Sterically encumbered tris(trialkylsilyl) phosphine precursors for quantum dot synthesis

Article abstract of DOI:10.1021/acs.inorgchem.0c02440

The synthesis of nanomaterials with a narrow size distribution is challenging, especially for III-V semiconductor nanoparticles (also known as quantum dots). Concerning phosphides, this issue has been largely attributed the use of overly reactive precursors. The problem is exacerbated due to the narrow range of competent reagents for III-V semiconductor syntheses. We report the use of sterically encumbered tris(triethylsilyl) phosphine and tris(tributylsilyl) phosphine for InP quantum dot (QD) synthesis among others. The hypothesis was that these reagents are less reactive than the near-ubiquitous precursor tris(trimethylsilyl) phosphine and can be used to create more homogeneous materials. It was found that the InP products' quantum yields and emission color saturation (fwhm) were improved, but not to the levels realized in CdSe QDs. Regardless, these reagents have other positive attributes; they are less pyrophoric and can be applied toward the synthesis of II-V semiconductors and organophosphorus compounds. Concerning safe practices, we demonstrate that ammonium bifluoride is an effective replacement for highly toxic HF for the post-treatment of III-V semiconductor quantum dots.

Full text of DOI:10.1021/acs.inorgchem.0c02440

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Article
Sterically Encumbered Tris(trialkylsilyl) Phosphine Precursors for
Quantum Dot Synthesis
Hashini B. Chandrasiri, Eun Byoel Kim, and Preston T. Snee*
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ABSTRACT: The synthesis of nanomaterials with a narrow size
distribution is challenging, especially for III−V semiconductor
nanoparticles (also known as quantum dots). Concerning
phosphides, this issue has been largely attributed the use of overly
reactive precursors. The problem is exacerbated due to the narrow
range of competent reagents for III−V semiconductor syntheses.
We report the use of sterically encumbered tris(triethylsilyl)
phosphine and tris(tributylsilyl) phosphine for InP quantum dot
(QD) synthesis among others. The hypothesis was that these
reagents are less reactive than the near-ubiquitous precursor
tris(trimethylsilyl) phosphine and can be used to create more
homogeneous materials. It was found that the InP products’
quantum yields and emission color saturation (fwhm) were
improved, but not to the levels realized in CdSe QDs. Regardless, these reagents have other positive attributes; they are less
pyrophoric and can be applied toward the synthesis of II−V semiconductors and organophosphorus compounds. Concerning safe
practices, we demonstrate that ammonium bifluoride is an effective replacement for highly toxic HF for the post-treatment of III−V
semiconductor quantum dots.
emiconductor quantum dots (QDs) have utility in
employed for metal phosphide synthesis.23 Tris(trimethylsilyl)
phosphine, TMS3P, is a common reagent for InP QDs,
although its use results in suboptimal size distributions and
emission color saturation (spectral fwhm). Several groups have
noted that TMS3P is overly reactive,18,20 resulting in poorly
defined nucleation and subsequent focused growth steps
necessary for producing high quality materials.15 Several
schemes have been employed to subvert this issue. For
example, Peng and co-workers first nucleated InP QDs at a
high temperature and then alternatively injected TMS3P and
indium precursors for subsequent growth at a lower temper-
ature.24 The product size distributions were narrowed as a
result. Recent publications have reported improved results
using similar, albeit somewhat more complex schemes,25,26
especially Ramasamy et al., who demonstrated ∼120 meV
emission line widths that are competitive with what can be
realized from CdSe QDs (∼80 meV). Other attempts to create
monodisperse InP QDs employed modified phosphorus anion
precursors. For example, Harris and Bawendi synthesized a
1
2−4 biomedical imaging,5,6 and biosens-
Soptical, electronic,
ing7−9 applications due to their many unique properties.
Several protocols have been developed for the synthesis of
QDs largely based on the rapid injection method for II−VI
CdE (E = S, Se, Te) dots reported in 1993.10 In fact, the
amount of effort put forth in synthetic development has
resulted in protocols to produce materials with narrow
emission profiles and ∼100% quantum yields, especially for
semiconducting PbSe and CdSe. However, the presence of
lead and cadmium11 has instigated significant research into QD
toxicity12 and diminished the prospects for their commercial
use. This has spurred the development of the colloidal
synthesis of III−V semiconductor nanoparticles, such as
InP,13 which can span the visible region of the electromagnetic
spectrum.14 Generally, good size distributions are realized by
an initial nucleation of nanoparticles followed by focused
growth, whereby smaller QDs grow faster than larger ones
(and thus “catch up”).15−17 Unfortunately, synthesis of
monodisperse InP QDs is a problem that has been attributed
to the difficulty in separating the nucleation and growth
phases.18−20 Magic-sized clusters may initially form during
preparation, which minimizes synthetic tunability by precursor
design and thus creates roadblocks for the preparation of
monodisperse materials.21,22
Received: August 14, 2020
Various phosphorus precursors including elemental Pred, P4,
alkyl phosphines, and aminoalkyl phosphines have been
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Strem Chemical. Chloroform, dichloromethane (ACS reagent, >
99.5%),], n-hexane, methanol, 1-octadecene (ODE, 90%), oleic acid
(90%, recrystallized before use), 2-propanol, sodium hydroxide (97%
reagent grade), and triethylamine (≥99%) were from Sigma-Aldrich.
Chloroform was dried over molecular sieves before use. Zinc stearate
was from Acros Organics. Cadmium oxide (>99%), coumarin-102
(97%, dye standard), and myristic acid (98.5%) were from Fluka.
Size-exclusion solid support (Biobeads, S-X1) was purchased from
Biorad. The water-solubilizing agent 40% octylamine-modified
poly(acrylic acid)37 was prepared as described in ref 38.
(Me3Ge)3P precursor due to the reduced lability of the
germane ligand compared to TMS.19 Unfortunately, the
improvement in InP QD properties was modest. Gary et al.
attempted to tune the reactivity of (R3Si)3P-type phosphorus
precursors by modulation of the R groups’ electronic and steric
properties. Specifically, they coordinated the P3− anion with
various substituted triarylsilyl ligands; this added bulk and
affected the polarization of the P−Si bonds. However, it was
found that the QDs synthesized with substituted triarylsilyl-
phosphines exhibited final size distributions that were not
improved because of the slow rate of nucleation.20 The use of
bulky R groups was also reported in other publications,27,28
again with modest results on minimizing polydispersity.
A recent advancement in InP synthesis concerns the
inclusion of zinc carboxylates in the core preparation. In fact,
zinc additives may be present in overwhelming abundance. As
zinc carboxylates are minimally productive precursors for
Zn3P2,29,30 they instead participate in the reaction mechanism
to increase the monodispersity and enhance the luminescence
of the core InP products. While a variety of roles for zinc in
InP QD production have been proposed, recently Koh et al.
demonstrated complexation of zinc with TMS3P to mollify the
phosphorus reactivity.31 It is notable that Ramasamy et al. used
a Zn−P complex for growing as-prepared InP QDs in their
report.25
Synthesis of Tris(triethylsilyl) Phosphine, TES3P. Warning!
These syntheses involve highly pyrophoric reagents and products. These
procedures require expertise with air-free techniques and should not be
attempted by a novice.
This preparation is a derivative of that reported by Holz et al.39 as
summarized in Scheme 1. We have found that the best results were
Scheme 1. A Summary of the Methods Used to Prepare
Sterically Encumbered Phosphorus Reagents
The works of Gary et al. and Koh et al. prompted our
investigation into the use of sterically encumbered triakylsilyl
phosphine ligands for InP QD synthesis in the presence of zinc
additives. In this regard, two alkylsilylphosphines, tris-
(triethylsilyl) phosphine and tris(tributylsilyl) phosphine,
were synthesized. These reagents are shown to be less reactive
than TMS3P. Combined with zinc carboxylate additives, their
use resulted in improved properties; however, the emission
color saturation was not as narrow as can be achieved with
CdSe QDs. Regardless, it was found that these precursors are
significantly less pyrophoric than TMS3P, for which we
recommend their usage. They are competent precursors for
preparing II−V nanomaterials as well as molecular organo-
phosphide compounds.
This report also examines the enhancement of InP emission
that occurs with surface etching with using fluorides.22,23 HF
treatment of InP quantum dots is a photochemical reaction
that can dramatically enhance brightness.32−34 However, the
handling of HF acid should be avoided due to the extreme
hazards associated with exposure. Several alternatives, such as
NH4F and in situ etchants with salts [BF4]− and [PF6]−, have
also been suggested.32,35 In this study, XPS and optical data
demonstrate that ammonium bifluoride, NH4HF2, is an
effective alternative to HF. Ammonium bifluoride etched InP
QDs were used as substrates for ZnSeS shell growth to prepare
water-soluble core/shell nanoparticles.8,36
obtained when performing the initial steps in an inert-atmosphere
glovebox. To a dried three-neck 500 mL round-bottom flask was
added 6.25 g of potassium (156 mmol) and 4.78 g of sodium metal
(208 mmol) that were cut and washed with hexanes prior to use.
Stirring for a few minutes allowed the Na/K alloy to form. Next, 250
mL dry DME was added followed by 3.8 g of red phosphorus (123
mmol) to produce a red suspension. The round-bottom was fitted
with a reflux condenser, an addition funnel, and a filtration column
attached to a Schlenk flask as shown in the SI, Scheme S1. The system
was sealed, brought into a fume hood, attached to a dry N2 source,
and then heated to reflux in an oil bath. After stirring overnight, the
suspension became black due to activation of the phosphorus. It was
allowed to return to room temperature. Next, a solution of 61 g of
chlorotriethylsilane (405 mmol) in 65 mL of dry DME was prepared
in a glovebox and was injected into the addition funnel through a
septum. The solution was then brought back to reflux for ∼48 h. At
this time, a gray precipitate formed, and the solution became visibly
viscous. After cooling to room temperature, the material was passed
through a fritted glass filter while under N2, and the solid material was
washed two to three times with either dry hexane, pentane, or DME
to capture more product. The resultant solution was sequestered in a
Schlenk flask that was dried in vacuuo to remove volatiles including
solvents before being sealed to store the crude product in a glovebox.
(Warning!The solid byproduct isolated in filtration contains unreacted
sodium, potassium, and activated phosphorus. It should be diluted with
hexane and slowly deactivated with butanol. The solid will ignite if
quenched too quickly or if mishandled.)
EXPERIMENTAL SECTION
■
Materials. Ammonium hydroxide solution (certified ACS plus),
1,2-dimethoxyethane (DME, certified), and toluene (certified ACS)
were obtained from Fisher. DME was dried over molecular sieves
prior to use. Ammonium bifluoride (95%), indium acetate (99.99%),
potassium metal (98%), red amorphous phosphorus (∼100 mesh,
98.9%), and sodium metal (99%) were obtained from Alfa Aesar.
Chlorotriethylsilane was purchased from Oakwood Chemical.
N,N,N′,N′-Tetramethyldiaminomethane (>99%), tributylchlorosilane
(>97%), triethylsilyl trifluoromethanesulfonate, and trifluorometha-
nesulfonic anhydride were purchased from TCI America. Diethyl zinc
(95%) and tri-n-octylphosphine (TOP, 97%) was purchased from
The crude product was distilled. A distillation system was
assembled in a glovebox and loaded with the crude product. It was
taken out and attached to a dry N2/vacuum (Schlenk line) source in a
fume hood, and the solution was heated incrementally under a
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vacuum. Generally, the receiving vessel was cooled in a liquid nitrogen
bath to lower the pressure, generally ∼100 mTorr. An unidentified
byproduct was observed distilling at ∼40 °C (the temperature was
measured at the distillation head, while the Corning PC-420D hot
plate reading was typically 2× higher). This was discarded. The tris-
triethylsilyl phosphine product distilled over at 110 → 120 °C when
the oil bath was set to 270 → 290 °C. In this example, 28 g of product
was recovered (60% yield) with ∼80% purity. Several preparations
were performed throughout this study, and it was found that the
product purity was variable over a range of 50% → 80%. We also
found that the purities increased as we gained experience performing
the reaction. The byproducts were bis(triethylsilyl) phosphine
hydride, TES2PH, and triethylsilyl phosphine dihydride, TESPH2
(see examples of 31P NMR in the SI, Figure S1). All attempts to
modify the reaction to prevent the formation of these byproducts
were thwarted. High purity was realized via the reaction of the
distilled products with silyl triflates as discussed below.
Ligation of TES2PH and TESPH2 with Triethylsilyl Triflate for
High Purity TES3P. The hydride byproducts can be converted to the
ligand-saturated product via ligation with the silyl triflate as shown in
Scheme 1.40 All reactions were performed in an inert atmosphere
glovebox. Crude tris(triethylsilyl) phosphine was characterized with
31P NMR to determine the initial purity and the subsequent reaction
stoichiometry. In this example, 1.2 g of an 80% pure tris(triethylsilyl)
phosphine product was added to 5 mL of dry dichloromethane with
additional 0.30 g of triethylsilyl trifluoromethanesulfonate (∼0.26 mL,
1.13 mmol) and 0.171 g of excess triethylamine (0.235 mL, 1.69
mmol). After stirring overnight, pure tris-triethylsilyl phosphine was
isolated by vacuum distillation as revealed by NMR characterizations
in the SI (Figure S2). 1H NMR (400 MHz, CDCl3): δ 1.01 (t,
H3C−), 0.80 ← 0.70 (b, −CH2−Si). 13C (400 MHz, CDCl3): δ 8.30
(d, J = 5.5 Hz), 8.23. 31P (400 MHz, CDCl3): δ −288.0.
in some P−H bond formation) and silica (extreme degradation), the
product was isolated by size exclusion gel chromatography using a
polystyrene solid phase with a dry toluene eluant. The first eluted
fraction (0.2 g) achieved the best level of purity according to NMR
characterizations (Figure S5 of the SI). 1H NMR (400 MHz, CDCl3):
δ 1.45 ← 1.25 (b, −CH2CH2−), 0.95 ← 0.85 (b, H3C−), 0.80 ←
0.70 (b, −CH2−Si). 13C NMR (400 MHz, CDCl3): δ 26.8, 17.2, 17.1,
13.7. 31P (400 MHz, CDCl3): δ −279.4 ppm.
Synthesis of InP QDs. The procedure of Koh et al.31 was
modified for InP core QD synthesis. In a typical preparation, 58 mg of
indium acetate (0.2 mmol), 144 mg of myristic acid (0.63 mmol), and
400 mg of zinc stearate (0.63 mmol) were added into a 50 mL three-
necked round-bottom flask with 10 mL of ODE. The mixture was
degassed at 110 °C for 1 h. Afterward, the solution was backfilled with
N2, and the heating mantle was removed. During this time, 0.2 mmol
of a phosphorus precursor solution (TMS3P, TES3P or TBS3P) in 1.5
mL of TOP was prepared in a glovebox. This was injected at ∼80 °C
into the reaction solution, which was immediately heated to 300 °C
within 4 min. A growth time of 15 min continued after the solution
reached 300 °C, during which time the QDs experienced some
increase in size as shown in Figure S6. Many of these parameters were
varied in the course of this investigation, see the Discussion section.
Ammonium Bifluoride Treatment. InP core QDs were
precipitated by adding 2-propanol followed with methanol.
Centrifugation resulted in a pellet that was dissolved in hexane, and
subsequently added to a solution of 177 mg of myristic acid in 10 mL
of ODE in a three-neck round-bottom flask. The hexane was removed
at 50 °C under a vacuum until the pressure reached equilibrium.
Afterward, a small chip (∼10 mg) of NH4HF2 was added to the
solution that was stirred overnight (generally ∼14 h) at 50 °C under a
N2 atmosphere.
Synthesis of InP/ZnSeS QDs. The quantity of InP QDs was
determined using the regression of Xie et al.43 In this example, we
attempted to grow a five monolayer ZnS shell on 5.4 × 10−7 moles of
∼2 nm diameter QDs. First, 0.327 g (0.52 mmol) of zinc stearate was
added into the ammonium bifluoride treated core dots (prepared with
TBS3P) under a high N2 flow. The temperature was raised to 50 °C
and degassed for 1 h. Next, the temperature was raised to 200 °C
under N2, upon which 0.1 mL of 1.0 M TOPSe in 0.9 mL of TOP was
added into the vessel by manual injection. Afterward, 17 mg (0.53
mmol) of elemental sulfur in 5 mL of ODE was injected through a
septum using a syringe injector over the course of ∼1.5 h.
Synthesis of Tris-tributylsilyl Phosphine, TBS3P. Tris-
(tributylsilyl) phosphine was prepared as for the ethyl derivative
using tributylchlorosilane. However, the distillation process simply
removed low molecular weight byproducts and solvents as the
product(s) did not vaporize under any condition. The yields were
typically ∼50% as characterized with 31P NMR, see Figure S3 for a
typical example. The major byproducts are bis(tributylsilyl)
phosphine hydride, TBS2PH, and to a lesser extent tributylsilyl
phosphine dihydride, TBSPH2. The low yield necessitated additional
ligation with the silyl triflate as outlined below.
Preparation of Tributylsilyl Triflate. This procedure is based on
ref 41. First, tributylsilanol was prepared by the reaction of
tributylchlorosilane with aqueous ammonium hydroxide exactly as
outlined in ref 42. 13C NMR (400 MHz, CDCl3 ppm): δ 26.64, 25.52,
15.48, 13.77. Next, 1 g of tributylsilanol (4.62 mmol) and 1.34 g of
trifluoromethanesulfonic anhydride (4.75 mmol) were mixed for 4 h
at 45 °C in an oil bath. The temperature was lowered to ambient, and
1.086 g of tributylchlorosilane (1.2 mL, 4.62 mmol) was added. A
vacuum was applied to remove any high vapor pressure byproducts,
and 13C NMR analysis demonstrated high purity of the resulting
tributylsilyl triflate product as shown in Figure S4 of the SI. 13C NMR
(400 MHz CDCl3): δ 123.19, 120.03, 116.88, 113.72, 26.05, 24.23,
13.46, 13.38.
Water Solubilization of InP/ZnSeS QDs. As-prepared InP/
ZnSeS QDs were processed via precipitation and weighed after
drying. Next, ∼5 times the QDs’ weight of 40% octylamine-modified
poly(acrylic acid) was added, and the QD/polymer mixture was
dissolved with ∼2 mL of chloroform and was shaken until clear
(sometimes a drop of methanol assisted in realizing optical clarity).
The solvent was removed under a vacuum, and the resultant film of
polymer encapsulated QDs was solubilized in 0.1 M aqueous NaOH.
Some samples require sonication and filtration using a 0.2 μm filter;
all samples were purified with dialysis using Millipore Amicon Ultra
100 K MWCO centrifugal filters against deionized water.
Synthesis of Cd3P2, Zn3P2, and Tris(dimethylaminomethyl)-
phosphine. The procedures of Xie et al.,44 Mobarok et al.,45 and
Prishchenko et al.46 were used to prepare Cd3P2 and Zn3P2 materials
as well as tris(dimethylaminomethyl)phosphine, respectively, as
described in the SI.
Ligation of TBS2PH and TBSPH2 with Tributylsilyl Triflate
for High Purity TBS3P. 31P NMR was used to ascertain the molar
ratios of the singly and doubly ligated phosphine hydride byproducts
to the target tris(tributylsilyl) phosphine. The resulting stoichiometry
was used to determine the mass of triflate needed to fully ligate the
byproducts. In this example, the starting material was 50% pure, which
was typical. First, 0.934 g of tributylsilyl triflate (2.68 mmol), 2.84 g of
the tributylsilyl phosphine(s) mixture, and 0.403 g of triethylamine
(∼0.56 mL, 3.98 mmol) were mixed in 5 mL of dry dichloromethane.
After stirring overnight, all byproducts were removed under a vacuum,
Characterization. X-ray photoelectron spectroscopy (XPS) was
performed on samples that were precipitated and drop cast onto clean
Si wafers. XPS spectra were obtained with a KRATOS AXIS-165
surface analysis system. The energy spectra were adjusted using the C
1s 284.6 eV peak as a reference. Transmission electron microscopy
(TEM) analyses were performed using a JEOL JEM-3010 microscope
operating at 300 keV. TEM grids were prepared by drop casting
processed nanoparticle solutions onto 300-mesh carbon-coated Cu
1
and the resulting products were characterized by H, 13C, and 31P
1
NMR. The presence of byproducts was significantly minimized;
however, we endeavored to enhance the purity tris(tributylsilyl)
phosphine further. As the product could not be distilled under a
vacuum and was found to be unstable in alumina (exposure resulted
grids from Ted Pella. H, 13C, and 31P NMR spectra were measured
with a Bruker Avance DPX400 spectrometer. Air-sensitive TES3P and
TBS3P were loaded into airtight NMR tubes in a glovebox and diluted
with dry NMR solvents. For X-ray powder diffraction (XRD) analysis,
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QD samples were precipitated, resuspended in hexane, and drop-
casted on Si wafers. X-ray diffraction measurements were performed
using a D8 Advance ECO Bruker XRD diffractometer with Cu Kα (λ
= 1.54056 Å) radiation. Absorption spectra were obtained with a
Varian Cary 300 Bio UV/vis. PL emission measurements were taken
on a HORIBA Fluoromax-400 or a customized Fluorolog (HORIBA
Jobin Yvon) modular spectrometer for NIR measurements. Coumarin
102, rhodamine 101, and fluorescein were used as reference dyes.
Absolute quantum yield measurements were also performed using a
HORIBA Quanta-ϕ F-3029 integrated sphere at the Center for
Nanoscale Materials at Argonne National Laboratory. It was found
that the integrating sphere QYs are generally higher than those
obtained with dye standards. We nonetheless include the dye
standard’s data, which likely represent a lower limit.
Results using the highest purity reagents are shown in Figure
1, where it can be seen that the InP absorption spectra of QDs
Pyrophoricity Assessment of Tris(trialkylsilyl) Phosphines.
To assess the pyrophoricity of phosphorus reagents, TMS3P, TES3P,
and TBS3P were exposed to increasing oxygen levels until ignition
using the system shown in Scheme S2.
Figure 1. Absorption and emission spectra of InP QDs prepared using
a variety of phosphorus sources. The emission fwhm in each spectrum
is indicated in meV.
RESULTS AND DISCUSSION
■
created using TBS3P, TES3P, and TMS3P are rather featureless.
Furthermore, the emission spectra fwhm vary between ∼70−
90 nm (260−295 meV). The highest molecular weight reagent
TBS3P generated the narrowest emission (260 meV); however,
it also had the lowest quantum yield (2%) with a best result of
8%. Despite these low values, the brightness of these samples
was enhanced upon etching with NH4HF2 and the addition of
a shell as discussed below. Although the range of quantum
yields realized with pure TES3P straddles that from TMS3P, we
generally found better quantum yields with TES3P. For
example, when increasing the Zn/(In,P) > 3, longer growth
times or higher injection temperatures resulted in QYs on the
order of 25−29% as discussed below.
This effort was initiated to expand the toolkit of phosphorus
precursors in the hope of finding a “magic reagent” to create
monodisperse InP QDs. In this regard, we examined sterically
encumbered phosphorus reagents and the use of zinc additives
to prepare the nanomaterials. Two alkylsilyl phosphines,
tris(triethylsilyl) phosphine (TES3P) and tris(tributylsilyl)
phosphine (TBS3P), were synthesized according to Scheme 1
following the methods reported in ref 39. The modified
procedure also produced (SiR3)nPH3−n byproducts in
abundance despite our best efforts to prevent protonolysis.47
The ethyl derivative could be purified via distillation, although
this is impossible for the butyl species due to its high molecular
weight. Fortunately, all the byproducts were converted into the
fully ligated targeted compounds via a simple reaction with the
corresponding alkylsilyl triflate. The purity of tris(tributylsilyl)
phosphine was further enhanced with size exclusion
chromatography. And although we demonstrate that these
precursors are not “magic reagents” for monodisperse InP QD
synthesis, they have numerous positive attributes including less
pyrophoricity than the ubiquitous TMS3P as discussed in the
Supporting Information. Furthermore, they are competent
precursors for a variety of nanomaterials as well as for
organophosphorus compounds.
InP core and InP/ZnSeS core/shell QDs were synthesized
using TBS3P and TES3P in the presence of zinc additives and
were compared to those realized by TMS3P. The hypothesis
was that the enhanced steric properties and potential
coordination to zinc would render the phosphorus less reactive
such that well-defined nucleation and growth processes would
create monodisperse products. A typical procedure was derived
from that reported by Koh et al., in which indium acetate,
myristic acid, and zinc stearate were degassed in ODE.31
Excessive myristic acid was avoided to minimize direct
conversion of the ligated phosphorus into a hydride.47
According to the “atomic valve” approach of Koh et al., the
addition of high quantities of zinc stearate has the dual effect of
suppressing InP magic clusters and slowing the reaction of the
phosphorus precursor. After the injection of phosphorus
precursor at 80 °C, the reaction solution was rapidly heated
to 300 °C. During this time, the solution became slightly
yellow at 100 °C, the intensity of which increased as the
temperature was raised to 300 °C. InP QDs were allowed to
grow for 15 min.
The effect of reagent purity was investigated. Given the
difficulty of preparing ∼100% pure reagents, we performed InP
QD syntheses as a function of the TES3P purity after
preparation. As shown in Figure 2A (top), although the InP
QDs prepared with 90% pure TES3P have a respectably narrow
emission, this spectrum also noticeably red tails. This sample
also had the lowest emission efficiency. Higher purity reagents
resulted in better QYs, yet the color saturation suffered. This
was a trend seen in many investigations, such as when
evaluating the effect of increasing the growth time with 90%
pure TES3P as shown in Figure 2B. Growing the InP QDs for a
longer period red-shifted the spectra and resulted in higher
emission efficiency (QY ∼ 29%), yet the spectral fwhm
increased at the same time. Similar behavior was seen in InP
34
̈
QDs made by Moller and co-workers. This same trend was
again observed when increasing the phosphorus precursor
injection temperature as shown in Figure 2C. Higher
temperatures promoted red-shifted absorption and emission
spectra, and higher quantum yields were observed. It is
interesting that a similar observation was made by Lucey et al.
despite the fact that they did not use zinc additives in their InP
QD preparations.48 Regardless, the higher QY samples also
suffered a loss of color saturation.
The effects of increasing zinc additives on the properties of
InP QDs prepared with 90% pure TES3P were ascertained.
With no or low levels of zinc (<0.2 mmol), the InP QDs were
nonemissive. Higher zinc amounts allowed for the observation
of emission, although there were minimal-to-no effects on the
optical bandgap to a certain point. However, at a Zn/(In,P)
ratio of ≥3, the bandgap of the QDs decreased ∼225 meV, and
the resultant materials had fairly high quantum yields (∼25%).
These observations of higher quantum yields and lower fwhm
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Figure 2. (A) The effect of increasing TES3P reagent purity affords higher quantum yields yet broader emission spectra. The emission fwhm in
each spectrum is indicated in meV. (B) The effects of increasing growth time and (C) increasing injection temperature were to enhance quantum
yields at the expense of color purity. The best QY of 29% for core InP was realized with a 30 min growth time, 90% pure TES3P, and an Zn/(In,P)
ratio of 3.
due to passivation of surface defects are similar to those of Koh
et al.;31 however, they did not observe a substantial Zn/(In,P)
ratio dependence on the optical properties as demonstrated in
Figure 3.
interesting, according to Prishenko et al.,46 the reaction
proceed via a Zn−P molecular intermediate, similar to Koh
et al.’s “atomic-valve” hypothesis.31
Etching and Core/Shell QDs. The surface treatment of
semiconductor materials with etchants is common and is in
fact necessary for observing emission from InP QDs prepared
without the use of zinc additives. HF is frequently used for this
purpose (see ref 50), which is unfortunate as the use of HF
should be strenuously avoided due to its many hazards. This
prompted the study of an alternative, ammonium bifluoride
(NH4HF2), which is used as a wheel cleaner in automatic car
washes. In a typical experiment, core InP QDs were treated
with ammonium bifluoride under an oxygen-free atmosphere
overnight. During the etching process, it was found that excess
ligands must be present to maintain colloidal stability.
Afterward, the samples must be handled air-free, and as such
the samples were processed for optical characterization in a
glovebox. It was found that the absorption spectra are blue-
shifted as shown in Figure 4, and the quantum yields increased
Figure 3. Effect of the addition of increasing levels of zinc first afford
an increase in quantum yield and at higher levels result in smaller QDs
with sharper excitonic features.
It is interesting to note that a ratio of 3 is the coordination
valency of the phosphorus anion and suggests that zinc may be
sequestered by phosphorus until the Zn/P ratio is raised to
allow free zinc cations to enhance the size distribution and
brighten the growing InP QDs. Alternatively, the presence of
unbound phosphorus could be detrimental. Overall, these
observations demonstrate that the zinc to phosphorus ratio is
more important than zinc to indium that has generally been
the focus of most research reports.
When exploring the utility of TES3P for the preparation of
other III−V QDs, it was found that GaP quantum dots could
not be prepared using a previously published method.49 We
interpret this result as testimony to the lowered reactivity of
the reagent, which prompted further study on other materials
and compounds.
I I − V S e m i c o n d u c t o r s a n d T r i s -
(dimethylaminomethyl)phosphine. The competency of
TES3P for the synthesis of II−V metal phosphides, specifically
Cd3P2 and Zn3P2, was investigated using previously published
protocols. Success was demonstrated in both cases as shown in
the powder XRD patterns of the products, see Figures S7 and
S8. To venture outside the arena of materials, the synthesis of
tris(dimethylaminomethyl)phosphine was investigated using
TES3P and zinc chloride as a catalyst. The NMR spectra shown
in Figure S9 confirmed the formation of the product. Most
Figure 4. Absorption and emission spectra as a function of NH4HF2
etching and shell growth on InP QDs as prepared with TBS3P.
2 to 4 times; see Table S1 of the Supporting Information. XPS
spectroscopy was used to evaluate the effect of ammonium
bifluoride exposure to compare against HF treatment as
reported by others. As shown in Figure 5A, InP QDs incubated
with ammonium bifluoride have an XPS F 1s peak at 684.3 eV
and, coupled with blue-shifted In 3d transitions (Figure 5B),
demonstrate that fluoride is chemically bound to indium.51
Zinc’s XPS features are unperturbed by ammonium bifluoride
treatment (Figure S10), which is interesting as the zinc is likely
surface-bound.52 The growth of an oxidized P 2p 133.3 eV
peak (Figure 5C) indicates oxygen sensitivity, which is a result
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Figure 5. XPS spectra of core and NH4HF2 treated InP QDs. (A) Fluorine is present on the ammonium bifluoride treated InP QD surface. (B)
The blue shift of the In 3d peaks is likely due to In−F bond formation as per ref 51. (C) Inadvertent air exposure when loading the processed
sample into the XPS resulted in oxidation of the phosphorus as evident from the growth of the P 2p 133.3 eV peak.
of this sample’s air exposure when loading it into the XPS
spectrometer. This is consistent with our observations on
emission quenching when the etched InP QDs were processed
outside of a glovebox, see Table S1. Other XPS character-
izations are found in the SI. To summarize, the treatment of
InP core QDs with ammonium bifluoride appears identical to
that realized with HF etching.
CONCLUSION
■
Methods to prepare pure high molecular weight phosphorus
precursors for InP, Cd3P2, and Zn3P2 quantum dots and
organophosphorus compounds are reported. These reagents
are less pyrophoric than TMS3P, which is commonly employed
for metal phosphide syntheses. With regard to safe chemistry,
the use of ammonium bifluoride as an alternative to HF
treatment for semiconductor processing is also presented.
The combination of sterically encumbered phosphorus
reagents with zinc additives did not result in monodisperse
size distributions for InP quantum dots. Although disappoint-
ing, this information contributes to our general knowledge of
the factors that govern controlled monomer delivery rates in
the quest to optimize the size distributions of InP and other
III−V quantum dots. Regardless, the nanomaterial products
created with these reagents had better optical properties
compared to those generated with commercially available
TMS3P. Core/shell InP/ZnSeS QDs are robust such that they
can be water solubilized, making them candidates for
fluorescent labels in biological applications.
InP QDs prepared with TES3P and TBS3P were shelled with
a higher bandgap semiconductor to verify that the materials are
good substrates for overcoating. This is important because core
materials are not generally efficacious in most applications due
to a lack of stability and suboptimal optical properties. To this
end, core dots were etched with ammonium bifluoride and
then were heated to 200 °C with additional zinc stearate,
whereupon a small quantity of trioctylphosphine selenide was
added. Next, a ZnS shell was grown at the same temperature
by the addition of elemental sulfur dissolved in ODE using a
syringe injector. In general, we have found that the slow
introduction of shell precursors produces better results, likely
due to suppression of nucleation of nanoparticles composed of
the shell materials. Optical spectra for core, post NH4HF2
treated cores, and core/shell QDs for one set of InP dots
prepared with TBS3P are shown in Figure 4. The ammonium
bifluoride etching obviously blue shifts and broadens both
absorption and emission, while the shelling appears to red shift
the absorption beyond the original core. This is sensible as the
2 nm core diameter (based on the regression by Xie43)
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02440.
increased to 3.2
0.4 nm for the core/shell species as
Details on the synthesis of pyrophoric chemicals and
characterization; NMR, optical, XPS, and XRD spectra;
and tabulated optical data (PDF)
measured by TEM, see Figure S11. However, the emission of
the core/shell did not energetically track the absorption,
resulting in a smaller Stokes’ shift. This is likely due to
suppression of hole trap sites that have been proposed to be
responsible for the wide energy gap between absorbing and
emitting states.53,54 These results also suggest that the shell
alloys into the core, which was further corroborated with XRD
data that demonstrate a crystal structure for InP/ZnSeS that
conforms to neither InP, ZnSe, nor ZnS, see Figure S12.
It was found that core/shell QDs prepared using our
procedure have emission efficiencies in the 30% → 40% range,
which represents an increase from the core dots that have QYs
typically between 2% and 15%. The emissions were typically
blue-shifted (Table S1). Core/shell InP/ZnSeS dots were
water-solubilized using 40% octylamine-modified poly(acrylic
acid)37 to demonstrate potential bioimaging and sensing
applications. Encapsulating the core/shell QDs in the polymer
resulted in a ∼50% reduction of quantum yield, and the
resultant dispersion was stable on a several-month-long time
scale.
AUTHOR INFORMATION
■
Corresponding Author
Preston T. Snee − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607-7061, United
States; orcid.org/0000-0002-6544-6826; Email: sneep@
uic.edu
Authors
Hashini B. Chandrasiri − Department of Chemistry, University
of Illinois at Chicago, Chicago, Illinois 60607-7061, United
States
Eun Byoel Kim − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607-7061, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02440
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Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(11) Directive 2002/95/EC, Off. J. Eur. Union L037; pp 19−23;
February 13, 2003.
Funding
The University of Illinois at Chicago and the American
Chemical Society Petroleum Research Fund.
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Bayesian network resource for meta-analysis: Cellular toxicity of
quantum dots. Small 2019, 15 (34), 1900510.
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Functionalized quantum dots for biosensing and bioimaging and
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by startup funds to P.T.S. from the
University of Illinois at Chicago. Acknowledgment is made to
the Donors of the American Chemical Society Petroleum
Research Fund for partial support of this research. Use of the
Center for Nanoscale Materials, an Office of Science user
facility, was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357. Also, Richard Schaller
and Xufeng Zhang are acknowledged for assistance with
quantum yield measurements.
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ABBREVIATIONS
■
ODE, 1-octadecene; QD, quantum dot; QY, quantum yield;
PL, photoluminescence; TES3P, tris(triethylsilyl) phosphine;
TBS3P, tris(tributylsilyl) phosphine; TMS, tri(methylsilyl);
TMS3P, tris(trimethylsilyl) phosphine
(19) Harris, D. K.; Bawendi, M. G. Improved precursor chemistry
for the synthesis of III-V quantum dots. J. Am. Chem. Soc. 2012, 134
(50), 20211−20213.
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indium phosphide quantum dot nucleation and growth utilizing
triarylsilylphosphine precursors. Chem. Mater. 2014, 26 (4), 1734−
1744.
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Two-step nucleation and growth of InP quantum dots via magic-sized
cluster intermediates. Chem. Mater. 2015, 27 (4), 1432−1441.
(22) Gary, D. C.; Flowers, S. E.; Kaminsky, W.; Petrone, A.; Li, X. S.;
Cossairt, B. M. Single-crystal and electronic structure of a 1.3 nm
indium phosphide nanocluster. J. Am. Chem. Soc. 2016, 138 (5),
1510−1513.
(23) Li, H.; Jia, C.; Meng, X. W.; Li, H. B. Chemical synthesis and
applications of colloidal metal phosphide nanocrystals. Front. Chem.
2019, 6, 6.
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dots with high-purity photo- and electroluminescence. Chem. Mater.
2018, 30 (11), 3643−3647.
(26) Xu, Z. H.; Li, Y.; Li, J. Z.; Pu, C. D.; Zhou, J. H.; Lv, L. L.;
Peng, X. G. Formation of size-tunable and nearly monodisperse InP
nanocrystals: Chemical reactions and controlled synthesis. Chem.
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