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In this talk I will discuss recent collaborative work between the University of Washington, Notre Dame, and Oak Ridge National Laboratory in developing a new electron microscopy technique to perform nano-ellipsometry measurements on individual nanoparticles. Focus will be made on inverting the low-loss electron energy-loss spectrum to retrieve the complex-valued and frequency-dependent dielectric functions of a series of individual tin-doped indium oxide nanocrystals with tin doping concentration ranging from 1−10 atomic percent. This method, devoid from ensemble averaging, illustrates the potential for nanoscale electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Optically levitated nanoparticles provide excellent systems to sense minute forces and explore quantum effects in a large system. However, optically levitated nanoparticles are prone to heating and require cooling and temperature stabilization to reach sensitivities necessary to study small forces and quantum effects. This problem can be solved by trapping nanocrystals doped with rare-earth ions that can be anti-Stokes cooled by tens of degrees. The efficiency of the anti-Stokes depends on gas pressure and can counter heating due to optical absorption. Cooling nanocrystals allows for thermally stabilizing nanocrystal systems to measure minute forces and quantum effects.

Colloidal quantum dots, which have demonstrated its usage in optoelectronics in the visible range, are promising as infrared active materials as well. The wavelength-tunability by many variables including the nanocrystal size allows easy access to the target wavelength. Mostly, the bandgap transition is used for energy conversion. Self-doped quantum dots are the nanocrystal in which other electronic transitions are available under steady-state and ambient conditions. This talk will focus on the synthesis of the self-doped Ag2Se, HgSe QDs and their various properties including carrier dynamics studied by mid-IR spectroscopy. Additionally, several applications based on the intraband transition will be discussed.

Superfluorescence (SF) is a unique optical phenomenon that consists of an ensemble of emitters coupling collectively to produce a short but extremely intense burst of light. SF has also only been realized in extreme conditions (at low temperatures of around 6 K). Moreover, no anti-Stokes shift SF has been discovered in either an ensemble of nanoparticles or at bulky crystal levels. We report on a new lanthanidedoped upconversion nanoparticles (UCNPs) as a medium to achieve cavity free anti-Stokes shifted SF at room temperature, culminating in rapid, intense, and narrow spectral peaks of upconverted SF. This is the first time that SF has been discovered in a single nanocrystal regime and is the smallest-ever SF media. We observed the resultant UCNP SF with an extremely narrow spectral width at single nanocrystal-level (full-width at half-maximum, FWHM = 2 nm), and to have a significantly shortened lifetime (τ = 46 ns, 10,000-fold accelerated radiative decay, when compared to the lifetime of τ = 455.8 μs of normal upconversion luminescence (UCL). The significantly upspeeded upconverted SF lifetimes at tens of nanoseconds scale should break through the key limitation in normal UCL. This will open up the opportunity to carry out high speed bioimaging using upconversion nanoparticles without compromising the imaging quality. In addition, our ultrafast upconverted SF will achieve fine temporal resolution control of highly dynamic physiological processes that have been constrained by normal UCL.

Semiconductor nanocrystal core/shell quantum dots (QDs) have successfully extended their original fundamental research into many practical applications. But core/shell QDs may still not satisfying enough in practical applications because of the existence of photoblinking, multi-exponential PL decay behavior, and Forster resonance energy transfer (FRET) between QDs. Herein we report an approach to synthesize a series of alloyed core/shell QDs by a “low-temperature injection and high-temperature growth” precisely controlled method. By probing shell-thickness dependent performance, ZnCdSe-based core/shell QDs not only with nonblinking but also single photoluminescence decay channel and suppressed FRET have been successfully prepared. As emitters, such ZnCdSe- based QLEDs exhibit high external quantum efficiencies, low-efficiency roll-off at high current density, and long operational lifetime.

Metal halide perovskites in the form of nanocrystals are highly efficient light emitters at visible-NIR wavelengths. In this work, the optical properties of single nanocrystals and ensembles will be discussed, as also several applications in nanophotonics. At low temperatures, single nanocrystals can be also single photon emitters if blinking and spectral diffusion is conveniently reduced. In the case of nanocrystal assemblies, stimulated emission can be observed with thresholds lower than 10 μJ/cm2 under nanosecond laser excitation at low temperatures, whose physical origin is attributed to single exciton recombination. Finally, the coupling of perovskite nanocrystals to the optical modes of hyperbolic metaldielectric metamaterials has been studied and demonstrated an important Purcell enhancement of the exciton radiative emission by more than a factor three for CsPbI3 and around factor two for FAPbI3 when the distance between the emitters and HMM is 10 nm.

Here we use optical trapping to isolate single Yb/Er-doped upconversion nanocrystals in plasmonic double nanohole apertures and show that the geometry of the aperture can be tuned to give high emission rate en- hancement. The double nanohole apertures show additional enhancement over the rectangular apertures that were previously demonstrated by our group, producing enough enhancement to observe emission at 400 nm and 1550 nm with 980 nm excitation—not seen in our group’s previous work with rectangular apertures. A facile method for tuning the geometry of double nanohole apertures by adjusting the plasma etching time in the colloidal lithography fabrication process is discussed. We find that a double nanohole with a cusp separation of 32 nm yields the greatest emission enhancement with multiple plasmonic resonances which enhance both the excitation and emission wavelengths. The emission enhancement for the DNH with 32 nm cusp separation was found to be a factor of 54, 44, and 31 greater than the rectangular apertures used in our group’s previous work, for wavelengths of 650 nm, 550 nm, and 400 nm. This result shows that double nanohole apertures can be tuned for emission enhancement as required by specific applications.

The polymer-salt method was applied to synthesize nanoscale Gd2O3:Nd3+ phosphors in the form of thin films on the inner surfaces of capillaries which organize the structure of a silica hollow-core anti-resonant optical fiber. To obtain luminescing centers, the preform of a hollow-core anti-resonant optical fiber was impregnated with a homogeneous mixture of Gd(NO3)3 and NdCl3 dissolved in water and organic solvent (polyvinylpyrrolidone). This procedure was followed by a few post-processing steps, including drying of the impregnated preform in normal conditions and its thermal treatment at temperature 1000 °C. As a result, Gd2O3:Nd3+-based thin films were produced inside the capillaries. Finally, the modified preform was drawn into the hollow-core anti-resonant optical fiber of 120 μm in diameter at temperature 1850 °C. The analysis of crystallographic structure of the initial Gd2O3:Nd3+ nanopowder and the same nanophosphor inside the fabricated fiber revealed the absence of structural and phase transformations of synthesized nanocrystals with an average size 35 nm after drawing. The data on spectral-luminescent properties of the fabricated fiber confirmed the presence of Gd2O3:Nd3+ nanophosphors in silica glass with the main emission peak at wavelength 1064 nm. Presented method of modifying the structure of a hollow-core anti-resonant optical fiber allows formation of active silica layers without using technologically complicated and expensive CVD processes.

This work is devoted to the study of phase transitions in CsPbBr3 perovskite nanocrystals nucleated in borogermanate glass. Perovskite nanocrystals are obtained by bulk crystallization in a glass matrix. A series of glass samples with nanocrystals of different sizes is investigated. Upon heating, the temperature dependence of the 1S exciton intensity for the entire series of samples shows the presence of three phase transitions: in the range 130-150, 440-450, and 510-515°C. The first phase transition refers to the transition from a tetragonal to a cubic structure. The last phase transition is associated with the melting of the crystalline phase. The phase transition at 440°С is declared for the first time. Upon cooling, the temperature curve shows the presence of only one phase transition in the region of 330°C, which is associated with the crystallization onset of CsPbBr3 perovskite nanocrystals in glass matrix.

One photon up-conversion photoluminescence is an optical phenomenon whereby the thermal energy of a fluorescent material is used to increase the energy of an emitted photon compared with the energy of the photon that was absorbed. When this occurs with near unity efficiency, the emitting material undergoes a net decrease in temperature—so called optical refrigeration. Because the up-conversion is thermally activated, the yield of up-converted photoluminescence is also a reporter of the temperature of the emitter. Taking advantage of this optical signature, we have shown that cesium lead trihalide nanocrystals are cooled by as much as 66 K during the up-conversion of 532 nm CW laser excitation. Our work is the first demonstration of optical cooling of colloidal semiconductor nanocrystals, as well as a new record for optical cooling of any semiconductor system, highlighting the intrinsic advantages of colloidal nanocrystals for this goal.

The surface of native polydimethylsiloxane (PDMS) was modified into black structures by irradiation of femtosecond laser pulses. Material analysis conducted on the modified areas revealed the formation of silicon carbide (SiC) nanocrystals, as well as multilayer graphitic carbon. Furthermore, electrical conductivity measurements of structures fabricated using various fabrication parameters, suggest that the amount of defect in the formed graphitic carbon affects the electrical conductivity of the fabricated structures. A preliminary demonstration of strain sensing was performed using the structures fabricated, indicating the potential of the structures fabricated by the femtosecond-laser-based modification of PDMS for flexible device applications.

We investigated the effect of the partial replacement of Pb2+ with Mg2+ in CsPbX3 (X-halides) on photophysical properties and their potential applications in light-emitting diodes (LEDs). The new composites exhibited a slight variation in the lattice dimension without altering their structure and morphology. However, the replacement of Pb2+ with Mg2+ imparted enhancement in photoluminescence and bandgap energies with maintaining the high luminous property. As a result, the application of these nanocrystals as the color conversion layers (3D printing-based design) in the fabrication of white LEDs generated white light with excellent color characteristics.

We present a super-resolving orientation microscopy method for single-particle imaging. In this method, the fluorescence signal from particles is imaged to obtain orientation and localization information simultaneously with higher throughput than photon counting methods. We demonstrate the technique by resolving nanocrystal quantum dot orientations within small clusters. The effects of orientation on coupling efficiency for energy transfer is investigated.

The introduction of non-toxic guest cations (zinc, Zn2+) in the synthesis of CsPbX3 NCs leads to the realization of lead-reduced CsPb1-xZnxX3 NCs, confirming the substitution of Pb2+ up to 15%. As-synthesized NCs exhibits lattice contraction accompanied by increased bandgap, blue-shifted photoluminescence emissions, and high quantum yield. Besides, the blue, green, yellow, and red-emitting CsPb1-xZnxX3 NCs (at x=15%) obtained by adjusting the composition of different halides when used as color conversion layers emits warm to cool white light with remarkable color qualities. Notably, this development will promote the halide perovskite NCs one step closer to their practical applications.

Silicon nanoparticles suspended in deionized water were obtained by the laser ablation of solids in liquids technique. A silicon wafer target was ablated with a Nd:YVO4 pico-second laser emitting at 1064 nm with 10 ps pulse width at a repetition rate of 402 kHz with an energy per pulse of 106 µJ. The effect of fluence changes in nanoparticle size and optical properties was studied. The fluence was varied from 2 to 6 J/cm2 by attenuating the beam by means of optical attenuators. Results show a strong relation between nanoparticles size and fluence values. Optical characterization gives bad gap values higher than that of bulk Si, indicating the existence of quantum confinement effects produced by size reduction of Si nanocrystals.

A thorough investigation of copper oxide, specifically cupric oxide (CuO), is performed in the following work with a focus on CuO’s ultrafast free-carrier dynamics and bandstructure. An above-bandgap control beam and below-bandgap signal beam are utilized in transient absorption spectroscopy to gain insight on CuO nanocrystals’ recombination and relaxation dynamics at varying control beam fluences. The authors witnessed three distinct time constants, the first of which changed with control beam fluence between 330 and 630 fs, while the second and third remained constant at 2 ps and 50 ps, respectively. The first time constant is attributed to momentum relaxation from valence band carrier-carrier scattering and exciton-exciton annihilation. The second time constant is attributed to energy relaxation from valence band carrier-phonon scattering. The third time constant is attributed to trapping and recombination as a result of the CuO nanocrystals’ increased trap state density. The findings of this work provide a basis for future research on this emerging CuO nanocrystal system.

As the field of semiconducting quantum dots (QDs) continues to mature, the dispersity of nanocrystal sizes present in a synthesized sample is still an obstacle. Because the properties of QDs are size dependent, it is crucial to produce monodisperse QD samples to understand structure-property relations. Magic-sized clusters (MSCs) circumvent the polydispersity seen in QDs, as growth is discrete and limited to only certain sized clusters. In spite of their promise, MSCs remain poorly studied. MSCs typically exhibit broad emission with low photoluminescence quantum yields (PLQY). This presentation will describe our efforts towards CdSe MSCs with sharp, high efficiency PLQY through the growth of a passivating shell.

Beyond the eponymous snow slides, avalanching processes – where a compounding nonlinear cascade of events is triggered by a small perturbation – are found today in a surprising variety of phenomena including the popping of champagne bubbles, nuclear explosions, lasing, neuronal networking, and even financial crises. In optics, photon avalanching (PA) behavior garnered significant interest more than three decades ago as a method for achieving efficient upcoverted lasing, and, due to its extreme nonlinearity, was more recently proposed as a mechanism for realizing facile superresolution imaging if satisfactory nanomaterials could be designed. However, a demanding set of criteria has limited PA in Ln-based systems only to fragile bulk materials and aggregates, and mostly to cryogenic temperatures. Here, we have leveraged a combination of advanced theoretical modeling, synthesis, and photophysical characterization to create robust photon avalanching nanoparticles.

In this talk, I will present our recent efforts on the development of new materials and processing approaches to achieve efficient and stable perovskite light emitting diodes (LEDs) with tunable colors. First, surface passivation of metal halide perovskite nanocrystals will be discussed, which could not only reduce surface defects, but also protect the surface from the penetration of degradation agents (e.g. moisture). Second, a novel approach to achieve efficient blue emissions from hollow perovskite nanocrystals via quantum size effects will be introduced. Lastly, processing engineering to prepare metal halide perovskite thin films with desired morphological and electronic properties for LEDs will be discussed.

Transient absorption spectroscopy can measure exciton dynamics and provide insight into the electronic structure of nanocrystals (NCs). This spectroscopy, however, is typically limited by the long timescales required for acquisition of transient spectra, preventing the accurate measurement of systems that are not at a structural equilibrium. The structure of NCs changes during their synthesis on a shorter timescale than that required for measurement, making it difficult to study the evolving photophysics of NCs during growth. Here, we leverage a single-shot transient absorption (SSTA) spectrometer, capable of recording transient spectra with excellent signal-to-noise in less than a minute, to measure exciton dynamics in growing NCs. The presence of internal electric fields caused by surface-trapped carriers is evident in the TA lineshape, where a distinct Stark effect is observed revealing that growing NCs are poorly passivated. This work will enable a range of future experiments to study charge carrier behavior in rapidly evolving NC systems.

Cellulose Nanocrystals are readily produced from many plant and bacterial sources and have been studied extensively for low cost self-assembled optical elements. Nanocrystals are known to form a chiral nematic phase, which allows for production of films with chiral character. I will discuss recent advances in understanding how to control the chirality of cellulose nanocrystal solutions. The Debye length of a nanocrystal solution is typically around 4nm for pH between 1.5 and 10 and abruptly drops to around 1 nm for pH lower or higher than this range. With a Debye Length of 1nm, the solution can only form nematic phase, which is more useful for production of waveplates and other birefringent optics. Additionally I will discuss the behavior of cellulose solutions with amorphous material remaining in solution which form discotic- like systems.

In this work, we demonstrated that through the addition of large organic cation PEABr, the quality of cesium lead halide thin films has been enhanced by controlling the crystallization process and thus the domain size and morphology. As a result, by increasing the PEABr concentration to 40%, the perovskite thin films with a small domain size, highly oriented crystal structure and low defect density demonstrated a lowest amplified spontaneous emission threshold of 7 µJ/cm2. However, further increasing PEABr concentration will result in a slower energy transfer, which limits the population inversion process and thus showed a higher ASE threshold. Using this strategy, by tuning the halide ratio, the ASE emissions were observed over the broad visible range (from 490 nm to 680 nm), demonstrating its wide wavelength-tunability.

Upconversion nanoparticles (UCNP) is a lanthanide ion-doped nanocrystal that has a natural nonlinear photo-response from their upconverting energy transfer process. The nonlinearity can be further modified by changing the doping element and concentration. Here we present a strategy that applies UCNPs as near-infrared (NIR) nonlinear fluorescence probe for in-depth super-resolution imaging. We present a method that takes advantage of “non-diffractive” Bessel beam, further employs the photon-saturation of the NIR emission from UCNPs, so that enabling super-resolution mapping of single nanoparticles located 55 μm inside a spheroid, with a resolution of 98 nm, without adaptive optics compensation. We further apply the photon-conversion of UCNPs for a high efficient NIR nonlinear structured illumination microscopy (NIRNSIM) for a rapid in-depth super-resolution imaging. With 10 kW/cm2 continuous wave (CW) excitation, NIR-NSIM achieves a resolution of 130 nm, 1/7th of the excitation wavelength, and a frame rate of 1 fps, through 50 μm biological tissues.

Nanocellulose has a great potential as a renewable material due to its high mechanical strength, high Young’s modulus, low density and eco-friendliness. Once a bulk material is made with it, then the bulk material made with nanocellulose can be a renewable bulk material, which is eco-friendly, lightweight and strong. This paper aims at testing the feasibility bulk material processing by using nanocellulose, specifically cellulose nanocrystal (CNC). The fabrication is carried out through steam with high temperature and high pressure to form hydrogen bonds between CNCs, followed by heat and pressure molding. Crystalline structures of the prepared bulk materials are investigated by using X-ray diffraction and morphology and mechanical properties are investigated by using scanning electron microscope and dynamic mechanical analysis. Also, machining behavior for the nanocellulose bulk material is tested by using end mill to see its manufacturing possibility. In addition, the surface roughness is measured by using optical surface profiler with endmill machining part. Machining heat generation is investigated by thermal imaging camera between endmill tool and machined surface of the sample.

In this contribution, we report synthesis of water-soluble Ag−In−S /ZnS quantum dots (QDs) using low toxic precursors as well as investigation of their optical properties. The QDs were successfully obtained directly in an aqueous environment in two ways: traditional one-pot water synthesis and hydrothermal synthesis in the presence of thioglycolic acid as a stabilizing and reactivity controlling agent. The effect of hydrothermal treatment on water-soluble Ag−In−S/ZnS QDs was shown and it was found that they are resistant for 4 hours autoclaving.

In this work, the existence of different crystal field sites for the rare-earth-doped tin dioxide nanopowder and RE-doped SiO2-SnO2 glass-ceramics is investigated. The slightly different crystal field symmetries have been resolved by using site-selective fluorescence line-narrowing spectroscopy. The obtained results show that a variety of optically non equivalent sites exist for the europium ion in the tin dioxide oxide structure associated to different allowed positions of the oxygen vacancies, whereas additional spectral disorder is found in the case of the glass-ceramic matrix. Ultrafast spectroscopy performed on Eu3+-doped tin dioxide nanocrystals shows that host-rare earth energy transfer occurs at a transfer rate of about 1.5×106 s-1. Similar experiments carried out for the Er3+-doped glass-ceramic system also validate the hypothesis that both host and matrix-excited RE emissions are decoupled due to the different origins of the involved physical mechanisms.

Cs-based perovskite nanocrystals (PNCs) possess alluring optoelectronic properties through compositional and structural versatility, tunable bandgap, high photoluminescence (PL) quantum yield (QY) and facile chemical synthesis. However, PL properties of solid-state samples suffer from environmental and operational degradation. Here we report alumina (AlOx) encapsulation of 0D Cs4PbBr6 nanocrystal thin films and individual nanoparticles using a modified atomic layer deposition (ALD) method with concurrent exposure of both Al and water precursors in the gas flow. We observed stronger PL intensity, increased PL lifetimes and much improved long-term stability at both film and single PNC level. These findings provide roadmap for ALD utilization to create solid-state perovskite devices.

In this work, we probe the photodegradative behaviour of CsPbBr3 perovskite nanocrystals under illumination intensities in excess of 1 W=cm2. In doing so, we uncover optical behaviours unique to this extreme form of degradation namely a pronounced period of increasing photoluminescent intensity at the outset of degradation along with a red-shifted emission lobe. We also compare the photochemical lifetimes of CsPbBr3 to the relating organic-inorganic hybrid of FAPbBr3 and show that FAPbBr3 can withstand such high intensities for approximately ten times longer than CsPbBr3. This marks out FAPbBr3 as a potential successor to CsPbBr3 in optoelectronic applications.

Establishing the optical refrigeration of semiconductors remains a longstanding goal due to potential applications in optoelectronics. Apart from stringent materials requirements, required to realize condensed phase laser cooling, namely the need to have near unity emission quantum yields, a practical challenge involves accurately measuring specimen temperatures in a non-contact fashion. Common all-optical approaches developed in response to this need include: pump– probe luminescence thermometry (PPLT) and differential luminescence thermometry (DLT). In this study, we compare and contrast PPLT and DLT to a newly developed up-conversion emission thermometry to establish the most robust approach for measuring semiconductor nanocrystal (NC) temperatures. Using high external quantum efficiency CdSe/CdS core/shell NCs, we reveal that up-conversion emission thermometry possesses higher accuracy than either PPLT or DLT. Up-conversion emission thermometry can also be used on specimens such as CsPbBr3 NCs with temperature-insensitive band gaps.

Coating inorganic nanocrystals (e.g., quantum dots and gold nanoparticles) with polymer ligands presenting many lipoic acid (LA) anchoring groups provides them with excellent colloidal stability in aqueous media. Here we exploit the natural swelling of polymer macromolecules, which imposes a configuration that leaves a fraction of the anchors on the polymerstabilized nanocolloids free or uncoordinated and target them for conjugation using thiol-to-maleimide chemistry. This allows easy surface functionalization of the nanocrystals, without the need to introduce additional reactive groups. We apply a photoligation strategy to coat QDs and AuNPs, followed by coupling with maleimide-modified dyes. We then use optical absorption and resonance energy transfer measurements, to extract estimates for the fraction of accessible LAs per nanocrystal. To further prove the effectiveness of this approach, we construct a ratiometric pH sensing probe made of QDSNARF conjugates. The combination of the multi-coordinating ligand design and in-situ photoligation yields colloidally stable nanocrystals, presenting several thiol reactive sites. Our results are promising and could advance the integration of nanomaterials in biological sensing and imaging applications.

Highly dispersed and uniform α-NaYF4 micron flowers with about 500nm size was synthesized by a simple wet-chemical at a low temperature, then NaYF4:Eu3+@Au hybrid structures with various amount Au nanoparticles (Au NPs) are prepared and single NaYF4:Eu3+@Au hybrid particle is taken to investigate the influence of noble metal nanoparticle on the luminescence emission and present corresponding mechanism. It is found that luminescence emissions of NaYF4:Eu3+ was quenched by introducing Au nanoparticles, and with increase of Au nanoparticles, the fluorescence quenching was more and more serous while emission intensity ratio of electric dipole to magnetic dipole transition of Eu3+ gradually become larger.

Rare-earth doped NaLaF4 nanocrystals were synthesized by solvothermal method in ethanol/oleic acid/water reaction system. The structure and porphology of samples were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM) and transmission electron microscopy(TEM). By adjusting the amount of NaOH in the precursor, we obtained NaLaF4:Er3+/Yb3+/Gd3+nanocrystals with a rich variety of morphologies, including nanoprisms, nano-dumbbells, nanorods and nanowires. A possible mechanism of formation of the NaLaF4 nanocrystals was proposed based on the time dependent experiments. The upconversion luminescence properties of rare-earth doped NaLaF4 nanocrystals with different morphology was discussed. The experimental results demonstrated that the NaLaF4:Er3+/Yb3+/Gd3+ nanocrystals with different morphologies exhibited similar up-conversion luminescence, with sharp emission peaks near 540 nm and 650 nm, and red color stronger than green light.

This work represents an effective method for synthesis and doping of ZnSe nanocrystals with elements of group 3 and transition elements. The structure of crystals and their average size are determined. The spectra of optical density and photoluminescence were studied, and a comparative analysis of characteristics of the obtained nanoparticles with their bulk analogs was carried out.

Ordered nanostructured crystals of thin perovskites films are of great interest to researchers because of the dimensional-dependence of their photoelectronic properties for developing the perovskites with novel properties. In this presentation, both top-down and bottom-up approaches for fabricating nanostructured perovskite films are demonstrated. First, a variety of micro/nanopatterns of a perovskite film are fabricated by either microimprinting or transfer-printing a thin spin-coated precursor film in soft-gel state with a topographically pre-patterned polymer mold, followed by thermal treatment for complete conversion of the precursor film to a perovskite one. Second, we also demonstrate a simple and robust route, involving the controlled crystallization of the perovskites templated with a self-assembled block copolymer (BCP), for fabricating nanopatterned perovskite films with various shapes and nanodomain sizes. When the precursor ion solution of a perovskite and poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) was spin-coated on the substrate, a nanostructured BCP was developed by microphase separation. Spontaneous crystallization of the precursor ions preferentially coordinated with the P2VP domains yielded ordered nanocrystals with various nanostructures. The nanopatterned perovskites showed significantly enhanced photoluminescence (PL) with high resistance to both humidity and heat due to geometrically confining crystals in and passivation with the P2VP chains. The self-assembled perovskite films with high PL performance provided a facile control of color coordinates by color conversion layers in blue-emitting devices for cool-white emission.

Lead-halide perovskites are currently the highest-performing solution-processable semiconductors for solar energy conversion, with record efficiencies rapidly approaching that of the Shockley-Queisser limit for single-junction solar cells. Further progress in the development of lead-halide perovskite solar cells must overcome this limit, which largely stems from the ultrafast relaxation of high-energy hot carriers above the bandedge. In this contribution, we use a highly-specialized pump-push-probe technique to unravel the key parameters which control hot carrier cooling in bulk and nanocrystal (NC) lead bromide perovskites with different material composition, NC diameter and surface treatment. All samples exhibit slower cooling for higher hot carrier densities, which we assign to a phonon bottleneck mechanism. By comparing this density-dependent cooling behavior in the different samples, we find that the weak quantum confinement of electronic states and the surface defects in the NCs play no observable role in the hot carrier relaxation. Meanwhile, in accordance with our previous observations for bulk perovskites, we show that the cation plays a critical role towards carrier cooling in the perovskite NCs, as evidenced by the faster overall cooling in the hybrid FAPbBr3 NCs with respect to the all-inorganic CsPbBr3 NCs. These observations highlight the crucial role of the cations toward the phononic properties of lead-halide perovskites, and further point towards the defect tolerance of these emerging solution-processed semiconductors.

The Luminescent Solar Concentrator (LSC) is a photovoltaics concept that can easily be used for integration in buildings. As a window component the transmitted spectrum of the LSC affects the color quality of daylight in the interior of a building. We evaluate eight state of the art nanocrystal luminophores as candidates for LSC windows applications, using ray-trace Monte Carlo simulations. LSCs are assessed with respect to different color quality criteria, using transparency ranging from 90% to 50%. We find that LSC device efficiency is inversely proportional to transparency. Also, luminophores with a wide absorption bandwidth in the visible have higher color temperatures and high color rendering index, leading to good visual comfort at appreciable device efficiency.

Lead-halide perovskite nanocrystals (NCs) have just emerged as a novel type of semiconductor nanostructure possessing great potentials in the optoelectronic, photovoltaic and quantum-information-processing applications. This renders it extremely necessary to have a comprehensive understanding of their electronic energy-level structures, which mysteriously exhibit either a doublet or a triplet exciton peak at the single-particle level. Here we show that transition from doublet to triplet excitons in single CsPbI3 NCs can be triggered by reinforcing quantum confinement in the same batch of sample upon being stored in the ambient environment. Besides size reduction and blue-shifted emission, this enhanced quantum confinement is also manifested by the suppressed emission of multiple and charged excitons in single CsPbI3 NCs with a triplet-exciton configuration. We propose that the doublet and triplet excitons should correspond respectively to the weak and strong quantum confinement regimes of single CsPbI3 NCs, with the electron-hole exchange interaction and the Rashba effect determining the exact energy-level alignments and the fine-structure splitting values.

Wide-band gap semiconductor nanoparticles has been the focus of interest recently, due to their validity for energy creations, decomposition of harmful substances, boosting useful chemical reactions etc. In this work, we will evaluate optical characteristics of a single semiconductor nanoparticle via broadband-UV Rayleigh scattering spectroscopy and photoluminescence (PL) spectroscopy. Rayleigh scattering spectroscopy reveal the bandgap energies while PL spectroscopy provide the information on exciton generation efficiencies as well as existence of surface defects. In our microscopy setup, a broadband white light source (LDLS) was collimated and obliquely illuminated on the sample to realize dark-field illumination to distinguish the position of individual particles in the microscopic image. Scattering from a single nanocrystal was collected by an reflection-type objective lens (NA0.5) and introduced to a spectrometer and detected by an EMCCD camera. The spectrometer was designed specifically for UV-DUV broadband spectroscopy and imaging. For photoluminescence (PL) measurements, we introduce 320 nm (CW) laser for excitation. The sample is enclosed in a temperature-controlled cell ranging from room temperature to 77K. We especially focus on titanium dioxide (TiO2), a typical photocatalyst, and tangusten trioxide (WO3) which is one of the candidate for decomposition of water into oxygen and hydrogen by a visible or longer wavelength light. The band structure of nano-particles is changed when the size is smaller than several tens of nanometers, due to crystallinity and quantum size effects. PL of single zinc oxide (ZnO) nanoparticles were also measured together with the temperature effects. The spectra obtained from a single nanoparticle is different from aggregates both for exciton PL and defects PL.

Devices based on small-gap mercury chalcogenide semiconductor nanocrystal inks have recently demonstrated increasingly high performance photodetection in the short and mid-wave infrared. These new colloidal inks are generating increasing interest because they could provide higher operating temperatures and vastly reduced costs compared to the current epitaxially-grown devices. However, in order to further increase detector operation temperatures and use these materials as infrared light sources, more detailed understandings of the carrier dynamics are required. Described here are picosecond mid-infrared absorption and emission studies of HgTe and HgSe colloidal quantum dots focusing on multicarrier nonradiative relaxation. Comparisons of the interband and intraband transitions in intrinsic and n-type systems reveal phenomena such as suppression of Auger relaxation in nanoparticles vs. bulk materials and slow or absent Auger relaxation in n-type quantum dots. Yet, the measured lifetimes are still limited by other nonradiative pathways that appear unique to small-gap nanomaterials. The deeper understanding of nonradiative relaxation in small-gap nanocrystals afforded by these experiments provides a path towards realizing high performance infrared photodetection near room temperature and robust mid-infrared light emission with colloidal quantum dots.