11 research outputs found
Tunable Band Gap Emission and Surface Passivation of Germanium Nanocrystals Synthesized in the Gas Phase
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 GeāCl 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
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 GeāCl 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
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 āinkā, 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
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 āinkā, 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
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
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
Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<sub>2</sub>, where A = cesium (Cs<sup>+</sup>), methylammonium (MA<sup>+</sup>), formamidinium (FA<sup>+</sup>); Aā² = A or H<sup>+</sup>; Xā² = 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, ...ā.
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> NCsīøan 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 2.85ā2.49
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 Xā²
sites separating haloplumbateĀ(II) layers within the Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<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 Chatelierās
principle tune the structure in Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<sub>2</sub> NCs from exclusively <i>n</i> = 1 to <i>n</i> = ā 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
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
Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<sub>2</sub>, where A = cesium (Cs<sup>+</sup>), methylammonium (MA<sup>+</sup>), formamidinium (FA<sup>+</sup>); Aā² = A or H<sup>+</sup>; Xā² = 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, ...ā.
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> NCsīøan 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 2.85ā2.49
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 Xā²
sites separating haloplumbateĀ(II) layers within the Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<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 Chatelierās
principle tune the structure in Aā²<sub>2</sub>A<sub><i>n</i>ā1</sub>Pb<i><sub>n</sub></i>Br<sub>3<i>n</i>ā1</sub>Xā²<sub>2</sub> NCs from exclusively <i>n</i> = 1 to <i>n</i> = ā 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
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 ExcitonāPlasmon Metafluids with Narrow Transparency Windows
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 200ā3300 nm
and feature a tunable narrow (ā¼100 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 (āoptical
wallā) in the UV and blue-green range of the spectrum. Water
itself acts as an opaque āwallā in the near-infrared
to infrared. We explore the limits of the concept of a āsimpleā
metafluid by computationally testing and validating the effective
medium approach based on the BeerāLambert 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
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