Ultrafast optical properties of stoichiometric and non-stoichiometric refractory metal nitrides TiNx, ZrNx, and HfNx.

Refractory metal nitrides have recently gained attention in various fields of modern photonics due to their cheap and robust production technology, silicon-technology compatibility, high thermal and mechanical resistance, and competitive optical characteristics in comparison to typical plasmonic materials like gold and silver. In this work, we demonstrate that by varying the stoichiometry of sputtered nitride films, both static and ultrafast optical responses of refractory metal nitrides can efficiently be controlled. We further prove that the spectral changes in ultrafast transient response are directly related to the position of the epsilon-near-zero region. At the same time, the analysis of the temporal dynamics allows us to identify three time components: the "fast" femtosecond one, the "moderate" picosecond one, and the "slow" at the nanosecond time scale. We also find out that the non-stoichiometry does not significantly decrease the recovery time of the reflectance value. Our results show the strong electron-phonon coupling and reveal the importance of both the electron and lattice temperature-induced changes in the permittivity near the ENZ region and the thermal origin of the long tail in the transient optical response of refractory nitrides.

Role of Self-Trapped Excitons in the Broadband Emission of Lead-Free Perovskite-Inspired Cu2AgBiI6.

The perovskite-inspired Cu2AgBiI6 (CABI) absorber shows promise for low-toxicity indoor photovoltaics. However, the carrier self-trapping in this material limits its photovoltaic performance. Herein, we examine the self-trapping mechanism in CABI by analyzing the excited-state dynamics of its absorption band at 425 nm, which is responsible for the self-trapped exciton emission, using a combination of photoluminescence and ultrafast transient absorption spectroscopies. Photoexcitation in CABI rapidly generates charge carriers in the silver iodide lattice sites, which localize into the self-trapped states and luminesce. Furthermore, a Cu-Ag-I-rich phase that exhibits similar spectral responses as CABI is synthesized, and a comprehensive structural and photophysical study of this phase provides insights into the nature of the excited states of CABI. Overall, this work explains the origin of self-trapping in CABI. This understanding will play a crucial role in optimizing its optoelectronic properties. It also encourages compositional engineering as the key to suppressing self-trapping in CABI.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures.

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.

Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy.

Transient absorption measurements were conducted on pristine and monoexciton saturated CsPbBr3 nanocrystals varying in size within the regime of a strong quantum confinement. Once the difference spectra were translated to absolute transient changes in absorption cross section, a single exciton is shown to completely bleach the band edge absorption peak and induce a new absorption roughly two times weaker ∼100 meV to the blue. Difference spectra obtained during Auger recombination of biexciton demonstrate that the addition of a second exciton, rather than double the effect of a first, bleaches the blue-induced absorption band without producing a net stimulated emission at the band edge. Accompanied by high time resolution transient absorption spectra pumping at the lowest exciton band, these results identify the blue-induced absorption as the second transition to 1Se1Sh which is shifted in energy due to unusually strong and promptly rising biexciton repulsion. Possible mechanisms giving rise to this repulsion and prospects for applying it to enhance optical gain applications of these particles are discussed.

Testing the fate of nascent holes in CdSe nanocrystals with sub-10 fs pump-probe spectroscopy.

Numerous studies have reported that transient absorption spectra in core CdSe nanocrystals do not register state filling in 1Sh, an absence which has profound consequences in light-emitting applications. It has been assigned alternatively to rapid hole trapping, or to distribution over a dense degenerate valence band manifold which includes dark states. Here we attempt to observe early contributions of nascent holes to the bleaching of the band edge exciton transition by conducting 1Se1Sh pump-1Se1Sh probe spectroscopy with <10 fs laser pulses on organic ligand passivated CdSe crystals. The results show no rapidly hole-state filling effects in transient absorption measurements even at the earliest delay, despite the use of pulses which are capable of resolving all dissipation mechanisms reflected in the homogeneous 1Se1Sh bandwidth. This proves that neither hole trapping nor rapid redistribution of the nascent hole over energetically available valence band states can explain the absence of hole contributions to band edge bleaching, calling for a mechanistic review of this phenomenon.

Hydrogen bond assisted photoinduced intramolecular electron transfer and proton coupled electron transfer in an ultrafast time domain using a ruthenium-anthraquinone dyad.

Quinones play a significant role as primary electron acceptors in the natural photosynthetic system of photosystem II, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, a ruthenium(ii) polypyridyl complex 1 coupled to an anthraquinone (AQ) functionality through a rigid imidazole (Im) spacer has been synthesized to examine the effect of H-bonding on both the thermal and photoinduced electron transfer reactions. The anthraquinone moiety of complex 1 is fused to a benzi-imidazole system bearing C[double bond, length as m-dash]OAQHNIm based H-bonding at one side of the anthraquinone moiety so that intramolecular hydrogen bonding from the imidazole group to the nearby quinone carbonyl can occur. The hydrogen bond formation involving the semiquinone radical anion produced through the photoinduced reduction process in Ru-im-AQ and the imidazole proton in complex 1 resulted in a significant positive shift of one electron reduction potential of complex 1. The kinetics for the formation of the charge-separated states was explored by using femtosecond transient absorption spectroscopy. Hydrogen bonding between water and the reduced anthraquinone accounted for thermodynamic and kinetic stabilization of these charge-separated states. An attempt has been made to assess the relative importance of the driving force and solvent polarity, in the rates of photoinduced electron transfer in complex 1. The 490 nm transient absorption band of anthraquinone radical anions (AQ˙-) and a broad absorption in the 580-750 nm region having maxima at ∼690 nm have been observed and this is attributed to the generation of a transient Ru3+-species of the corresponding complex 1. Addition of water entails an acceleration of electron transfer rates by a factor of 3.33. The system investigated may serve as a model for the mechanistic diversity of PCET reactions in general with water as a primary proton donor. Furthermore, our studies are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as a variant form of PCET.

Improving the Power-Conversion Efficiency through Alloying in Common Anion CdZnX (X=S, Se) Nanocrystal Sensitized Solar Cells.

In this paper, we have investigated the possibility of utilizing CdZnS and CdZnSe alloy nanocrystals (NCs) as sensitizers in quantum-dot solar cells (QDSCs). The alloy NCs were synthesized by a high-temperature hot injection method and subsequently characterized through high photoluminescence quantum yield, along with larger size compared to binary NCs. Femtosecond transient absorption measurements revealed long-lived charge carriers in the alloy structure due to more structural rigidity and less defect states. Finally, the solar-cell efficiencies of the CdZnS (CdZnSe) NCs were found to be 3.05 % (3.69 %) as compared to 1.23 % (3.12 %) efficiencies for CdS (CdSe) NCs. Thus, common anion ternary NCs have been successfully utilized for solar-cell assembly and can be helpful for constructing tandem solar cells to harvest the high-energy portion of solar radiation.

Correlating Charge-Carrier Dynamics with Efficiency in Quantum-Dot Solar Cells: Can Excitonics Lead to Highly Efficient Devices?

The photovoltaic performance of quantum-dot solar cells strongly depends on the charge-carrier relaxation and recombination processes, which need to be modulated in a favorable way to obtain maximum efficiency. Recently, significant efforts have been devoted to investigate the carrier dynamics of nanocrystal sensitizers, both in solution and deposited on TiO2 photoanodes, with the aim to correlate the excitonics with solar-energy conversion efficiency. This Minireview summarizes some proof of the concepts that efficiency can be directly correlated to the exciton dynamics of quantum-dot solar cells. The presented findings are based on CdSeS alloy, CdSe/CdS core/shell, Au/CdSe nanohybrids, and Mn-doped CdZnSSe nanocrystals, where the favourable excitonic processes are optimized to enhance the efficiency. Future prospects and limitations are addressed as well.

Concurrent Ultrafast Electron- and Hole-Transfer Dynamics in CsPbBr3 Perovskite and Quantum Dots.

Ultrafast charge-transfer (i.e., electron and hole) dynamics has been investigated between the cesium lead bromide (CsPbBr3, CPB) perovskite nanocrystals (NCs) and cadmium selenide (CdSe) quantum dots (QDs) as a new composite material for photocatalytic and photovoltaic applications. The CPB NCs have been synthesized and characterized by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) pattern. The redox levels (i.e., conduction band (CB) and valence band (VB)) of the CPB NCs and CdSe QDs suggest the feasibility of photoexcited electron transfer from CPB NCs to CdSe QDs and photoexcited hole transfer from CdSe QDs to CPB NCs, and it has been confirmed by both steady-state and time-resolved spectroscopy. To investigate the electron- and hole-transfer dynamics in ultrafast time scale, we have performed femtosecond up-conversion and femtosecond transient absorption studies. The measured electron-transfer time from CPB NCs to CdSe QDs and hole-transfer time from CdSe QDs to CPB NCs were found to be 550 and 750 fs, respectively. Interestingly, the charge-transfer process found to be restricted in CPB/CdSe@CdS core-shell system where electron transfer from CPB NCs to core shell takes place, but the hole transfer from core shell to CPB NCs found to be restricted due to CdS shell making the process thermodynamically nonviable. Our observation has suggested that after the photoexcitation of CPB NCs/CdSe QDs composite system, a charge-separated state is formed where the electrons are localized in CB of CdSe QDs and holes are localized in VB of CPB NCs. This makes the composite system a better material for efficient light harvesting and photocatalytic material as compared to the individual ones.

Direct Correlation of Excitonics with Efficiency in a Core-Shell Quantum Dot Solar Cell.

Shell thickness dependent band-gap engineering of quasi type II core-shell material with higher carrier cooling time, lower interfacial defect states, and longer charge carrier recombination time can be a promising candidate for both photocatalysis and solar cell. In the present investigation, colloidal CdSe@CdS core-shells with different shell thickness (2, 4 and 6 monolayer CdS) were synthesized through hot injection method and have been characterized by high resolution transmission electron microscope (HRTEM) followed by steady state absorption and luminescence techniques. Ultrafast transient absorption (TA) studies suggest longer carrier cooling, lower interfacial surface states, and slower carrier recombination time in CdSe@CdS core-shell with increasing shell thickness. By TA spectroscopy, the role of CdS shell in power conversion efficiency (PCE) has been explained in detail. The measured PCE was found to initially increase and then decrease with increasing shell thickness. Shell thickness has been optimized to maximize the efficiency after correlating the shell controlled carrier cooling and recombination with PCE values and a maximum PCE of 3.88 % was obtained with 4 monolayers of CdS shell, which is found to be 57 % higher than compared to bare CdSe QDs.

Boosting the Efficiency of Quantum Dot-Sensitized Solar Cells through Formation of the Cation-Exchanged Hole Transporting Layer.

In search of a viable way to enhance the power conversion efficiency (PCE) of quantum dot-sensitized solar cells, we have designed a method by introducing a hole transporting layer (HTL) of p-type CuS through partial cation exchange process in a postsynthetic ligand-assisted assembly of nanocrystals (NCs). High-quality CdSe and CdSSe gradient alloy NCs were synthesized through colloidal method, and the charge carrier dynamics was monitored through ultrafast transient absorption measurements. A notable increase in the short-circuit current concomitant with the increase in open-circuit voltage and the fill factor led to 45% increment in PCE for CdSe-based solar cells upon formation of the CuS HTL. Electrochemical impedance spectroscopy further revealed that the CuS layer formation increases recombination resistance at the TiO2/NC/electrolyte interface, implying that interfacial recombination gets drastically reduced because of smooth hole transfer to the redox electrolyte. Utilizing the same approach for CdSSe alloy NCs, the highest PCE (4.03%) was obtained upon CuS layer formation compared to 3.26% PCE for the untreated one and 3.61% PCE with the conventional ZnS coating. Therefore, such strategies will help to overcome the kinetic barriers of hole transfer to electrolytes, which is one of the major obstacles of high-performance devices.

Hot-electron transfer from the semiconductor domain to the metal domain in CdSe@CdS{Au} nano-heterostructures.

Semiconductor-metal hybrid nanostructures are recognized as great materials due to their high level of light-induced charge separation, which has direct relevance in photocatalysis and solar energy conversion. To understand the mechanism of charge separation processes, hybrid CdSe@CdS{Au} nano-heterostructures containing Au nanoparticles (NPs) with different sizes were synthesized, and the ultrafast charge-transfer dynamics were monitored using femtosecond transient absorption spectroscopy. Steady-state optical absorption studies suggest the formation of charge-transfer complexes between core shell nanocrystals (NCs) and Au NPs. Steady-state and time-resolved luminescence spectroscopy suggest electron transfer from the photo-excited CdSe@CdS core shell QDs NCs to the Au NPs within the heterostructure. The ultrafast interfacial electron-transfer dynamics in the heterostructures were monitored by femtosecond transient absorption spectroscopy. The results revealed that both hot and thermalized electrons are transferred from the core shell QDs to the metal NPs with time constants of 150 and 300 fs, respectively. Hot-electron transfer from QDs to Au NPs was found to take place predominantly in the heterostructures depending on the sizes of the metal NPs. The photo-degradation of rhodamin B in the presence of the CdSe@CdS{Au} heterostructures under visible-light radiation suggests that the hot electrons in the heterostructures play a major role in photocatalytic degradation.

Proton-Coupled Electron-Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen-Bond Stabilization on Photoinduced Electron Transfer.

The proton-coupled electron-transfer (PCET) reaction is investigated for a newly synthesized imidazole-anthraquinone biomimetic model with a photoactive RuII -polypyridyl moiety that is covalently coupled to the imidazole fragment. Intramolecular H-bonding interactions between imidazole and anthraquinone moieties favor the PCET process; this can be correlated to an appreciable positive shift in the one-electron reduction potential of the coordinated anthraquinone moiety functionalized with the imidazole fragment. This can also be attributed to the low luminescence quantum yield of the RuII -polypyridyl complex used. The dynamics of the intramolecular electron-transfer (ET) and PCET processes are studied by using femtosecond transient absorption spectroscopy. The steady-state spectroscopic studies and the results of the time-resolved absorption studies confirm that H-bonded water molecules play a major role in both ET and PCET dynamics as a proton relay in the excited state. The electron-transfer process is followed by a change in the H-bonding equilibrium between AQ and imidazole in acetonitrile solvent, and protonation of AQ.- by water leads to PCET in the presence of water. A slower forward and backward electron-transfer rate is observed in the presence of D2 O compared with that in H2 O. These results provide further experimental support for a detailed understanding of the PCET process.

Photoinduced ultrafast charge separation in colloidal 2-dimensional CdSe/CdS-Au hybrid nanoplatelets and corresponding application in photocatalysis.

Multicomponent hybrid nanocrystals (HNC) consisting of a semiconductor and metallic domains are an important class of nanostructured materials demonstrating useful applications and interesting basic knowledge. In this scenario, Au nanoparticle (NP) islands of ∼2 nm have been grown on unique two dimensional (2D) CdSe/CdS core@shell hexagonal nanoheteroplatelets of 20 nm diameter to form unprecedented 2D CdSe/CdS-Au HNCs and detailed optical characterization has been carried out to determine the dimensionality based electron transfer dynamics on the ultrafast scale. Steady state optical absorption studies show that upon growing Au NPs onto the 2D nanoplates, a new band appears in the red region of the spectra (500-800 nm), which suggests a strong interaction between the exciton of the core-shell and the plasmon of the metal NPs. Fluorescence studies showed the quenching of emission of the semiconductor domains upon the growth of the metallic domains. Detailed optical and TRPL studies suggested efficient charge transfer from the 2D CdSe/CdS to the Au domains, irrespective of excitation wavelength. Femtosecond transient absorption studies suggest that the electron transfer from the 2D hybrid nanocrystals to the metal domain is on an ultrafast time scale (∼800 fs). No evidence is observed for charge transfer from the 2 nm Au domains to the semiconductor seeds. The broad absorption in the visible region of the hybrid nanocrystals and the ultrafast charge transfer facilitates very efficient photo-catalytic reactions under direct sun light, as a case study.

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

We have synthesized three AgxInS2 (AIS) ternary nanocrystals (NCs), where x 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 ( = ∼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.

Effect of Molecular Coupling on Ultrafast Electron-Transfer and Charge-Recombination Dynamics in a Wide-Gap ZnS Nanoaggregate Sensitized by Triphenyl Methane Dyes.

Wide-band-gap ZnS nanocrystals (NCs) were synthesized, and after sensitizing the NCs with series of triphenyl methane (TPM) dyes, ultrafast charge-transfer dynamics was demonstrated. HRTEM images of ZnS NCs show the formation of aggregate crystals with a flower-like structure. Exciton absorption and lumimescence, due to quantum confinement of the ZnS NCs, appear at approximately 310 and 340 nm, respectively. Interestingly, all the TPM dyes (pyrogallol red, bromopyrogallol red, and aurin tricarboxylic acid) form charge-transfer complexes with the ZnS NCs, with the appearance of a red-shifted band. Electron injection from the photoexcited TPM dyes into the conduction band of the ZnS NCs is shown to be a thermodynamically viable process, as confirmed by steady-state and time-resolved emission studies. To unravel charge-transfer (both electron injection and charge recombination) dynamics and the effect of molecular coupling, femtosecond transient absorption studies were carried out in TPM-sensitized ZnS NCs. The electron-injection dynamics is pulse-width-limited in all the ZnS/TPM dye systems, however, the back electron transfer differs, depending on the molecular coupling of the sensitizers (TPM dyes). The detailed mechanisms for the above-mentioned processes are discussed.

Restriction of molecular twisting on a gold nanoparticle surface.

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications.