11 research outputs found

    Tunable Band Gap Emission and Surface Passivation of Germanium Nanocrystals Synthesized in the Gas Phase

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    The narrow bulk band gap and large exciton Bohr radius of germanium (Ge) make it an attractive material for optoelectronics utilizing band-gap-tunable photoluminescence (PL). However, realization of PL due to quantum confinement remains scarcely reported. Instead, PL is often observed from surface trap states and is independent of nanocrystal (NC) size. Here, we demonstrate tunable band gap PL by chemically passivating the Ge NC surface. The exchange of native Geā€“Cl surface groups with alkyl groups using Grignard reagents leads to the first instance of tunable band gap emission from free-standing Ge NCs synthesized in the gas phase. Ge NCs between 4.8 and 10.2 nm in diameter exhibit near-infrared emission featuring spectral line widths that are at least a factor of 2 narrower than any previous report

    Tunable Band Gap Emission and Surface Passivation of Germanium Nanocrystals Synthesized in the Gas Phase

    No full text
    The narrow bulk band gap and large exciton Bohr radius of germanium (Ge) make it an attractive material for optoelectronics utilizing band-gap-tunable photoluminescence (PL). However, realization of PL due to quantum confinement remains scarcely reported. Instead, PL is often observed from surface trap states and is independent of nanocrystal (NC) size. Here, we demonstrate tunable band gap PL by chemically passivating the Ge NC surface. The exchange of native Geā€“Cl surface groups with alkyl groups using Grignard reagents leads to the first instance of tunable band gap emission from free-standing Ge NCs synthesized in the gas phase. Ge NCs between 4.8 and 10.2 nm in diameter exhibit near-infrared emission featuring spectral line widths that are at least a factor of 2 narrower than any previous report

    Thermodynamic Driving Force in the Spontaneous Formation of Inorganic Nanoparticle Solutions

    No full text
    Nanoparticles are the bridge between the molecular and the macroscopic worlds. The growing number of commercial applications for nanoparticles spans from consumer products to new frontiers of medicine and next-generation optoelectronic technology. They are most commonly deployed in the form of a colloid, or ā€œinkā€, which are formulated with solvents, surfactants, and electrolytes to kinetically prevent the solid particulate phase from reaching the thermodynamically favored state of separate solid and liquid phases. In this work, we theoretically determine the thermodynamic requirements for forming a single-phase solution of spherical particles and engineer a model system to experimentally demonstrate the spontaneous formation of solutions composed of only solvent and bare inorganic nanoparticles. We show molecular interactions at the nanoparticle interface are the driving force in high-concentration nanoparticle solutions. The work establishes a regime where inorganic nanoparticles behave as molecular solutes as opposed to kinetically stable colloids, which has far-reaching implications for the future design and deployment of nanomaterial technologies

    Thermodynamic Driving Force in the Spontaneous Formation of Inorganic Nanoparticle Solutions

    No full text
    Nanoparticles are the bridge between the molecular and the macroscopic worlds. The growing number of commercial applications for nanoparticles spans from consumer products to new frontiers of medicine and next-generation optoelectronic technology. They are most commonly deployed in the form of a colloid, or ā€œinkā€, which are formulated with solvents, surfactants, and electrolytes to kinetically prevent the solid particulate phase from reaching the thermodynamically favored state of separate solid and liquid phases. In this work, we theoretically determine the thermodynamic requirements for forming a single-phase solution of spherical particles and engineer a model system to experimentally demonstrate the spontaneous formation of solutions composed of only solvent and bare inorganic nanoparticles. We show molecular interactions at the nanoparticle interface are the driving force in high-concentration nanoparticle solutions. The work establishes a regime where inorganic nanoparticles behave as molecular solutes as opposed to kinetically stable colloids, which has far-reaching implications for the future design and deployment of nanomaterial technologies

    Morphological Control of In<sub><i>x</i></sub>Ga<sub>1ā€“<i>x</i></sub>P Nanocrystals Synthesized in a Nonthermal Plasma

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    We explore the growth of In<sub><i>x</i></sub>Ga<sub>1ā€“<i>x</i></sub>P nanocrystals (<i>x</i> = 1, InP; <i>x</i> = 0, GaP; and 1 > <i>x</i> > 0, alloys) in a nonthermal plasma. By tuning the reactor conditions, we gain control over the morphology of the final product, producing either 10 nm diameter hollow nanocrystals or smaller 3 nm solid nanocrystals. We observe the gas-phase chemistry in the plasma reactor using plasma emission spectroscopy to understand the growth mechanism of the hollow versus solid morphology. We also connect this plasma chemistry to the subsequent native surface chemistry of the nanocrystals, which is dominated by the presence of both dative- and lattice-bound phosphine species. The dative phosphines react readily with oleylamine in an L-type ligand exchange reaction, evolving phosphines and allowing the particles to be dispersed in nonpolar solvents. Subsequent treatment by HF causes the solid InP<sub>1.5</sub> and In<sub>0.5</sub>Ga<sub>0.5</sub>P<sub>1.3</sub> to become photoluminescent, whereas the hollow particles remain nonemissive

    Dynamic Evolution of 2D Layers within Perovskite Nanocrystals via Salt Pair Extraction and Reinsertion

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    Metal halide perovskite (MHP) semiconductors exhibit unprecedented optoelectronic properties coupled with low formation energies that enable scalable, cost-efficient solution processing. The low formation energies additionally facilitate dynamic transformation of the chemical composition and crystal structure of the MHP material. In this work, we show that CsBr salt is selectively extracted from CsPbBr<sub>3</sub> nanocrystals (NCs) to yield PbBr<sub>2</sub> NCs. The PbBr<sub>2</sub> NCs are then exposed to different glacial acetic acid ABr salt solutions to generate a variety of emissive compounds with the generic structure Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub>, where A = cesium (Cs<sup>+</sup>), methylammonium (MA<sup>+</sup>), formamidinium (FA<sup>+</sup>); Aā€² = A or H<sup>+</sup>; Xā€² = Br<sup>ā€“</sup> or acetate (CH<sub>3</sub>COO<sup>ā€“</sup>); and <i>n</i> is the number of lead halide layers, where <i>n</i> = 1, 2, 3, ...āˆž. We systematically vary the ratios of PbBr<sub>2</sub>/ABr/CH<sub>3</sub>COOH and show that certain ratios result in isolable single-phase APbBr<sub>3</sub> NCsī—øan effective A-site cation exchange from the parent CsPbBr<sub>3</sub> NCs. Importantly, time-resolved photoluminescence (PL) spectroscopy shows the dynamic evolution of many additional species as evidenced by blue-shifted emission peaks from 2.85ā€“2.49 eV for MA<sup>+</sup>-based structures. We assign these species to <i>n</i> = 1, 2, 3, 4, and 5 quasi-two-dimensional network (2DN) sheets, in which CH<sub>3</sub>COO<sup>ā€“</sup> anions and Br<sup>ā€“</sup> anions compete for the <i>c</i>-axis Xā€² sites separating haloplumbateĀ­(II) layers within the Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub> NCs. Finally, we demonstrate the degree of CH<sub>3</sub>COO<sup>ā€“</sup> incorporation, and thus the 2DN layer thickness and PL energy, is controlled in the early reaction times by kinetic factors. After a longer time (3 h), thermodynamic forces dictated by Le Chatelierā€™s principle tune the structure in Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub> NCs from exclusively <i>n</i> = 1 to <i>n</i> = āˆž depending on the PbBr<sub>2</sub>/ABr/CH<sub>3</sub>COOH ratio

    Dynamic Evolution of 2D Layers within Perovskite Nanocrystals via Salt Pair Extraction and Reinsertion

    No full text
    Metal halide perovskite (MHP) semiconductors exhibit unprecedented optoelectronic properties coupled with low formation energies that enable scalable, cost-efficient solution processing. The low formation energies additionally facilitate dynamic transformation of the chemical composition and crystal structure of the MHP material. In this work, we show that CsBr salt is selectively extracted from CsPbBr<sub>3</sub> nanocrystals (NCs) to yield PbBr<sub>2</sub> NCs. The PbBr<sub>2</sub> NCs are then exposed to different glacial acetic acid ABr salt solutions to generate a variety of emissive compounds with the generic structure Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub>, where A = cesium (Cs<sup>+</sup>), methylammonium (MA<sup>+</sup>), formamidinium (FA<sup>+</sup>); Aā€² = A or H<sup>+</sup>; Xā€² = Br<sup>ā€“</sup> or acetate (CH<sub>3</sub>COO<sup>ā€“</sup>); and <i>n</i> is the number of lead halide layers, where <i>n</i> = 1, 2, 3, ...āˆž. We systematically vary the ratios of PbBr<sub>2</sub>/ABr/CH<sub>3</sub>COOH and show that certain ratios result in isolable single-phase APbBr<sub>3</sub> NCsī—øan effective A-site cation exchange from the parent CsPbBr<sub>3</sub> NCs. Importantly, time-resolved photoluminescence (PL) spectroscopy shows the dynamic evolution of many additional species as evidenced by blue-shifted emission peaks from 2.85ā€“2.49 eV for MA<sup>+</sup>-based structures. We assign these species to <i>n</i> = 1, 2, 3, 4, and 5 quasi-two-dimensional network (2DN) sheets, in which CH<sub>3</sub>COO<sup>ā€“</sup> anions and Br<sup>ā€“</sup> anions compete for the <i>c</i>-axis Xā€² sites separating haloplumbateĀ­(II) layers within the Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub> NCs. Finally, we demonstrate the degree of CH<sub>3</sub>COO<sup>ā€“</sup> incorporation, and thus the 2DN layer thickness and PL energy, is controlled in the early reaction times by kinetic factors. After a longer time (3 h), thermodynamic forces dictated by Le Chatelierā€™s principle tune the structure in Aā€²<sub>2</sub>A<sub><i>n</i>ā€“1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā€“1</sub>Xā€²<sub>2</sub> NCs from exclusively <i>n</i> = 1 to <i>n</i> = āˆž depending on the PbBr<sub>2</sub>/ABr/CH<sub>3</sub>COOH ratio

    Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

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    The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D <sup>1</sup>Hā€“<sup>29</sup>Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D <sup>1</sup>Hā€“<sup>29</sup>Si HETCOR and dipolar 2D <sup>1</sup>Hā€“<sup>1</sup>H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of <sup>1</sup>H and <sup>29</sup>Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected <sup>1</sup>Hā€“<sup>29</sup>Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surface hydrides. Varying the <sup>29</sup>Si evolution time in refocused INEPT experiments and fitting the oscillation of the NMR signals allows for the relative populations of the different surface hydrides to be estimated. This analysis confirms that monohydride species are the predominant surface species on the as-synthesized Si NCs. A reduction in the populations of the di- and trihydrides is observed upon functionalization with alkyl groups, consistent with our previous hypothesis that the trihydride, or silyl (*SiH<sub>3</sub>), group is primarily responsible for initiating surface functionalization reactions. Density functional theory (DFT) calculations were used to obtain quantum chemical structural models of the Si NC surface and reproduce the observed <sup>1</sup>H and <sup>29</sup>Si chemical shifts. The approaches outlined here will be useful to obtain a more detailed picture of surface structures for Si NCs and other hydride-passivated nanomaterials

    Broadband Absorbing Excitonā€“Plasmon Metafluids with Narrow Transparency Windows

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    Optical metafluids that consist of colloidal solutions of plasmonic and/or excitonic nanomaterials may play important roles as functional working fluids or as means for producing solid metamaterial coatings. The concept of a metafluid employed here is based on the picture that a single ballistic photon, propagating through the metafluid, interacts with a large collection of specifically designed optically active nanocrystals. We demonstrate water-based metafluids that act as broadband electromagnetic absorbers in a spectral range of 200ā€“3300 nm and feature a tunable narrow (āˆ¼100 nm) transparency window in the visible-to-near-infrared region. To define this transparency window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped silicon nanocrystals as opaque optical absorbers (ā€œoptical wallā€) in the UV and blue-green range of the spectrum. Water itself acts as an opaque ā€œwallā€ in the near-infrared to infrared. We explore the limits of the concept of a ā€œsimpleā€ metafluid by computationally testing and validating the effective medium approach based on the Beerā€“Lambert law. According to our simulations and experiments, particle aggregation and the associated decay of the window effect are one example of the failure of the simple metafluid concept due to strong interparticle interactions

    Degradation of Highly Alloyed Metal Halide Perovskite Precursor Inks: Mechanism and Storage Solutions

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    Whereas the promise of metal halide perovskite (MHP) photovoltaics (PV) is that they can combine high efficiency with solution-processability, the chemistry occurring in precursor inks is largely unexplored. Herein, we investigate the degradation of MHP solutions based on the most widely used solvents, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). For the MHP inks studied, which contain formamidinium (FA<sup>+</sup>), methylammonium (MA<sup>+</sup>), cesium (Cs<sup>+</sup>), lead (Pb<sup>2+</sup>), bromide (Br<sup>ā€“</sup>), and iodide (I<sup>ā€“</sup>), dramatic compositional changes are observed following storage of the inks in nitrogen in the dark. We show that hydrolysis of DMF in the precursor solution forms dimethylammonium formate, which subsequently incorporates into the MHP film to compromise the ability of Cs<sup>+</sup> and MA<sup>+</sup> to stabilize FA<sup>+</sup>-based MHP. The changes in solution chemistry lead to a modification of the perovskite film stoichiometry, band gap, and structure. The solid precursor salts are stable when ball-milled into a powder, allowing for the storage of large quantities of stoichiometric precursor materials
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