Amplified Spontaneous Emission from Electron-Hole Quantum Droplets in Colloidal CdSe Nanoplatelets.

Two-dimensional cadmium selenide nanoplatelets (NPLs) exhibit large absorption cross sections and homogeneously broadened band-edge transitions that offer utility in wide-ranging optoelectronic applications. Here, we examine the temperature-dependence of amplified spontaneous emission (ASE) in 4- and 5-monolayer thick NPLs and show that the threshold for close-packed (neat) films decreases with decreasing temperature by a factor of 2-10 relative to ambient temperature owing to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. Interestingly, for pump intensities that exceed the ASE threshold, we find development of intense emission to lower energy in particular provided that the film temperature is ≤200 K. For NPLs diluted in an inert polymer, both biexcitonic ASE and low-energy emission are suppressed, suggesting that described neat-film observables rely upon high chromophore density and rapid, collective processes. Transient emission spectra reveal ultrafast red-shifting with the time of the lower energy emission. Taken together, these findings indicate a previously unreported process of amplified stimulated emission from polyexciton states that is consistent with quantum droplets and constitutes a form of exciton condensate. For studied samples, quantum droplets form provided that roughly 17 meV or less of thermal energy is available, which we hypothesize relates to polyexciton binding energy. Polyexciton ASE can produce pump-fluence-tunable red-shifted ASE even 120 meV lower in energy than biexciton ASE. Our findings convey the importance of biexciton and polyexciton populations in nanoplatelets and show that quantum droplets can exhibit light amplification at significantly lower photon energies than biexcitonic ASE.

Light-Induced Transient Lattice Dynamics and Metastable Phase Transition in CH3NH3PbI3 Nanocrystals.

Methylammonium lead iodide (MAPbI3) perovskite nanocrystals (NCs) offer desirable optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Structural rearrangements of MAPbI3 in response to photoexcitation, such as lattice distortions and phase transitions, are of particular interest, as these engender long carrier lifetime and bolster carrier diffusion. Here, we use variable temperature X-ray diffraction (XRD) and synchrotron-based transient X-ray diffraction (TRXRD) to investigate lattice response following ultrafast optical excitation. MAPbI3 NCs are found to slowly undergo a phase transition from the tetragonal to a pseudocubic phase over the course of 1 ns under 0.02-4.18 mJ/cm2 fluence photoexcitation, with apparent nonthermal lattice distortions attributed to polaron formation. Lattice recovery exceeds time scales expected for both carrier recombination and thermal dissipation, indicating meta-stability likely due to the proximal phase transition, with symmetry-breaking along equatorial and axial directions. These findings are relevant for fundamental understanding and applications of structure-function properties.

Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology.

We report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2-6 monolayer thickness) and core/shell motif. Using electro-optical modulation of co- and cross-polarization pump-probe combinations, we sensitively observe spin-polarized transitions. Core-only nanoplatelets exhibit few-picosecond spin lifetimes that weakly increase with layer thickness. The spectral content of differenced spin-polarized signals indicate biexciton binding energies that decrease with increasing thickness and smaller values than previously reported. Shell growth of CdS with controlled thicknesses, which partially delocalize the electron from the hole, significantly increases the spin lifetime to ∼49 ps at room temperature. Implementation of ZnS shells, which do not alter delocalization but do alter surface termination, increased spin lifetimes up to ∼100 ps, bolstering the interpretation that surface termination heavily influences spin coherence, likely due to passivation of dangling bonds. Spin precession in magnetic fields both confirms long coherence lifetime at room temperature and yields the excitonic g factor.

Acceleration of Biexciton Radiative Recombination at Low Temperature in CdSe Nanoplatelets.

Colloidal semiconductor nanocrystals offer bandgap tunability, high photoluminescence quantum yield, and colloidal processing of benefit to optoelectronics, however rapid nonradiative Auger recombination (AR) deleteriously affects device efficiencies at elevated excitation intensities. AR is understood to transition from temperature-dependent behavior in bulk semiconductors to temperature-independent behavior in zero-dimensional quantum dots (QDs) as a result of discretized band structure that facilitates satisfaction of linear momentum conservation. For nanoplatelets (NPLs), two-dimensional morphology renders prediction of photophysical behaviors challenging. Here, we investigate and compare the temperature dependence of excited-stated lifetime and fluence-dependent emission of CdSe NPLs and QDs. For NPLs, upon temperature reduction, biexciton lifetime surprisingly decreases (even becoming shorter lived than trion emission) and emission intensity increases nearly linearly with fluence rather than saturating, consistent with dominance of radiative recombination rather than AR. CdSe NPLs thus differ fundamentally from core-only QDs and foster increased utility of photogenerated excitons and multiexcitons at low temperatures.

Photothermal behaviour of titanium nitride nanoparticles evaluated by transient X-ray diffraction.

The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface.

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation.

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.

Transient Lattice Response upon Photoexcitation in CuInSe2 Nanocrystals with Organic or Inorganic Surface Passivation.

CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2-) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.

Engineering Directionality in Quantum Dot Shell Lasing Using Plasmonic Lattices.

We report how the direction of quantum dot (QD) lasing can be engineered by exploiting high-symmetry points in plasmonic nanoparticle (NP) lattices. The nanolaser architecture consists of CdSe-CdS core-shell QD layers conformally coated on two-dimensional square arrays of Ag NPs. Using waveguide-surface lattice resonances (W-SLRs) near the Δ point in the Brillouin zone as optical feedback, we achieved lasing from the gain in CdS shells at off-normal emission angles. Changing the periodicity of the plasmonic lattices enables other high-symmetry points (Γ or M) of the lattice to overlap with the QD shell emission, which facilitates tuning of the lasing direction. We also increased the thickness of the QD layer to introduce higher-order W-SLR modes with additional avoided crossings in the band structure, which expands the selection of cavity modes for any desired lasing emission angle.

Synthesis of Type I PbSe/CdSe Dot-on-Plate Heterostructures with Near-Infrared Emission.

Zero-dimensional PbSe quantum dots are heterogeneously nucleated and grown onto two-dimensional zincblende CdSe nanoplatelets. Electron microscopy shows ad-grown dots predominantly decorate edges and corners of the nanoplatelets. Spectroscopic characterizations relate type I electronic alignment as demonstrated via photoluminescence excitation spectroscopy enhancement of near-infrared emission. Transient photoluminescence and absorption convey ultrafast transfer of excitons to the lower energy semiconductor dots. These structures combine benefits of large absorption cross sections of nanoplatelets and efficient near-infrared emission of PbSe with quantum confinement tuning of energy gap.

Photoinduced, reversible phase transitions in all-inorganic perovskite nanocrystals.

Significant interest exists in lead trihalides that present the perovskite structure owing to their demonstrated potential in photovoltaic, lasing, and display applications. These materials are also notable for their unusual phase behavior often displaying easily accessible phase transitions. In this work, time-resolved X-ray diffraction, performed on perovskite cesium lead bromide nanocrystals, maps the lattice response to controlled excitation fluence. These nanocrystals undergo a reversible, photoinduced orthorhombic-to-cubic phase transition which is discernible at fluences greater than 0.34 mJ cm-2 through the loss of orthorhombic features and shifting of high-symmetry peaks. This transition recovers on the timescale of 510 ± 100 ps. A reversible crystalline-to-amorphous transition, observable through loss of Bragg diffraction intensity, occurs at higher fluences (greater than 2.5 mJ cm-2). These results demonstrate that light-driven phase transitions occur in perovskite materials, which will impact optoelectronic applications and enable the manipulation of non-equilibrium phase characteristics of the broad perovskite material class.