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Abstract:

Second-harmonic generation (SHG) is a powerful surface-specific probe for centrosymmetric materials, with broad relevance to energy and biological interfaces. Plasmonic nanomaterials have been extensively utilized to amplify this nonlinear response. Yet, material instability has constrained most studies to gold, despite the significance of plasmonic metals such as copper for catalysis. Here, we demonstrate stable and anisotropic SHG from monocrystalline copper, overcoming long-standing challenges associated with surface degradation. By leveraging an on-substrate synthesis approach that yields atomically flat and oxidation-resistant Cu microflakes, we enable reliable SHG measurements and reveal a strong cross-polarized response with C 3v surface symmetry. The SHG signal remains stable over 3 h of continuous femtosecond excitation, highlighting the remarkable optical robustness of the Cu microflakes. These results reinforce the viability of monocrystalline Cu as a robust platform for nonlinear nanophotonics and surface-sensitive spectroscopy, expanding the range of copper-based optical applications.

Abstract:

Lead halide perovskite nanocrystals have emerged as promising candidates for classical light-emitting devices and single-photon sources, owing to their high photoluminescence quantum yield, narrow emission line width and tunable emission. Judicious choice of ligands to passivate nanocrystal surfaces has proven to be critical to the structural stability and optoelectronic performance of such nanocrystals. While many ligands have been deployed, the resulting quality of the nanocrystal surface can be difficult to assess directly. Here, we demonstrate ultralow frequency Raman spectroscopy as a powerful tool to resolve surface-sensitive changes in size and ligand choice in perovskite nanocrystals. By investigating a size series of CsPbBr3 nanocrystals from the strong (5 nm) to the weak (28 nm) confinement range, we show that the line width of Raman-active modes provides a highly selective metric for surface disorder and quality. We further examine a series of 28 nm diameter nanocrystals with four different zwitterionic ligands, unravelling clear links between varying steric effects and surface quality evident from Raman analysis. Photoluminescence and THz photoconductivity probes reveal an evident correlation of charge-carrier dynamics and radiative emission yields with ligand chemistry and surface quality inferred from phonon broadening. We further show that surface defects preferentially trap hot charge carriers, which affects exciton stability and radiative emission yields. Overall, our approach offers powerful insights into optimizing nanocrystal-ligand boundaries to enhance the performance of nanoscale quantum light sources and optoelectronic devices.

Abstract:

The optoelectronic properties of covalent organic frameworks (COFs) can be controlled by the design of their molecular building blocks and assembly. Here, a facile and efficient synthetic route is reported for the novel thiophene‐enriched tetrathienonaphthalene (TTN)‐based node 4,4′,4″,4′″‐(naphtho[1,2‐b:4,3‐b′:5,6‐b″:8,7‐b″′]tetrathiophene‐2,5,8,11‐tetrayl)tetraaniline (TTNTA) for constructing imine‐linked COFs. Utilizing TTNTA, highly crystalline, thiophene‐enriched donor–donor (D–D) and donor–acceptor (D–A) COFs, denoted as TT COF and BDT(BT)2 COF, are synthesized using two distinct aldehyde‐functionalized linear linkers: [2,2′‐bithiophene]‐5,5′‐dicarbaldehyde (TT) and 7,7′‐(4,8‐diethoxybenzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(benzo[c][1,2,5]thiadiazole‐4‐carbaldehyde) (BDT(BT)2), respectively. Highly crystalline and oriented TTNTA COF films on various substrates via a solvothermal method enabled further comprehensive optical and electronic characterizations. Optical‐pump terahertz‐probe spectroscopy revealed effective charge‐carrier mobility values φμ = 0.34 ± 0.04 and 0.18 ± 0.02 cm2V−1s−1 for TT and BDT(BT)2 COF films, respectively. These results reveal distinct charge‐transport characteristics and provide mechanistic insights into their ultrafast charge‐carrier dynamics. The COFs are demonstrated to be photoactive, showing promising potential as photocathodes without co‐catalysts in photoelectrochemical water splitting, with notable photocurrent densities of 10 and 15.3 µA cm−2 after 1 h illumination, respectively. This work highlights the potential of TTNTA‐based COFs in optoelectronic applications and provides insights into the design of thiophene‐enriched COFs with high crystallinity and photoactive behavior.

Abstract:

Mixed‐halide perovskites are ideal mid‐ and wide‐gap absorbers for multijunction solar cells, but stable photovoltaic performance is severely hampered by halide segregation. This study reveals that crystalline film quality and halide segregation are critically affected by bromide fraction x in CH3NH3Pb(I1−xBr x )3 because of macrostrain and ordered‐phase formation. X‐ray diffractometry across stoichiometries spanning 22 bromide fractions demonstrates that central compositions near x = 0.5 form two macrostrained phases, which exhibit halide segregation under light at different rates. While the overall amplitude of phase segregation follows a broadly symmetric distribution in compositional space, maximized near x = 0.5, the potentially ordered compositions of CH3NH3PbIBr2 and CH3NH3PbI2Br diverge sharply, presenting particularly stable and unstable scenarios, respectively. Notably, halide segregation is shown to occur even below the widely quoted perceived threshold of x = 0.2. Such analysis highlights promising approaches to mitigate halide segregation, through engineering of macrostrained phases and local atomistic ordering. Together, these observations provide crucial benchmarks for proposed models of halide segregation and establish new routes toward segregation‐resistant materials for multijunction perovskite‐based photovoltaics.

Abstract:

Colloidal lead halide perovskite nanocrystals (LHP NCs) are promising semiconductor materials for optoelectronic applications due to their strong quantum confinement, near-unity photoluminescence quantum yields, and tunable emission characteristics. However, their modest thermal stability remains a challenge, particularly at smaller core diameters due to enhanced phonon scattering at inorganic core-organic ligand interfaces. In this study, we directly quantify size-dependent thermal conductivity (κ) in lecithin-capped CsPbBr₃ NC thin films using a transducer-free, vibrational pump-visible probe (VPVP) spectroscopy technique. A mid-infrared pump thermally excites the ligand shell, while a broadband probe tracks transient reflectance change correlated to lattice temperature decay. Finite-element modeling of the decay dynamics yields κ values from 0.13 to 0.16 W·m^-1·��^-1 for NC films with sub-10 nm core diameter, significantly lower than those of its bulk counterpart. A steep κ suppression with decreasing NC size emphasizes the dominant role of ligand shells and boundary effects in thermal transport.