Synthesis of Phosphonic Acid Ligands for Nanocrystal Surface Functionalization and Solution Processed Memristors

Article abstract of DOI:10.1021/acs.chemmater.8b03768

Here, we synthesized 2-ethylhexyl, 2-hexyldecyl, 2-[2-(2-methoxyethoxy)ethoxy]ethyl, oleyl, and n-octadecyl phosphonic acid and used them to functionalize CdSe and HfO₂ nanocrystals. In contrast to branched carboxylic acids, postsynthetic surface functionalization of CdSe and HfO₂ nanocrystals was readily achieved with branched phosphonic acids. Phosphonic acid capped HfO₂ nanocrystals were subsequently evaluated as memristors using conductive atomic force microscopy. We found that 2-ethylhexyl phosphonic acid is a superior ligand, combining a high colloidal stability with a compact ligand shell that results in a record-low operating voltage that is promising for application in flexible electronics.

Full text of DOI:10.1021/acs.chemmater.8b03768

Article
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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Synthesis of Phosphonic Acid Ligands for Nanocrystal Surface
Functionalization and Solution Processed Memristors
Jonathan De Roo,*,†,‡ Zimu Zhou,§ Jiaying Wang,§ Loren Deblock,† Alfred J. Crosby,∥
Jonathan S. Owen,‡ and Stephen S. Nonnenmann*,§
†Department of Chemistry, Ghent University, Gent B-9000, Belgium
‡Department of Chemistry, Columbia University, New York, New York 10027, United States
§Department of Mechanical and Industrial Engineering, University of MassachusettsAmherst, Amherst, Massachusetts 01003,
United States
∥Polymer Science and Engineering Department, University of MassachusettsAmherst, Amherst, Massachusetts 01003, United
States
S
* Supporting Information
ABSTRACT: Here, we synthesized 2-ethylhexyl, 2-hexyldecyl, 2-[2-
(2-methoxyethoxy)ethoxy]ethyl, oleyl, and n-octadecyl phosphonic
acid and used them to functionalize CdSe and HfO2 nanocrystals. In
contrast to branched carboxylic acids, postsynthetic surface
functionalization of CdSe and HfO2 nanocrystals was readily
achieved with branched phosphonic acids. Phosphonic acid capped
HfO2 nanocrystals were subsequently evaluated as memristors using
conductive atomic force microscopy. We found that 2-ethylhexyl
phosphonic acid is a superior ligand, combining a high colloidal
stability with a compact ligand shell that results in a record-low
operating voltage that is promising for application in flexible electronics.
properties across length scales (a few nanometers to 100
μm) relevant to both individual NCs and the nanoribbon
assemblies. Therefore, we used c-AFM analysis to show that
operating parameters such as set/reset voltage and LRS/HRS
ratios of the nanoribbons inversely scale with the length of the
ligand used to stabilize the NCs in solution.9 Therefore, short
ligands are desired, but ligands shorter than dodecanoic acid
result in unstable dispersions, thereby restraining the minimum
set voltage that could be used to switch the ribbons of NCs.
With respect to ligands, the oleyl fragment is ubiquitous in
the nanocrystal field; many syntheses utilize either oleylamine
or oleic acid as the surfactant.17−20 The unsaturation provides
a high dispersibility in organic solvents and offers a distinct
signal in liquid 1H nuclear magnetic resonance (NMR)
spectroscopy. The latter is especially appealing for surface
chemistry studies because liquids NMR spectroscopy is the
method of choice for monitoring ligand binding and exchange
on colloidal NCs.21,22 Peng and coworkers recently showed
that NCs with short, branched ligands are more dispersible
than NCs with long, straight chain ligands.23 They used 2-
ethylhexanethiol as a champion ligand to obtain high
conductance in solution processed thin films of CdSe NCs.
Thiols bind well to CdSe and metal NCs but are poor ligands
INTRODUCTION
■
Surface engineering of colloidal nanocrystals (NCs) is a
prerequisite for advanced applications such as solar cells,1,2
thermoelectrics,3 field effect transistors,4 smart windows,5
superconductors,6,7 catalysis,8 and memristors.9 In particular,
threshold memristors garner intense research interest because
they emulate synaptic responses in neuromorphic comput-
ing.10 They also exhibit faster switching speeds, lower power
consumption, higher scalability, and greater 3D stackability
than standard complementary metal oxide semiconductor
(CMOS) circuits, thus promoting further miniaturizing
beyond Moore’s law.11,12 In such threshold switching, a bias
is applied, resulting in current flow and a significant amount of
internal Joule heating. Once above the metal−insulator
transition,13 the device is activated and maintains a low
resistance state (LRS) until the voltage is removed and the
device resets to an insulating high resistance state (HRS). A
memristor device comprises an insulating oxide layer
sandwiched between two metal electrodes, typically fabricated
by a vacuum deposition processes. However, these conven-
tional approaches are limited due to expensive fabrication
equipment and small deposition areas.14 A facile and cost-
effective alternative is the solution deposition of monodisperse,
highly crystalline inorganic NCs (e.g., SrTiO3, HfO2) into
ribbons of nanocrystals via evaporative assembly using flow
coating.9,15,16 In addition, conductive atomic force microscopy
(c-AFM) has the ability to locally interrogate electrical
Received: September 5, 2018
Revised: October 4, 2018
© XXXX American Chemical Society
A
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for metal oxides. Whereas carboxylic acids have a reasonably
high affinity for metal oxides,24,25 exchange of native oleate
ligands for 2-hexyldecanoic acid proved to be unfavorable
because of the steric encumbrance of the branched aliphatic
chain.26 Interestingly, phosphonic acid ligands are known to
quantitatively displace carboxylate ligands on CdSe,27,28
PbSe,29 and ZrO230 NCs and also bind well to flat surfaces.25,31
Saturated phosphonic acids are also used in the synthesis of
wurtzite CdSe NCs.32−34 However, postsynthetic functional-
ization of NCs still mostly focuses on carboxylic acid or thiol
ligands, even though the latter typically quench the photo-
luminescence quantum yield.35,36
Faster reactions and lower temperatures are accessible using
the Michaelis−Becker approach, wherein a dialkyl phosphite
46
anion is prepared from CsCO3 or sodium hydride in THF47
or Na in hexane48 in place of the trin-alkylphosphite as the
nucleophile. Following this precedent, we prepared diethyl
phosphonate esters from sodium diethylphosphite and the
appropriate alkyl bromide in dimethylformamide solution
(Scheme 3).49 Reactions reached completion within a few
Scheme 3. Appel Reaction Converts Alcohols in
Bromides,50,51 Michaelis−Becker Reaction Converts
Bromides to Phosphonate Esters, and Finally, Phosphonate
Esters Are Hydrolyzed to Phosphonic Acids
Here, we explored the functionalization of NCs with
phosphonic acid derivatives 1−5 (Scheme 1) and their
Scheme 1. Target Phosphonic Acid Ligands Tailored
towards Nanocrystal Surfaces
hours at room temperature for linear alkyl bromides and at 70
°C for branched substrates. Only compound 3 could not be
synthesized in high purity using the Michaelis−Becker reaction
(neither in DMF, nor in THF) due to slow conversion and the
formation of various uncharacterized byproducts. In this case,
the Michaelis−Arbuzov method was used, and the ethyl
phosphonate ester byproduct was removed from 3 by
distillation.
fabrication into memristor devices. We selected novel,
branched phosphonic acids (1 and 2), a versatile polyethylene
glycol34,37−40 derivative (3), as yet unreported oleylphos-
phonic acid (4), and a prototypical straight chain derivative
(5). The convenient synthesis of these phosphonic acids is
followed by surface functionalization methods that are facile,
even for the branched substrates. Finally, using these ligands,
we deposit HfO2 NCs into ribbons of NCs by a simple flow
coating process and measure their local memristive current−
voltage response with c-AFM.
Pure phosphonate ester derivatives 1−4 are isolated in
greater than 70% yield by distillation and provided pure
samples of phosphonic acid upon treatment with bromo-
trimethylsilane and methanol (Scheme 3).52 5 was purified by
recrystallization as the phosphonic acid. Via single crystal
diffraction, it was found that 5 crystallizes in the monoclinic
crystal structure and forms a stacked arrangement (Figure 1A).
The acidic hydrogens form a hydrogen bond to the oxygen
lone pairs of the PO bond with the hydrogen bonds being
inequivalent, measuring 1.84(4) and 1.79(5) Å, (Figure 1B).
Compound 4 is a waxy solid and can in principle also be
recrystallized from hexane, but the yield is low (20%), and this
increases the relative amount of trans-2 compared to cis-2.
Next, we demonstrate the versatility of our compounds with
respect to nanocrystal surfaces. As a model system, cadmium
oleate capped CdSe NCs (d = 3.3 nm, Figure S1) were
synthesized according to Cao et al.53 and purified by
precipitation with methyl acetate (see Supporting Information
for experimental details). Figure 2A shows the characteristically
broadened NMR resonances of bound oleate ligands in CDCl3.
Although oleate ligands can be quantitatively replaced with
linear phosphonic acids,27,28 the question remains whether
branched phosphonic acids react identically. To investigate
this, we added 1.2 equiv of 1 to the CdSe NCs in CDCl3.
Consequently, the alkene resonance narrows, and its multi-
RESULTS AND DISCUSSION
■
Phosphonic acids are usually synthesized via the Michaelis−
Arbuzov reaction (Scheme 2).41,42 High temperatures (160
Scheme 2. Michaelis−Arbuzov Reaction
°C) are often required, and generally, only primary, sterically
unencumbered halides react (thus not suitable for target
compounds 1 and 2).43 The scope is also limited by side
reactions with an ethyl bromide coproduct that leads to ethyl
phosphonate ester byproducts. Alternative Michaelis−Arbuzov
conditions have been developed that address some of these
issues but remain ineffective for sterically congested sub-
strates44 or are only suitable for benzylic substrates.45
B
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single exponential (Figure S2), indicating that 1 is mostly
bound to the nanocrystal, and the amount of free ligand, if any,
is small (<5%). The diffusion coefficient of bound 1 is 133
μm2/s, much smaller that the diffusion coefficient of free 1,
550 μm2/s. The bound nature of 1 is further corroborated by a
broad 31P NMR resonance at 33 ppm (fwhm = 2000 Hz,
Figure 2B, inset). This compares to a line width of only 65 Hz
for free phosphonic acid. Following the exchange, the aerial
density of surface ligands54 was unchanged (4.4 0.5 oleates
per nm2 and 4.7 0.5 phosphonates per nm2). We conclude
that a complete one-for-one exchange of oleate for the
branched ligand 1 is achieved, and they exist as mono-
hydrogenphosphonate ligands when bound to the surface, in
line with earlier reports.27 The complete exchange with 1 is
interesting in light of the poor exchange reactivity of 2-
hexyldecanoic acid26 and suggests that the higher binding
affinity of the phosphonate headgroup for the surface
overcomes any steric issue associated with the branched
chain (note: the branch point is β to the phosphonate center,
while it at the α-position in 2-hexyldecanoate).
Using the same exchange strategy, CdSe nanocrystals are
readily functionalized with ligands 2−5 in chloroform solution.
Nanocrystals bound by 1, 2, 4, and 5 are dispersible in
chloroform, toluene, and hexane. As expected, ligand 3 is more
versatile and stabilizes NCs in a variety of solvents, including
methanol, ethanol, acetone, chloroform, and toluene; however,
hexane precipitates NCs functionalized with 3 from those
solvents.
Figure 1. Crystal structure of 5. (A) View along the [010] direction;
hydrogens are omitted for clarity. The aliphatic chains angle toward
the reader at an angle of 45°. The chain packing density is 3.3 nm−2.
(B) View along the [001] direction; hydrogen bonds are displayed.
Only the first carbon of the chain is depicted for clarity.
We utilize ligands 1− 5 to prepare memristors based on
hafnium oxide NCs. HfO2 NCs (d = 3.75 nm) are synthesized
in benzyl alcohol from hafnium(IV) tert-butoxide.55 As-
synthesized NCs are not stabilized by ligands and simply
washed with diethyl ether to remove organics. Subsequently,
the NCs are functionalized by the addition of ligands 1−5 and
purified (see Supporting Information for experimental details).
Transmission electron microscopy (TEM) images of these
nanoparticles reveal an interparticle distance that varies from
1.6 nm for 1 to 2.4 nm for 5 (Figures 3A and B). The NCs
with ligands 1, 2, 4, and 5 are well-purified by precipitation
1
with methanol, as attested by solution H NMR (Figure 3C)
and 31P NMR (Figure 3D), so only a monolayer of ligands is
attached to the NCs. HfO2 NCs capped with 3 can only be
precipitated using hexane; however, ligand 3 is also insoluble in
hexane and is not readily separated from the HfO2 NCs
(Figures 3C and D).
Thin films of hafnium oxide are typically used as a CMOS-
compatible high-κ dielectric gate oxide56 and in memristor
applications.57 HfO2 NCs, when assembled into ribbons of
nanoparticles, feature a low resistance state and a high
resistance state, interesting for resistive random access memory
(RRAM) applications.9,11,12 Using a mixture of dodecanoic
acid and 10-undecenoic acid as ligands adsorbed on the HfO2
NCs, we previously achieved a minimal threshold switching set
voltage of 1.2 0.3 V.9 In the same fashion, we assemble here
the phosphonic acid capped HfO2 NCs in ribbons of
nanoparticles (Figures S3−S8, see Supporting Information
for experimental details) and measured the switching proper-
ties in function of the ligands 1−5 (Figure 4). Use of a
conductive scan probe as a top electrode enables measure-
ments of nanoribbons in a geometry similar to the nano-
crossbar memristor or atomic switch arrays used in thin film-
based devices,11,12 where both the bias and current response
occur across the nanostack dimension.
Figure 2. (A) 1H NMR spectrum of oleate capped CdSe nanocrystals
in CDCl3 ([NC] = 261 μM, [oleate] = 39 mM). The ligand density is
4.4 0.5 nm−2. The inset shows the alkene resonance after addition
1
of 1.2 equiv of 1. (B) H NMR spectrum of CdSe nanocrystals in
CDCl3, after ligand exchange for 1 and purification. ([NC] = 124 μM,
[phosphonate] = 20 mM). The ligand density is 4.7 0.5 nm−2. The
inset shows the 31P NMR spectrum.
plicity, typical for free, unbound ligands, appears (Figure 2A,
inset). After purification (see Supporting Information), the 1H
NMR spectrum features only the broadened resonances of 1,
and no trace of oleate is detected (Figure 2B), indicating a
successful exchange of oleate for 1. To further confirm the
successful purification, we performed pulsed field gradient
experiments. The resulting diffusion decay is fitted with a
C
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high (4.4 0.7 V) for ligands 4 and 5 to a record low (1.0
0.3 V) for ligand 1. Also, bipolar switching is observed for
HfO2 nanocrystal assemblies with ligand 1 (Figure S9). Note
that ligand 3 violates the trend and displays higher set voltages
than expected for its ligand length. This may be caused by the
presence of the free ligand (which proved difficult to remove
from the nanocrystals as described above) or by the nature of
the polyethylene glycol chain. Note that the triangular cross-
section of the nanoribbons (Figures S3−S8) creates a
thickness-dependence in the memristive response. The thickest
portion of the nanoribbons is difficult to electroform using the
10 V bias supply of the microscope, and therefore, we
performed all conductive probe measurements in thinner areas
of equivalent thickness (∼80 nm, circle marker Figure S10) for
each nanoribbon tested. We anticipate that ribbons a few NCs
in height will exhibit optimal switching character.
Because it is desirable to minimize the set voltage for future
memory applications, the shortest possible ligand should be
used, which is 1 in this case of 3.75 nm HfO2 NCs. However,
larger HfO2 NCs such as the 5 nm NCs synthesized from
HfCl4 and benzyl alcohol24 cannot be stabilized by 1, and the
NCs precipitate after functionalization and purification. This is
rationalized by the van der Waals attraction between the cores
(Figure S11). In this case, colloidal stability is provided by 2,
which it is still considerably shorter than the standard ligands
(such as oleic acid) that are typically used. Together, these
results place the branched ligands 1 and 2 squarely in the focus
of electronic applications where solution processing is
combined with interparticle charge transfer after assembly.
Figure 3. (A) TEM image of HfO2 NCs functionalized with 1. (B)
1
TEM image of HfO2 NCs functionalized with 5. (C) Solution H
NMR and (D) solution 31P NMR spectra of HfO2 NCs functionalized
with 1−5 in CDCl3. The resonance with the asterisk is an unidentified
impurity.
CONCLUSION
■
Conductive atomic force microscopy (c-AFM) was used in
point spectroscopic mode to collect 40 I−V curves over four
separate, individual nanoribbons for each ligand type. We
observed the expected trend where operating voltage scales
with ligand size. Here, the set voltage varies from extremely
We developed a scalable synthesis of new phosphonic acid
surfactant ligands. We subsequently showed their superiority in
functionalizing nanocrystal surfaces. Whereas exchange for
branched carboxylic acids is disfavored by the branching, the
Figure 4. (A−E) I−V curve of the threshold switching of HfO2 nanoribbons using 1−5, respectively, as ligand. (F) Switching voltage in function of
the ligand chain length.
D
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high binding affinity, we expect the synthesized ligands to be
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.8b03768.
■
S
Crystallographic information for octadecylphosphonic
acid (CIF)
UV−vis spectrum of CdSe nanocrystals, DOSY decay
fittings, additional NMR spectra, AFM images of
nanoribbons, and switching results and calculations of
the interaction potential (PDF)
̀
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J.; Rijckaert, H.; Pollefeyt, G.; Bruneel, E.; Palau, A.; Coll, M.; Ricart,
S.; Van Tendeloo, G.; Puig, T.; Obradors, X.; Van Driessche, I.
Superconducting YBa2Cu3O7-δ Nanocomposites Using Preformed
ZrO2 Nanocrystals: Growth Mechanisms and Vortex Pinning
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: Jonathan.DeRoo@ugent.be.
*E-mail: ssn@umass.edu.
ORCID
Jonathan De Roo: 0000-0002-1264-9312
Jonathan S. Owen: 0000-0001-5502-3267
Stephen S. Nonnenmann: 0000-0002-5369-9279
■
Notes
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The authors declare no competing financial interest.
Today 2008, 11, 28−36.
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memories. Nat. Mater. 2007, 6, 833.
ACKNOWLEDGMENTS
■
The authors thank David A. Sambade for solving the crystal
structure of n-octadecylphosphonic acid. J.D.R. acknowledges
the Belgian American Education Foundation (B.A.E.F.),
Fulbright, Ghent University, and the COMPASS project
(H2020-MSCA-RISE-2015-691185) for financial support.
Z.Z., J.W., and S.S.N. were partially supported by University
of MassachusettsAmherst start-up funding, the UMass
Center for Hierarchical Manufacturing (CHM), and NSF
Nanoscale Science and Engineering Center (CMMI-1025020).
This work was supported by the United States Army Research
Laboratory and the United States Army Research Office under
contract W911NF-12-1-0594 (MURI). CCDC 1844778
contains the supplementary crystallographic data for this
paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
̈
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DOI: 10.1021/acs.chemmater.8b03768
Chem. Mater. XXXX, XXX, XXX−XXX