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Carrier spins in semiconductor nanocrystals are promising candidates for quantum information processing. Using a combination of time-resolved Faraday rotation and photoluminescence spectroscopies, we demonstrate optical spin polarization and coherent spin precession in colloidal CsPbBr3 nanocrystals that persists up to room temperature. By suppressing the influence of inhomogeneous hyperfine fields with a small applied magnetic field, we demonstrate inhomogeneous hole transverse spin-dephasing times (T2*) that approach the nanocrystal photoluminescence lifetime, such that nearly all emitted photons derive from coherent hole spins. Thermally activated LO phonons drive additional spin dephasing at elevated temperatures, but coherent spin precession is still observed at room temperature. These data reveal several major distinctions between spins in nanocrystalline and bulk CsPbBr3 and open the door for using metal-halide perovskite nanocrystals in spin-based quantum technologies., Comment: Accepted for publication at Nano Letters, 11/19/20

Yb3+-doped inorganic metal-halide perovskites (Yb3+:CsPbX3, X = Cl, Br) have recently been discovered to display highly efficient quantum cutting, in which the energy from individual blue or UV photons absorbed by the material is re-emitted in the form of pairs of near-infrared photons by Yb3+ dopants. Experimental photoluminescence quantum yields approaching 200{%} have been reported. As the first quantum-cutting materials that combine such high photoluminescence quantum yields with strong, broadband absorption in the visible, these materials offer unique opportunities for enhancing the efficiencies of solar technologies. Little is known about the fundamental origins of this quantum cutting, however. Here, we describe variable-temperature and time-resolved photoluminescence studies of Yb3+:CsPbCl3 in two disparate forms - colloidal nanocrystals and macroscopic single crystals. Both forms show very similar spectroscopic properties, demonstrating that quantum cutting is an intrinsic property of the Yb3+:CsPbX3 composition itself. Diverse Yb3+ speciation is observed in both forms by low-temperature photoluminescence spectroscopy, but remarkably, quantum cutting is dominated by the same specific Yb3+ species in both cases. Time-resolved photoluminescence measurements provide direct evidence of the previously hypothesized intermediate state in the quantum-cutting mechanism. This intermediate state mediates relaxation from the photogenerated excited state of the perovskite to the emissive excited state of Yb3+, and hence is of critical mechanistic importance. At room temperature, this intermediate state is populated within a few picoseconds and has a decay time of only ~ 7 ns in both nanocrystalline and single-crystal Yb3+:CsPbCl3. The mechanistic implications of these observations are discussed., Comment: Physical Review Materials, accepted for publication

The ability to produce nanowires through vapor- and solution-based processes has propelled nanowire material systems toward a wide range of technological applications. Conventional, vapor-based nanowire syntheses have enabled precise control over nanowire composition and phase. However, vapor-based nanowire growth employs batch processes with specialized pressure management systems designed to operate at high temperatures, limiting throughput. More recently developed solution-based nanowire growth processes have improved scalability but can require even more extensive pressure and temperature management systems. Here, we demonstrate a continuous-flow, solution-based nanowire growth process that utilizes the large Young-Laplace interfacial surface pressures and collective heating effects of colloidal metal nanocrystals under irradiation to drive semiconductor nanowire growth photothermally without the need for high-pressure or high-temperature equipment. In this process, a laser irradiates a solution containing metal nanocrystals and semiconductor precursors. Upon light absorption, the metal nanocrystals heat rapidly, inducing semiconductor precursor decomposition and nanowire growth. This process is performed on a benchtop in simple glassware under standard conditions. To demonstrate the generality of this technique, we synthesized three distinct semiconductor nanowire material systems: bismuth-seeded germanium nanowires, bismuth-seeded cadmium selenide nanowires, and indium-seeded germanium nanowires. The simplicity and versatility of this process opens the door to a range of experiments and technologies including in-line combinatorial identification of optimized reaction parameters, in situ spectroscopic measurements to study solution-based nanowire growth, and the potential production of nanowires with complex compositions or rationally incorporated dopants., Comment: 46 pages, 14 figures, submitted to Proceedings of the National Academy of Sciences of the United States of America

The development of color centers in diamond as the basis for emerging quantum technologies has been limited by the need for ion implantation to create the appropriate defects. We present a versatile method to dope diamond without ion implantation, by synthesis of a doped amorphous carbon precursor and transformation at high temperatures and high pressures. To explore this bottom-up method for color center generation, we rationally create silicon-vacancy defects in nanodiamond and investigate them for optical pressure metrology. In addition, we show that this process can generate noble gas defects within diamond from the typically-inactive argon pressure medium, which may explain the hysteresis effects observed in other high pressure experiments and the presence of noble gases in some meteoritic nanodiamonds. Our results illustrate a general method to produce color centers in diamond, and may enable the controlled generation of designer defects., Comment: 12 pages, 4 figures

Nanodiamonds have emerged as promising materials for quantum computing, biolabeling, and sensing due to their ability to host color centers with remarkable photostability and long spin-coherence times at room temperature. Recently, a bottom-up, high-pressure, high-temperature (HPHT) approach was demonstrated for growing nanodiamonds with color centers from amorphous carbon precursors in a laser-heated diamond anvil cell (LH-DAC) that was supported by a near-hydrostatic noble gas pressure medium. However, a detailed understanding of the photothermal heating and its effect on diamond growth, including the phase conversion conditions and the temperature-dependence of color center formation, has not been reported. In this work, we measure blackbody radiation during LH-DAC synthesis of nanodiamond from carbon aerogel to examine these temperature-dependent effects. Blackbody temperature measurements suggest that nanodiamond growth can occur at 16.3 GPa and 1800 K. We use Mie theory and analytical heat transport to develop a predictive photothermal heating model. This model demonstrates that melting the noble gas pressure medium during laser heating decreases the local thermal conductivity to drive a high spatial resolution of phase conversion to diamond. Finally, we observe a temperature-dependent formation of nitrogen vacancy centers and interpret this phenomenon in the context of HPHT carbon vacancy diffusion using CB{\Omega} theory., Comment: 5 Figures, 1 Table, Submitted

Advances in cryogenic sciences have enabled several observations of new low-temperature physical phenomena including superconductivity, superfluidity, and Bose-Einstein condensates. Heat transfer is also critical in numerous applications including thermal management within integrated microelectronics and the regulation of plant-growth and development. Here we demonstrate that single-beam laser-trapping can be used to induce and quantify the local refrigeration of aqueous media through analysis of the cold Brownian dynamics of individual Yb3+-doped yttrium lithium fluoride (YLF) crystals in an inhomogeneous temperature field via forward light scattering and back-focal-plane interferometry. A tunable, NIR continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm2. Heat is transported out of the crystal lattice (across the solid / liquid interface) by anti-Stokes photoluminescence following upconversion of Yb3+ excited states mediated by optical-phonon absorption. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 C below ambient conditions suggesting a range of potential future applications., Comment: 9 pages, 7 figures, preprint