26 research outputs found
Plasmonic Properties of Silicon Nanocrystals Doped with Boron and Phosphorus
Degenerately doped silicon nanocrystals
are appealing plasmonic materials due to siliconās low cost
and low toxicity. While surface plasmonic resonances of boron-doped
and phosphorus-doped silicon nanocrystals were recently observed,
there currently is poor understanding of the effect of surface conditions
on their plasmonic behavior. Here, we demonstrate that phosphorus-doped
silicon nanocrystals exhibit a plasmon resonance immediately after
their synthesis but may lose their plasmonic response with oxidation.
In contrast, boron-doped nanocrystals initially do not exhibit plasmonic
response but become plasmonically active through postsynthesis oxidation
or annealing. We interpret these results in terms of substitutional
doping being the dominant doping mechanism for phosphorus-doped silicon
nanocrystals, with oxidation-induced defects trapping free electrons.
The behavior of boron-doped silicon nanocrystals is more consistent
with a strong contribution of surface doping. Importantly, boron-doped
silicon nanocrystals exhibit air-stable plasmonic behavior over periods
of more than a year
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
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
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
Surface Structure and Silicon Nanocrystal Photoluminescence: The Role of Hypervalent Silyl Groups
We report a combined experimental
and theoretical study of the
relationship between the surface structure of silicon nanocrystals
synthesized in a nonthermal plasma reactor and their photoluminescence
(PL) yields. Upon heating to 160 Ā°C, a significant change in
the SiH stretch region of the vibrational spectrum is observed indicating
a decrease in surface SiH<sub>3</sub> groups, which correlates with
an increase in the PL yield. Effusion of SiH<sub><i>x</i></sub> and Si<sub>2</sub>H<sub>2<i>x</i></sub> from the
material is detected by residual gas analysis upon heating to temperatures
below 200 Ā°C, suggesting a weakly bound species. Analysis of
electron paramagnetic resonance spectra before and after heating points
to a small reduction in the density of dangling bonds upon heating
but this reduction does not correlate with the increase in PL yield.
Electronic structure calculations indicate that SiH<sub>3</sub><sup>ā</sup> groups may hypervalently bond to fully coordinated
surface silicon atoms, resulting in a relatively weak (0.70 eV) bond
that is consistent with the experimentally observed effusion at low
temperature. Furthermore, nonadiabatic molecular dynamics simulations
indicate that such hypervalent silyl defects provide efficient pathways
for nonradiative recombination via conical intersections that are
energetically accessible after near-infrared excitation
Phosphorus-Doped Silicon Nanocrystals Exhibiting Mid-Infrared Localized Surface Plasmon Resonance
Localized surface plasmon resonances
(LSPRs) enable tailoring of
the optical response of nanomaterials through their free carrier concentration,
morphology, and dielectric environment. Recent efforts to expand the
spectral range of usable LSPR frequencies into the infrared successfully
demonstrated LSPRs in doped semiconductor nanocrystals. Despite siliconās
importance for electronic and photonic applications, no LSPRs have
been reported for doped silicon nanocrystals. Here we demonstrate
doped silicon nanocrystals synthesized via a nonthermal plasma technique
that exhibits tunable LSPRs in the energy range of 0.07ā0.3
eV or mid-infrared wavenumbers of 600ā2500 cm<sup>ā1</sup>
Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals
In
this work, we present an all-gas-phase approach for the synthesis
of quantum-confined core/shell nanocrystals (NCs) as a promising alternative
to traditional solution-based methods. Spherical quantum dots (QDs)
are grown using a single-stage flow-through nonthermal plasma, yielding
monodisperse NCs, with a concentric core/shell structure confirmed
by electron microscopy. The in-flight negative charging of the NCs
by plasma electrons keeps the NC cores separated during shell growth.
The success of this gas-phase approach is demonstrated here through
the study of Ge/Si core/shell QDs. We find that the epitaxial growth
of a Si shell on the Ge QD core compressively strains the Ge lattice
and affords the ability to manipulate the Ge band structure by modulation
of the core and shell dimensions. This all-gas-phase approach to core/shell
QD synthesis offers an effective method to produce high-quality heterostructured
NCs with control over the core and shell dimensions
Controlled Doping of Silicon Nanocrystals Investigated by Solution-Processed Field Effect Transistors
The doping of semiconductor nanocrystals (NCs), which is vital for the optimization of NC-based devices, remains a significant challenge. While gas-phase plasma approaches have been successful in incorporating dopant atoms into NCs, little is known about their electronic activation. Here, we investigate the electronic properties of doped silicon NC thin films cast from solution by field effect transistor analysis. We find that, analogous to bulk silicon, boron and phosphorus electronically dope Si NC thin films; however, the dopant activation efficiency is only ā¼10<sup>ā2</sup>ā10<sup>ā4</sup>. We also show that surface doping of Si NCs is an effective way to alter the carrier concentrations in Si NC films
Near-Infrared Plasmonic Copper Nanocups Fabricated by Template-Assisted Magnetron Sputtering
In
this article we experimentally and theoretically study the plasmonic
properties of discrete copper nanocups fabricated by magnetron sputtering
on ordered, non-close-packed colloidal templates. Wide tunability
of the main plasmon resonance peak between 900 and 1500 nm, extending
the typical plasmon resonance range previously reported for other
copper nanostructures between 600 and 1000 nm, is achieved by varying
shell thickness and particle size in the colloidal template. The nature
of the plasmon resonance peaks is revealed from calculated charge
maps and electromagnetic field intensity maps. Good agreements are
found between experimental and calculated extinction spectra, which
validates the geometry model and suggests that the nanocups have a
well-defined shape. The main plasmon resonance peak exhibits a minor
red-shift and attenuation after 3 days of oxidation and eventually
stabilizes after 13 days. We also demonstrate that a potentially useful
optical material that blocks near-infrared but transmits visible light
can be constructed by mixing copper nanocups of three different sizes
at appropriate ratios