Alexander Mikhailovsky

Dirac and Weyl semimetals, such as cadmium arsenide (Cd₃As₂), have recently attracted attention for use in high-speed photodetectors that operate at longer infrared wavelengths, where conventional semiconductor-based photodetectors have a limited performance. In this Letter, we explore near-infrared (960 nm) photodetection in a Cd₃As₂/AlSb heterojunction. We show that Cd₃As₂/AlSb heterojunctions allow for an unbiased operation and demonstrate an enhanced responsivity and quantum efficiency compared to AlSb and Cd₃As₂ reference devices. To characterize their high-speed response, the photocurrent dynamics of the heterojunctions were also explored via time-resolved photocurrent (TRPC) measurements, revealing a photocurrent process occurring over 200–500 ps in the Cd₃As₂/AlSb heterojunctions that is not observed in the AlSb and Cd₃As₂ reference devices. This time is attributed to the recombination rate of carriers in the heterojunction.

As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti‐Stokes shift) while diminishing the cost of production. In this vein, studies employing metal‐free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion is utilized with the TADF sensitizer 2,4,5,6‐tetrakis(carbazol‐9‐yl)isophthalonitrile (4CzIPN), resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p‐terphenyl as the emitter. In this ternary blend, the sensitizer 4CzIPN is paired with an intermediate acceptor, 1‐methylnaphthalene, in addition to the emitter molecule, p‐terphenyl, yielding a normalized upconversion quantum yield of 7.6% while maintaining the 0.83 eV anti‐Stokes shift. These results illustrate the potential benefits of utilizing this strategy of energy‐funneling, previously used only with heavy‐metal based sensitizers, to increase the performance of these photon upconversion systems.

Simulation of electronic dynamics in realistically large molecular systems is a demanding task that has not yet achieved the same level of quantitative prediction already realized for its static counterpart. This is particularly true for processes occurring beyond the Born–Oppenheimer regime. Non-adiabatic molecular dynamics (NAMD) simulations suffer from two convoluted sources of error: numerical algorithms for dynamics and electronic structure calculations. While the former has gained increasing attention, particularly addressing the validity of ad hoc methodologies, the effect of the latter remains relatively unexplored. Indeed, the required accuracy for electronic structure calculations to reach quantitative agreement with experiment in dynamics may be even more strict than that required for static simulations. Here, we address this issue by modeling the electronic energy transfer in a donor–acceptor–donor (D–A–D) molecular light harvesting system using fewest switches surface hopping NAMD simulations. In the studied system, time-resolved experimental measurements deliver complete information on spectra and energy transfer rates. Subsequent modeling shows that the calculated electronic transition energies are “sufficiently good” to reproduce experimental spectra but produce over an order of magnitude error in simulated dynamical rates. We further perform simulations using artificially shifted energy gaps to investigate the complex relationship between transition energies and modeled dynamics to understand factors affecting non-radiative relaxation and energy transfer rates.

Inorganic phosphor materials play a crucial role in the creation of white light from blue and near-UV solid-state light-emitting diodes. Understanding the intricacies of the phosphor structure is key for setting the stage for improved, more efficient functionality. Average structure and coordination environment analysis of the robust and efficient green-emitting phosphor, β-SiAlON:Eu²⁺ (β-Si_6−zAl_zO_zN_8−zEu_0.009), is combined here with a range of property measurements to elucidate the role of Al content (z) in luminescence properties, including the red shift of emission and the thermal quenching of luminescence as a function of increasing Al content z. Average structure techniques reveal changes in polyhedral distortion with increasing z for the 9-coordinate Eu site in β-SiAlON:Eu²⁺. X-ray absorption near edge structure (XANES) is used to confirm that the majority of the activator Eu is in the Eu²⁺ state, exhibiting the symmetry-allowed and efficient 4f⁷5d⁰ → 4f⁶5d¹ transitions. Room temperature and temperature-dependent luminescence indicate a curious increase in thermal stability with increasing z over a small range due to an increasing barrier for thermal ionization, which is correlated to an increase in the quantum yield of the phosphor.

A membrane‐intercalating conjugated oligoelectrolyte (COE), PTTP, was designed and synthesized with the goal of providing red‐shifted absorption spectra relative to previously synthesized COE analogs. Specifically, electron‐rich and electron‐poor subunits were introduced in the conjugated backbone to modulate the band gap. PTTP exhibits maxima of absorption at 507 nm and of emission at 725 nm. PTTP can also efficiently function to generate singlet oxygen in situ (Φ_Δ≈20 %) and has appropriate topology and dimensions to interact with lipid membranes. The resulting rapid membrane insertion and sensitizing ability provide PTTP with a highly efficient antibacterial capability under a low light dose (0.6 J cm⁻²) toward Gram‐negative bacteria E. coli, making it a remarkably efficient optically mediated antimicrobial agent.

We demonstrate tuning of infrared Mie resonances by varying the carrier concentration in doped semiconductor antennas. We fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We subsequently demonstrate doping-dependent frequency shifts that follow simple Drude models, culminating in the emergence of plasmonic resonances at high doping levels and long wavelengths. These findings demonstrate the potential for actively tuning infrared Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable metamaterials.

We describe the use of cadmium telluride quantum dots (CdTe QDs) as antennas for the photosensitization of nitric oxide release from a ruthenium nitrosyl complex with visible light excitation. The CdTe QDs were capped with mercaptopropionic acid to make them water-soluble, and the ruthenium nitrosyl complex was cis-[Ru(NO)(4-ampy)(bpy)₂]³⁺ (Ru–NO; bpy is 2,2′-bipyridine, and 4-ampy is 4-aminopyridine). Solutions of these two components demonstrated concentration-dependent quenching of the QD photoluminescence (PL) as well as photoinduced release of NO from Ru–NO when irradiated by 530 nm light. A NO release enhancement of ∼8 times resulting from this association was observed under longer wavelength excitation in visible light range. The dynamics of the quenching determined by both PL and transient absorption measurements were probed by ultrafast flash photolysis. A charge transfer mechanism is proposed to explain the quenching of the QD excited states as well as the photosensitized release of NO from Ru–NO.

New methods connecting molecular structure, self-organization, and optoelectronic performance are important for understanding the current generation of organic photovoltaic (OPV) materials. In high power conversion efficiency (PCE) OPVs, light-harvesting small-molecules or polymers are typically blended with fullerene derivatives and deposited in thin films, forming a bulk heterojunction (BHJ), a self-assembled three-dimensional nanostructure of electron donors and acceptors that separates and transports charges. Recent data suggest micrometer-scale orientational order of donor domains exists within this complex nanomorphology, but the link to the optoelectronic properties is yet unexplored. Here we introduce polarization-dependent, photoconductive atomic force microscopy (pd-pcAFM) as a combined probe of orientational order and nanoscale optoelectronic properties (∼20 nm resolution). Using the donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), p-DTS(FBTTh2)2, we show significant spatial dependence of the nanoscale photocurrent with polarized light in both pristine and BHJ blends (up to 7.0% PCE) due to the local alignment of the molecular transition dipoles. By mapping the polarization dependence of the nanoscale photocurrent, we estimate the molecular orientation and orientational order parameter. Liquid crystalline disclinations are observed in all films, in agreement with complementary electron microscopy experiments, and the order parameter exceeds 0.3. The results demonstrate the utility of pd-pcAFM to investigate the optical/structural anisotropy that exists within a well-performing BHJ system and its relationship to optoelectronic properties on both the nanometer and micrometer length scales.