Christopher McCleese

Femtosecond Time-Resolved Transient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2

CH3NH3PbI3 perovskite layered films deposited on substrates with and without a titania support structure have been prepared and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. In addition, for the first time we could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red-shifted at ∼780 nm. We show that an additional bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to estimate relative amounts of PbI2 in the samples. Kinetic analysis reveals that perovskite films with less PbI2 show faster relaxation rates than those containing more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are due to a passivation effect, in line with other recently reported work.

Identification and characterization of the intermediate phase in hybrid organic–inorganic MAPbI3 perovskite

Perovskite films were prepared using single step solution deposition at different annealing temperatures and annealing times. The crystal structure, phases and grain size were investigated with XRD, XPS and SEM/EDX. The prepared films show a typical orientation of tetragonal perovskite phase and a gradual transition at room temperature from the yellow intermediate phase to the black perovskite phase. Films with high purity were obtained by sintering at 100 °C. In addition, the chemical composition and crystal structure of intermediate phase were investigated in detail. FTIR, UV-vis and NMR spectra revealed the occurance of DMF complexes. Interestingly, the intermediate phase could be transformed to the black perovskite phase upon X-ray irradiation. In addition, the recovery of the aged perovskite films from a yellow intermediate phase back to the black perovskite was shown to be viable via heating and X-ray irradiation.

Fluorescent carbon dots from milk by microwave cooking

Milk, containing carbohydrates and proteins, is one of the most popular sources of nutrition for human beings. Along with others, we have recently found that milk can also be used as a carbon source for the synthesis of carbon quantum dots. Here, we report the formation of nanometre-sized, highly-fluorescent, nitrogen-doped carbon dots from milk by microwave cooking. Carbon dots thus produced are well dispersed in aqueous solution, and could be easily taken up by HeLa cells without additional surface functionalization. The distribution of carbon dots in cells is confirmed by two-photon excited fluorescence intensity imaging and fluorescence lifetime imaging. Fortunately, no significant cytotoxicity of the milk-derived carbon dots was observed while the strong fluorescence makes them potentially useful for biomedical imaging and many other applications.

Reverse saturable absorbing cationic iridium(III) complexes bearing the 2-(2-quinolinyl)quinoxaline ligand: effects of different cyclometalating ligands on linear and nonlinear absorption

To develop broadband reverse saturable absorbing materials, four Ir(III) complexes containing the 2-(2-quinolinyl)quinoxaline (quqo) ligand and different cyclometalating ligands ((quqo)Ir(C^N)2PF6, where C^N = 2-[4-(7-benzothiazolylfluoren-2-yl)phenyl]pyridine (1), 2-(7-benzothiazolylfluoren-2-yl)pyridine (2), 2-[3-(7-benzothiazolylfluoren-2-yl)phenyl]pyridine (3), and 2-[3-(7-naphthalimidylfluoren-2-yl)phenyl]pyridine (4)), were synthesized. Their linear and nonlinear absorption and emission characteristics were systematically investigated via spectroscopic techniques (i.e. UV-vis absorption, emission, and fs and ns transient absorption), nonlinear optical characterization techniques (i.e. Z-scan and the time-resolved pump–probe technique with a phase object (PO pump–probe)), and theoretical calculations. The effect of quqo as a strong electron-withdrawing and extensively π-conjugated ligand, and the effect of the substitution position (meta versus para) at the 2-phenylpyridine ligand on the lowest-energy charge transfer absorption band and the triplet excited-state lifetimes of these complexes were explored. It was found that all these complexes possess a very weak but broad charge-transfer ground-state absorption band from 450 to 750 nm, where their excited states absorb strongly. Therefore, they exhibit the broadest optical window in the visible to the near-IR region among reverse saturable absorbers reported to date. The strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses was demonstrated for these complexes. In order to maintain the long-lived triplet excited state while red-shifting the charge-transfer ground-state absorption band, attaching the 7-R-fluoren-2-yl substituent at the 4-position of the cyclometalating phenyl ring instead of the 3-position of the cyclometalating phenyl ring is the solution.

Curing of degraded MAPbI3 perovskite films

Perovskite films were prepared using one-step solution deposition. The prepared films were degraded for 14 days under ambient atmosphere with controlled humidity of 60 ± 5%. The degraded films showed separated regions of PbI2-rich and methylammonium iodide (MAI)-rich phases. Through X-ray, UV and electron beam irradiation, a fast recovery of perovskite was initiated in the MAI regions and the photoproduct further spread into the PbI2 regions over a time frame of minutes. Ultraviolet and electron beam were also explored as irradiation sources and the same photoinduced perovskite formation was observed in each case. XRD results proved that the photoproduct is indeed tetragonal perovskite MAPbI3. A slower conversion process can be achieved by storing the degraded samples at 25% ± 5% humidity. It is also observed that samples degraded at high humidity can be recovered by thermal annealing at a temperature as low as 35 °C. In this work, recovery of the aged perovskite films from decomposed phases (intermediate phase and PbI2) is shown to be feasible, which paves a new way for sustainable perovskite-based solar cells.

Complete Conversion of PbI2 to Methyl Ammonium PbI3 Improves Perovskite Solar Cell Efficiency

One major disadvantage to the two-step deposition method of perovskite films is the incomplete conversion from PbI2 to perovskite resulting in the presence of a thick PbI2 layer, which hinders charge carrier transportation. In this study, a quaternary ammonium salt has been used to manipulate the crystallization of PbI2 in the first deposition step, which leads to facile incorporation of the methylammonium iodide into the Pb-I lattice and promotes the conversion of PbI2 to perovskite leading to improved device performance.

Prostate-Specific Membrane Antigen Targeted Gold Nanoparticles for Theranostics of Prostate Cancer

Prostate cancer is one of the most common cancers and among the leading causes of cancer deaths in the United States. Men diagnosed with the disease typically undergo radical prostatectomy, which often results in incontinence and impotence. Recurrence of the disease is often experienced by most patients with incomplete prostatectomy during surgery. Hence, the development of a technique that will enable surgeons to achieve a more precise prostatectomy remains an open challenge. In this contribution, we report a theranostic agent (AuNP-5kPEG-PSMA-1-Pc4) based on prostate-specific membrane antigen (PSMA-1)-targeted gold nanoparticles (AuNPs) loaded with a fluorescent photodynamic therapy (PDT) drug, Pc4. The fabricated nanoparticles are well-characterized by spectroscopic and imaging techniques and are found to be stable over a wide range of solvents, buffers, and media. In vitro cellular uptake experiments demonstrated significantly higher nanoparticle uptake in PSMA-positive PC3pip cells than in PSMA-negative PC3flu cells. Further, more complete cell killing was observed in Pc3pip than in PC3flu cells upon exposure to light at different doses, demonstrating active targeting followed by Pc4 delivery. Likewise, in vivo studies showed remission on PSMA-expressing tumors 14 days post-PDT. Atomic absorption spectroscopy revealed that targeted AuNPs accumulate 4-fold higher in PC3pip than in PC3flu tumors. The nanoparticle system described herein is envisioned to provide surgical guidance for prostate tumor resection and therapeutic intervention when surgery is insufficient.

Optoelectronic Dichotomy of Mixed Halide CH3NH3Pb(Br1–xClx)3 Single Crystals: Surface versus Bulk Photoluminescence

In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CH3NH3Pb(Br1- xCl x)3 (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite ( x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at lower percentages of halide substitution ( x ≤ 11%). The observed opposing trends, based on 1PE versus 2PE, highlight a dichotomy between extrinsic (surface) and intrinsic (bulk) effects of chloride substitution on the carrier dynamics in lead bromide perovskites. We discuss the physical relation between halide exchange and bulk carrier lifetimes in CH3NH3PbBr3 in terms of the Rashba effect. We propose that the latter is suppressed at the surface due to disorder in the alignment of the MA and that it increases in the bulk with Cl concentration because of the reduction in lattice parameters, which compresses the space available for the MA orientational degrees of freedom.

Excitonic Interactions in Bacteriochlorin Homo-Dyads Enable Charge Transfer: A New Approach to the Artificial Photosynthetic Special Pair

Excitonically coupled bacteriochlorin (BC) dimers constitute a primary electron donor (special pair) in bacterial photosynthesis and absorbing units in light-harvesting antenna. However, the exact nature of the excited state of these dyads is still not fully understood. Here, we report a detailed spectroscopic and computational investigation of a series of symmetrical bacteriochlorin dimers, where the bacteriochlorins are connected either directly or by a phenylene bridge of variable length. The excited state of these dyads is quenched in high-dielectric solvents, which we attribute to photoinduced charge transfer. The mixing of charge transfer with the excitonic state causes accelerated (within 41 ps) decay of the excited state for the directly linked dyad, which is reduced by orders of magnitude with each additional phenyl ring separating the bacteriochlorins. These results highlight the origins of the excited-state dynamics in symmetric BC dyads and provide a new model for studying the primary processes in photosynthesis and for the development of artificial, biomimetic systems for solar energy conversion.

Stable 2D Bisthienoacenes: Synthesis, Crystal Packing, and Photophysical Properties

Two novel 2D bisthienoacenes with annulated thiophene units at different positions were developed. Both 1,2- and 1,4-addition of the α,β-unsaturated ketone moieties lead to the major formation of four-fold alkylsilylethynyl substituted 2D heteroacenes (namely BTT-4TIPS and BTP-4TIPS). The photophysical, electrochemical properties, crystal packing structures, and charge carrier transport performances were investigated in detail.