Lead-halide perovskite nanocrystals are promising candidates for semiconductor laser cooling due to their near-unity photoluminescence quantum yields and efficient photon/phonon up-conversion process. This unexpected, efficient sub-gap energy up-conversion implies an unexpectedly strong electron-phonon interaction in perovskite nanocrystals. However, the underlying mechanism remains mostly unclear. Detailed experiments, along with theory, have now been conducted to elucidate the efficient up-conversion in CsPbBr3 NCs, utilizing a combination of techniques: photothermal absorption spectroscopy, up-conversion detuning spectroscopy, and ultrafast transient differential absorption spectroscopy.
Cesium lead bromide nanocrystals, in contrast to most other materials, exhibit near-unity photoluminescence quantum yields (PLQY). When excited below the band gap, they absorb the photons and show anti-Stokes photoluminescence (ASPL), emitting higher energy, band-gap photons. Simultaneous existence of near-unity PLQY and ASPL can be used to optically cool these materials. In this talk, I will report near-unity ASPL efficiencies in CsPbBr3 nanocrystals and attribute it to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals.
Achieving condensed phase optical refrigeration requires near-unity emission quantum yields (QYs). Colloidal CsPbBr3 nanocrystals (NCs) are promising candidates in this respect given near unity QY-values, achieved by post-synthetic surface treatment with quaternary ammonium bromide ligands. The origin of these QY enhancements, however, is not understood. Systematic nuclear magnetic resonance studies of the organic ligand passivation of near unity QY CsPbBr3 NC surfaces are therefore conducted to better reveal their surface-ligand interactions.
CsPbBr3 perovskite nanocrystals have been identified as a potential medium to realize condensed phase optical refrigeration. This is due to its near unity emission quantum yields and efficient anti-Stokes photoluminescence (ASPL). Despite much work on CsPbBr3’s optical response, the origin of its efficient ASPL remains unclear. We conduct detailed optical spectroscopy measurements in conjunction with theory to establish mechanistic insights into CsPbBr3’s up-conversion process. Experimental techniques utilized include: temperature-dependent and detuning energy-dependent ASPL measurements, temperature-tunable photothermal heterodyne absorption spectroscopy, and ultrafast transient differential absorption (TDA) spectroscopy.
Infrared photothermal heterodyne imaging (IR-PHI) is an ultrasensitive technique capable of achieving super-resolution chemical and morphological characterization of specimens via absorption of mid-infrared light. While early iterations of IR-PHI have involved point-by-point raster-scanning, here, we introduce a widefield modality to IR-PHI that utilizes ns-timescale infrared pump pulses synchronized to an ultrafast complementary metal-oxide-semiconductor camera to parallelize data acquisition. A 300-fold decrease in image acquisition time is realized, falling from 20 minutes to four seconds.
Infrared photothermal heterodyne imaging (IR-PHI) is an established all-optical, table-top approach for conducting super-resolution mid-infrared microscopy and spectroscopy on submicrometer-sized particles. The instrument’s capabilities are highlighted by its ability to operate in spectroscopically-crowded environments. This includes specimens obtained from environmental matrices where particulates with different morphologies, chemical compositions, and abundances exist. Here, proof-of-concept IR-PHI measurements have been conducted on anthropogenic micro- and nanoplastics (MNPs) derived from the breakdown of consumer products. In particular, IR-PHI is used to characterize MNPs extracted from steeped plastic teabags and floor dust from a household vacuum. IR-PHI results reveal the presence of complex MNP structures made of polyamide fibers and acrylonitrile butadiene styrene MNPs.
Semiconductor nanocrystals (NCs) are potential materials for verifiable demonstrations of semiconductor-based laser cooling. The key feature that makes NCs appealing for laser cooling is their near unity emission quantum yields (QYs). An unresolved issue regarding NC QYs, however, centers on the existence of an excitation energy dependent (EED) QY. Here, we study EED QYs on three NC systems, aimed at demonstrating NC-based laser cooling (CsPbBr3, CsPbI3, and CdSe/CdS core/shell NCs). We evaluate the impact of EED QYs using two approaches. The first involves direct QY measurements using an integrating sphere. The second entails photoluminescence excitation spectroscopy where changes to NC QYs with excitation energy can be assessed qualitatively.
CsPbBr3 nanocrystals possess optical properties amenable to achieving verifiable condensed phase laser cooling. This includes near unity emission quantum yields, a high tolerance to defects, and efficient up-conversion. We have previously demonstrated emission QYs that approach unity as well as up-conversion efficiencies that range from ~30% to 75%, depending on sub gap excitation energy. Emission up-conversion is also seen at the individual nanocrystal level. In this presentation, I will discuss our latest results on evaluating CsPbBr3 nanocrystals for the purpose of demonstrating condensed phase laser cooling.
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.
Infrared photothermal heterodyne imaging (IR-PHI) represents a convenient, table top approach for conducting super-resolution imaging and spectroscopy throughout the all-important mid infrared (MIR) spectral region. Although IR-PHI provides label-free, super-resolution MIR absorption information, it is not quantitative. In this study, we establish quantitative relationships between observed IR-PHI signals and relevant photothermal parameters of investigated specimens. Specifically, we conduct a size series analysis of different radii polystyrene (PS) beads to quantitatively link IR-PHI signal contrast to specimen heat capacity, thermo-optic coefficient, MIR peak absorption cross-section, and scattering cross-section at IR-PHI’s probe wavelength.
Much effort has gone into realizing laser cooling with solids over the last two decades. Multiple attempts have been made with systems that include rare-earth doped glasses, GaAs heterostructures, CdS nanobelts and hybrid perovskite nanoplatelets. Here we suggest that CsPbBr3 perovskite nanocrystals may eventually lead to verifiable demonstrations of condensed phase laser cooling. The highest emission quantum yield we have realized in CsPbBr3 nanocrystal ensembles is 99.9% at room temperature. This value lies above the critical quantum yield for CsPbBr3, needed to realize laser cooling. We also find that associated CsPbBr3 nanocrystal emission up-conversion efficiencies are large and are 75% and 32% for laser detuning energies of 24 meV and 105 meV.
The concept of condensed phase optical cooling has existed for nearly 90 years ever since Pringsheim proposed a conceptual approach for cooling solids through the annihilation of phonons via phonon-assisted photoluminescence (PL) up-conversion. In this process, energy is removed from the solid by the emission of photons with energies larger than those of incident photons. However, actually realizing optical cooling requires exacting parameters from the condensed phase medium such as near unity external quantum efficiencies as well as low background absorption. Until recently, solid state laser cooling has only been successfully realized in rare earth-doped solids.
In semiconductors, optical cooling has very recently been demonstrated in cadmium sulfide (CdS) nanobelts. Large internal quantum efficiencies, sub-wavelength thicknesses, which decrease light trapping, and low background absorption, all make near unity external quantum yields possible. Net cooling by as much as 40 K has therefore been possible with CdS nanobelts.
In this study, we describe a detailed investigation of the nature of efficient anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature-dependent PL up-conversion and optical absorption studies on individual NBs together with frequency-dependent up-converted PL intensity spectroscopies suggest that ASPL in CdS nanobelts is defect-mediated via the involvement of donor-acceptor states.
The concept of optical cooling of solids has existed for nearly 90 years ever since Pringsheim proposed a way to cool solids through the annihilation of phonons via phonon-assisted photoluminescence (PL) up-conversion. In this process, energy is removed from the solid by the emission of photons with energies larger than those of incident photons. However, actually realizing optical cooling requires exacting parameters from the condensed phase medium such as near unity external quantum efficiencies as well as existence of a low background absorption. Until recently, laser cooling has only been successfully realized in rare earth doped solids.
In semiconductors, optical cooling has very recently been demonstrated in cadmium sulfide (CdS) nanobelts as well as in hybrid lead halide perovskites. For the former, large internal quantum efficiencies, sub-wavelength thicknesses, which decrease light trapping, and low background absorption, all make near unity external quantum yields possible. Net cooling by as much as 40 K has therefore been possible with CdS nanobelts.
In this study, we describe a detailed investigation of the nature of efficient anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature-dependent PL up-conversion and optical absorption studies on individual NBs together with frequency-dependent up-converted PL intensity spectroscopies suggest that ASPL in CdS nanobelts is defect-mediated through involvement of defect levels below the band gap.
Photothermal microscopy has achieved single molecule sensitivity. However, the analytes are usually restricted to be natural absorbers in the visible light region. Mid-infrared (MIR) imaging, on the other hand, provides a wealth of information, but encounters difficulties of diffraction-limited spatial resolution and scarcity of ideal detectors. Here we present Mid-IR photothermal heterodyne imaging (MIR-PHI) microscopy as a high sensitivity, super-resolution mid-IR imaging technique. In MIR-PHI, a tunable Mid- IR pulsed laser at 150 kHz is used to excite a micron sized particle. Energy relaxation creates a temperature gradient around the particle, changing the refractive index of the surrounding solvent and creating a thermal lens. A collinear, counter propagating probe beam (a 532 nm CW laser) is modified by the thermal lens and generates a super-resolution photothermal image. We studied 1.1 μm polystyrene beads at the single particle level using this technique. Various solvents with different heat capacities and refractive indices are tested for the best image contrast. The wide applicability and potentially high sensitivity of this technique make it promising for biological imaging and identification.
The solution phase synthesis of narrow diameter (< 10 nm) CdSe NWs is described. Crystalline NWs with lengths between 1-10 mm are obtained using a seeded solution approach, whereby NW growth is catalyzed by Au/Bi core/shell NPs. A gold biphasic reduction step results in 1.5 (3) nm diameter Au NPs and is followed by the thermolysis of trialkylbismuthines to yield low melting, bimetallic particles with diameters less than 3 nm. These Au/Bi NPs are catalytically active towards the growth of similar diameter CdSe NWs (~7 nm) that exhibit unique quantum confinement effects since the bulk exciton Bohr radius of CdSe is 5.6 nm. Manipulating the Cd:Se ratio results in a transition from straight to branched NWs, yielding v-shapes, tripods, y-shapes, as well as higher order structures. Structural characterization shows NW growth along either the [111] or [0001] directions of zinc blende (ZB) and wurtzite (W) phases respectively for both straight and branched NWs. High resolution TEM imaging reveals that the NWs alternate between ZB and W along their length. A similar reaction scheme can be used to produce PbSe NWs with diameters less than 5 nm, demonstrating the generality of the technique.
We describe the synthesis of colloidal mercury chalcogenide quantum dots (QDs) using a combination of strong Hg(II) coordinating ligands and precursor phase separation. This synthetic strategy provides a means of controlling the growth kinetics of mercury based II-VI QDs and addresses some of the problems which have heretofore made the synthesis of such compounds difficult. In particular, the simultaneous use of mercury coordinating ligands and precursor phase separation overcomes both the rapid precipitation of bulk mercury chalcogenides that occurs when only weak ligands are used and the reduction of Hg(II) when a strong ligand/high temperature combination is pursued. In the case of both HgS and HgSe this scheme has yielded one of the first examples of mercury chalcogenide QDs to date. The linear absorption/emission of HgS is size-dependent and ranges from 500 nm to 800 nm with corresponding sizes between 1 to 5 nm in diameter. For HgSe the band edge absorption/emission are also size dependent, ranging from 600 to 900 nm. The zincblende phase of both HgS and HgSe QDs is determined from wide angle x-ray diffraction experiments and reveals potentially large (band edge) spectral tunabilities for either material given their zero or slightly negative (bulk) band gaps.
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