Efficient 2D to 0D Energy Transfer in HgTe Nanoplatelet-Quantum Dot Heterostructures through High-Speed Exciton Diffusion.

Large area absorbers with localized defect emission are of interest for energy concentration via the antenna effect. Transfer between 2D and 0D quantum-confined structures is advantageous as it affords maximal lateral area antennas with continuously tunable emission. We report the quantum efficiency of energy transfer in in situ grown HgTe nanoplatelet (NPL)/quantum dot (QD) heterostructures to be near unity (>85%), while energy transfer in separately synthesized and well separated solutions of HgTe NPLs to QDs only reaches 47 ± 11% at considerably higher QD concentrations. Using Kinetic Monte Carlo simulations, we estimate an exciton diffusion constant of 1-10 cm2/s in HgTe NPLs, the same magnitude as that of 2D semiconductors. We also simulate in-solution energy transfer between NPLs and QDs, recovering an R-4 dependence consistent with 2D-0D near-field energy transfer even in randomly distributed NPL/QD mixtures. This highlights the advantage of NPLs 2D morphology and the efficiency of NPL/QD heterostructures and mixtures for energy harvesting.

Exploring the design of superradiant J-aggregates from amphiphilic monomer units.

Excitonic chromophore aggregates have wide-ranging applicability in fields such as imaging and energy harvesting; however their rational design requires adapting principles of self-assembly to the requirements of excited state coupling. Using the well-studied amphiphilic cyanine dye C8S3 as a template-known to assemble into tubular excitonic aggregates-we synthesize several redshifted variants and study their self-assembly and photophysics. The new pentamethine dyes retain their tubular self-assembly and demonstrate nearly identical bathochromic shifts and lineshapes well into near-infrared wavelengths. However, detailed photophysical analysis finds that the new aggregates show a significant decline in superradiance. Additionally, cryo-TEM reveals that these aggregates readily form short bundles of nanotubes that have nearly half the radii of their trimethine comparators. We employ computational screening to gain intuition on how the structural components of these new aggregates affect their excitonic states, finding that the narrower tubes are able to assemble into a larger number of arrangements, resulting in more disordered aggregates (i.e. less superradiant) with highly similar degrees of redshift.

Spectrally Selective Time-Resolved Emission through Fourier-Filtering (STEF).

We demonstrate a method for separating and resolving the dynamics of multiple emitters without the use of conventional filters. By directing the photon emission through a fixed path-length imbalanced Mach-Zehnder interferometer, we interferometrically cancel (or enhance) certain spectral signatures corresponding to one emissive species. Our approach, Spectrally selective Time-resolved Emission through Fourier-filtering (STEF), leverages the detection and subtraction of both outputs of a tuned Mach-Zehnder interferometer, which can be combined with time-correlated single photon counting (TCSPC) or confocal imaging to demix multiple emitter signatures. We develop a procedure to calibrate out imperfections in Mach-Zehnder interferometry schemes. Additionally, we demonstrate the range and utility of STEF by performing the following procedures with one measurement: (1) filtering out laser scatter from a sample, (2) separating and measuring a fluorescence lifetime from a binary chromophore mixture with overlapped emission spectra, (3) confocally imaging and separately resolving the standard fluorescent stains in bovine pulmonary endothelial cells and nearly overlapping fluorescent stains on RAW 264.7 cells. This form of spectral balancing can allow for robust and tunable signal sorting.

Maximizing light-driven CO2 and N2 fixation efficiency in quantum dot-bacteria hybrids.

Integrating light-harvesting materials with microbial biochemistry is a viable approach to produce chemicals with high efficiency from the air, water, and sunlight. Yet it remains unclear whether all absorbed photons in the materials can be transferred through the material-biology interface for solar-to-chemical production and whether the presence of materials beneficially affect the microbial metabolism. Here we report a microbe-semiconductor hybrid by interfacing CO2/N2-fixing bacterium Xanthobacter autotrophicus with CdTe quantum dots for light-driven CO2 and N2 fixation with internal quantum efficiencies of 47.2 ± 7.3% and 7.1 ± 1.1%, respectively, reaching the biochemical limits of 46.1% and 6.9% imposed by the stoichiometry in biochemical pathways. Photophysical studies suggest fast charge-transfer kinetics at the microbe-semiconductor interfaces, while proteomics and metabolomics indicate a material-induced regulation of microbial metabolism favoring higher quantum efficiencies compared to the biological counterparts alone.

Establishing design principles for emissive organic SWIR chromophores from energy gap laws.

Rational design of bright near and shortwave infrared (NIR: 700-1000 SWIR: 1000- 2000 nm) emitters remains an open question with applications spanning imaging and photonics. Combining experiment and theory, we derive an energy gap quantum yield master equation (EQME), describing the fundamental limits in SWIR quantum yields (ϕ F ) for organic chromophores. Evaluating the photophysics of 21 polymethine NIR/SWIR chromophores to parameterize the EQME, we explain the precipitous decline of ϕ F past 900 nm through decreasing radiative rates and increasing nonradiative losses via high frequency vibrations relating to the energy gap. Using the EQME we develop an energy gap independent ϕ F NIR/SWIR chromophore comparison metric. We show electron donating character on polymethine heterocycles results in relative increases in radiative efficiency obscured by a simultaneous redshift. Finally, the EQME yields rational chromophore design insights shown by how deuteration (backed by our experimental results) or molecular aggregation increases SWIR ϕ F .

Approaching the intrinsic exciton physics limit in two-dimensional semiconductor diodes.

Two-dimensional (2D) semiconductors have attracted intense interest for their unique photophysical properties, including large exciton binding energies and strong gate tunability, which arise from their reduced dimensionality1-5. Despite considerable efforts, a disconnect persists between the fundamental photophysics in pristine 2D semiconductors and the practical device performances, which are often plagued by many extrinsic factors, including chemical disorder at the semiconductor-contact interface. Here, by using van der Waals contacts with minimal interfacial disorder, we suppress contact-induced Shockley-Read-Hall recombination and realize nearly intrinsic photophysics-dictated device performance in 2D semiconductor diodes. Using an electrostatic field in a split-gate geometry to independently modulate electron and hole doping in tungsten diselenide diodes, we discover an unusual peak in the short-circuit photocurrent at low charge densities. Time-resolved photoluminescence reveals a substantial decrease of the exciton lifetime from around 800 picoseconds in the charge-neutral regime to around 50 picoseconds at high doping densities owing to increased exciton-charge Auger recombination. Taken together, we show that an exciton-diffusion-limited model well explains the charge-density-dependent short-circuit photocurrent, a result further confirmed by scanning photocurrent microscopy. We thus demonstrate the fundamental role of exciton diffusion and two-body exciton-charge Auger recombination in 2D devices and highlight that the intrinsic photophysics of 2D semiconductors can be used to create more efficient optoelectronic devices.

Bright Chromenylium Polymethine Dyes Enable Fast, Four-Color In Vivo Imaging with Shortwave Infrared Detection.

Optical imaging within the shortwave infrared (SWIR, 1000-2000 nm) region of the electromagnetic spectrum has enabled high-resolution and high-contrast imaging in mice, non-invasively. Polymethine dyes, with their narrow absorption spectra and high absorption coefficients, are optimal probes for fast and multiplexed SWIR imaging. Here, we expand upon the multiplexing capabilities in SWIR imaging by obtaining brighter polymethine dyes with varied excitation wavelengths spaced throughout the near-infrared (700-1000 nm) region. Building on the flavylium polymethine dye scaffold, we explored derivatives with functional group substitution at the 2-position, deemed chromenylium polymethine dyes. The reported dyes have reduced nonradiative rates and enhanced emissive properties, enabling non-invasive imaging in mice in a single color at 300 fps and in three colors at 100 fps. Combined with polymethine dyes containing a red-shifted julolidine flavylium heterocycle and indocyanine green, distinct channels with well-separated excitation wavelengths provide non-invasive video-rate in vivo imaging in four colors.

Dielectric Screening Modulates Semiconductor Nanoplatelet Excitons.

The influence of external dielectric environments is well understood for 2D semiconductor materials but overlooked for colloidally grown II-VI nanoplatelets (NPLs). In this work, we synthesize MX (M = Cd, Hg; X = Se, Te) NPLs of varying thicknesses and apply the Elliott model to extract exciton binding energies-reporting values in good agreement with prior methods and extending to less studied cadmium telluride and mercury chalcogenide NPLs. We find that the exciton binding energy is modulated both by the relative effect of internal vs external dielectric and by the thickness of the semiconductor material. An analytical model shows dielectric screening increases the exciton binding energy relative to the bulk by distorting the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate NPLs are colloidal analogues of van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

Large-Area Synthesis and Patterning of All-Inorganic Lead Halide Perovskite Thin Films and Heterostructures.

All-inorganic lead halide perovskites have attracted tremendous interest for their excellent stability when compared with hybrid perovskites. Here we report a large-area growth of monocrystalline all-inorganic perovskite thin films and further patterning them into heterostructure arrays. We show that highly oriented CsPbBr3 microcrystal domains can be readily grown on muscovite mica substrates with a well-defined epitaxial relationship, which can further expand and eventually merge into large-area monocrystalline CsPbBr3 thin films with an excellent optical quality. Taking a step further, we show the large-area CsPbBr3 thin film can be further patterned and selectively transformed into CsPbI3 using a selective anion-exchange process to produce CsPbBr3-CsPbI3 lateral heterostructure arrays with spatially modulated photoluminescence emission and an apparent current rectification behavior. The capability to grow large-area CsPbBr3 monocrystalline thin films and heterostructure arrays defines a robust material platform for both the fundamental investigations and potential applications in optoelectronics.

Surface chemical trapping of optical cycling centers.

Quantum information processors based on trapped atoms utilize laser-induced optical cycling transitions for state preparation and measurement. These transitions consist of an electronic excitation from the ground to an excited state and a decay back to the initial ground state, associated with a photon emission. While this technique has been used primarily with atoms, it has also recently been shown to work for some divalent metal hydroxides (e.g. SrOH) and alkoxides (e.g. SrOCH3). This extension to molecules is possible because these molecules feature nearly isolated, atomic-like ground and first-excited electronic states centered on the radical metal atom. We theoretically investigate the extension of this idea to a larger scale by growing the alkyl group, R, beyond the initial methyl group, CH3, while preserving the isolated and highly vertical character of the electronic excitation on the radical metal atom, M. Theory suggests that in the limit as the size of the ligand carbon chain increases, it can be considered a functionalized diamond (or cubic boron nitride) surface. Several requirements must be observed for the cycling centers to function when bound to the surface. First, the surface must have a significant band gap that fully encapsulates both the ground and excited states of the cycling center. Second, while the surface lattice imposes strict limits on the achievable spacing between the SrO- groups, at high coverage, SrO- centers can interact, and show geometric changes and/or electronic state mixing. We show that the coverage of the diamond surface with SrO- cycling centers needs to be significantly sub-monolayer for the functionality of the cycling center to be preserved. Having the lattice-imposed spatial control of SrO- placements will allow nanometer-scale proximity between qubits and will eliminate the need for atom traps for localized cycling emitters. Our results also imply that a functionalization could be done on a scanning microscope tip for local quantum sensing or on photonic structures for optically-mediated quantum information processing.

Silicon incorporation in polymethine dyes.

Methods to red-shift fluorophores have garnered considerable interest due to the broad utility of low energy light. The incorporation of silicon into xanthene and coumarin scaffolds has resulted in an array of visible and near-infrared fluorophores. Here, we extend this approach to polymethine dyes, another popular fluorophore class, performing experimental and computational analyses. We found that when oxygen was replaced with SiMe2, bathochromic shifts of up to 121 nm and fluorophores with emission above 900 nm were achieved.

Mercury Chalcogenide Nanoplatelet-Quantum Dot Heterostructures as a New Class of Continuously Tunable Bright Shortwave Infrared Emitters.

Despite broad applications in imaging, energy conversion, and telecommunications, few nanoscale moieties emit light efficiently in the shortwave infrared (SWIR, 1000-2000 nm or 1.24-0.62 eV). We report quantum-confined mercury chalcogenide (HgX, where X = Se or Te) nanoplatelets (NPLs) can be induced to emit bright (QY > 30%) and tunable (900-1500+ nm) infrared emission from attached quantum dot (QD) "defect" states. We demonstrate near unity energy transfer from NPL to these QDs, which completely quench NPL emission and emit with a high QY through the SWIR. This QD defect emission is kinetically tunable, enabling controlled midgap emission from NPLs. Spectrally resolved photoluminescence demonstrates energy-dependent lifetimes, with radiative rates 10-20 times faster than those of their PbX analogues in the same spectral window. Coupled with their high quantum yield, midgap emission HgX dots on HgX NPLs provide a potential platform for novel optoelectronics in the SWIR.

Decay-Associated Fourier Spectroscopy: Visible to Shortwave Infrared Time-Resolved Photoluminescence Spectra.

We describe and implement an interferometric approach to decay-associated photoluminescence spectroscopy, which we term decay-associated Fourier spectroscopy (DAFS). In DAFS, the emitted photon stream from a substrate passes through a variable path length Mach-Zehnder interferometer prior to detection and timing. The interferometer encodes spectral information in the intensity measured at each detector enabling simultaneous spectral and temporal resolution. We detail several advantages of DAFS, including wavelength-range insensitivity, drift-noise cancellation, and optical mode retention. DAFS allows us to direct the photon stream into an optical fiber, enabling the implementation of superconducting nanowire single photon detectors for energy-resolved spectroscopy in the shortwave infrared spectral window (λ = 1-2 µm). We demonstrate the broad applicability of DAFS, in both the visible and shortwave infrared, using two Förster resonance energy transfer pairs: a pair operating with conventional visible wavelengths and a pair showing concurrent acquisition in the visible and the shortwave infrared regime.

Single-crystal-to-single-crystal intercalation of a low-bandgap superatomic crystal.

The controlled introduction of impurities into the crystal lattice of solid-state compounds is a cornerstone of materials science. Intercalation, the insertion of guest atoms, ions or molecules between the atomic layers of a host structure, can produce novel electronic, magnetic and optical properties in many materials. Here we describe an intercalation compound in which the host [Co6Te8(PnPr3)6][C60]3, formed from the binary assembly of atomically precise molecular clusters, is a superatomic analogue of traditional layered atomic compounds. We find that tetracyanoethylene (TCNE) can be inserted into the superstructure through a single-crystal-to-single-crystal transformation. Using electronic absorption spectroscopy, electrical transport measurements and electronic structure calculations, we demonstrate that the intercalation is driven by the exchange of charge between the host [Co6Te8(PnPr3)6][C60]3 and the intercalant TCNE. These results show that intercalation is a powerful approach to manipulate the material properties of superatomic crystals.

Lead halide perovskites: Crystal-liquid duality, phonon glass electron crystals, and large polaron formation.

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This "crystal-liquid" duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials.

Charge Saturation and Intrinsic Doping in Electrolyte-Gated Organic Semiconductors.

Electrolyte gating enables low voltage operation of organic thin film transistors, but little is known about the nature of the electrolyte/organic interface. Here we apply charge-modulation Fourier transform infrared spectroscopy, in conjunction with electrical measurements, on a model electrolyte gated organic semiconductor interface: single crystal rubrene/ion-gel. We provide spectroscopic signature for free-hole like carriers in the organic semiconductor and unambiguously show the presence of a high density of intrinsic doping of the free holes upon formation of the rubrene/ion-gel interface, without gate bias (Vg = 0 V). We explain this intrinsic doping as resulting from a thermodynamic driving force for the stabilization of free holes in the organic semiconductor by anions in the ion-gel. Spectroscopy also reveals the saturation of free-hole like carrier density at the rubrene/ion-gel interface at Vg < -0.5 V, which is commensurate with the negative transconductance seen in transistor measurements.