demonstrated by gradual increment of pump fluence as well as with different stoichiometry of Ag and In

Restriction of Molecular Rotation and Intramolecular Charge Distribution in the Photoexcited State of Coumarin Dyes on Gold Nanoparticle Surface

Effect of molecular structure on the excited state dynamics, molecular rotation, and intramolecular charge distribution in the excited states of two structurally similar coumarin dyes, namely, coumarin 343 (C-343) and 7-diethyl amino coumarin 3-carboxylic acid (D-1421), on the Au nanoparticle (NP) surface has been investigated using steady state and time-resolved emission spectroscopy. In the first excited state (S₁), both the coumarin dyes exist as a locally excited (LE) state in nonpolar solvent; however, in polar solvent, C-343 exists as intramolecular charge transfer (ICT) state and D-1421 predominantly exists as a twisted intramolecular charge transfer (TICT) state. Photoexcited C-343 molecules transfer energy to Au NP in aqueous solution; however, photoexcited D-1421 molecules do not transfer energy to Au NP due to poor overlap between emission band of D-1421 and plasmon absorption band of Au NP. Interestingly, emission intensity of the S₁ state of D-1421 drastically increases on the Au NP surface due to restriction of amino-group rotation which is responsible for the population of TICT states. Intramolecular charge distribution in the excited states for both C-343 and D-1421 dyes found to be restricted on Au NP surface

Electron Trap to Electron Storage Center in Specially Aligned Mn-Doped CdSe d‑Dot: A Step Forward in the Design of Higher Efficient Quantum-Dot Solar Cell

Specially aligned surface-accumulated Mn-doped CdSe (MnCdSe) quantum dots (QDs) have been synthesized to study the effect of dopant atom on charge-carrier dynamics in QD materials. EPR studies suggest that the ⁴T₁ state of Mn²⁺ lies above the conduction band of CdSe, and as a result no Mn-luminescence was observed from MnCdSe. Femtosecond transient absorption studies suggest that Mn atom introduces structural defects in surface-doped CdSe, which acts as electron trap center in doped QD for the photoexcited electron. Bromo-pyrogallol red (Br-PGR) were found to form strong charge-trasfer complex with both CdSe and MnCdSe QDs. Charge separation in both the CdSe/Br-PGR and MnCdSe/Br-PGR composites was found to take place in three different pathways by transferring the photoexcited hole of CdSe/MnCdSe QDs to Br-PGR, electron injection from photoexcited Br-PGR to the QDs, and direct electron transfer from the HOMO of Br-PGR to the conduction band of both the QDs. Hole-transfer dynamics are found to be quite similar (∼1.1 to 1.3 ps) for both of the systems and found to be independent of Mn doping. However, charge recombination dynamics was found to be much slower in the MnCdSe/Br-PGR system as compared with that in the CdSe/Br-PGR system, which confirms that the Mn dopant act as the electron storage center. As a consequence, the MnCdSe/Br-PGR system can be used as a better super sensitizer in quantum-dot-sensitized solar cell to increase efficiency further

Subpicosecond Exciton Dynamics and Biexcitonic Feature in Colloidal CuInS₂ Nanocrystals: Role of In–Cu Antisite Defects

Charge carrier dynamics of multinary quantum dots like CuInS₂ (CIS) nanocrystals (NCs) is not clearly understood, especially in ultrafast time scales. Herein we have synthesized colloidal CIS NCs that show defect-induced emission between donor (antisite) and acceptor (internal/surface) states as indicated from steady-state and time-resolved photoluminescence (PL) measurements. Subpicosecond transient absorption (TA) spectra of CIS NCs reveal a gradient of electronic states that exists above the conduction band edge. The electron cooling rate has been determined to be ∼0.1–0.15 eV/ps. The cascade of electron cooling dynamics was monitored after following the TA kinetics at different electronic states. Interestingly, the kinetics at the antisite state unveil a biexcitonic feature, which has been enlightened through a probe-induced biexciton mechanism. With progressively higher fluence (⟨<i>N</i>⟩), the biexciton binding energy increases, and the electron cooling to the antisite state considerably slows down. Extra energy released during Auger recombination of bi/multiexcitons are used to re-excite the electron to a further high energy level, resulting in longer electron cooling time to the antisite states

Solar Conversion Efficiency Performance of a High Temperature Alloy over a Low Temperature One: Comprehending Interfaces through <i>Excitonics</i> Study

To take account of the interface in the nanocrystal (NC) materials, we have synthesized high quantum yield gradient CdZnSSe alloy NC having minimal involvement of interface (G-300) through high temperature (300 °C) pyrolysis and investigated the charge carrier dynamics. The performance was unraveled through femtosecond transient absorption studies. A gradient alloy of CdZnSSe (G-250) at low alloying temperature (250 °C) was also synthesized where several interfaces were present in the form CdSe/CdS/ZnSe/ZnS within the alloy material along with other deep traps as well as surface defects. The successful formulation of minimal involvement of interface in G-300 alloy has been envisaged through its blue-shifted optical absorption spectrum as compared to the G-250 alloy due to interionic diffusion of less reactive Zn and S toward the core of the material at elevated reaction temperatures that widen the band gap. Unlike the G-250 analogue, no charge transfer (CT) state was observed in the G-300 alloy, which also suggests the nonexistence of a CdSe/CdS gradient type structure otherwise present for the G-250 analogue. The slow electron cooling time of 4 ps in the G-250 alloy is found to be absent in the G-300 alloy, which can be attributed to minimal involvement of gradient structure otherwise, where electron–hole decoupling leads to slower electron cooling. It has been observed that although the absorption cross-section of G-300 alloy is lower in the solar spectrum as compared to the G-250 analogue, photocurrent conversion efficiency (PCE) measurements of G-300 show promising 4.5% PCE due to smooth electron transfer to TiO₂ through the interface free NCs whereas the G-250 analogue shows only 3.5% PCE. Our investigation suggests that engineering with alloys having less gradient structure and without any boundary restrictions can lead us to new perceptions regarding the design and development of higher efficient quantum dot sensitized solar cell (QDSSC)