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Increasing the Cytotoxicity of Ru(II) Polypyridyl Complexes by Tuning the Electronic Structure of Dioxo Ligands.
Communication
pubs.acs.org/JACS
Hexaaminobenzene as a building block for a Family of 2D
Coordination Polymers
Nabajit Lahiri,† Neda Lotfizadeh,‡ Ryuichi Tsuchikawa,‡ Vikram V. Deshpande,*,‡ and Janis Louie*,†
†
Department of Chemistry and ‡Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, United
States
S Supporting Information
*
as the metal linkers, we have prepared several nitrogen-based
few-layered CPs.
A gas−liquid interfacial reaction was used wherein
hexaaminobenzene (HAB) was dissolved in degassed, deionized water, and an ethyl acetate solution of M(acac)2 (M = Ni,
Cu, Co, acac = acetylacetonate) was gently added on top of the
aqueous phase such that it covered approximately half of the
surface (Figure 1). The reaction was allowed to sit for 4 h to
ABSTRACT: A family of 2D coordination polymers were
successfully synthesized through “bottom-up” techniques
using Ni2+, Cu2+, Co2+, and hexaaminobenzene. Liquid−
liquid and air−liquid interfacial reactions were used to
realize thick (∼1−2 μm) and thin (<10 nm) stacked layers
of nanosheet, respectively. Atomic-force microscopy and
scanning electron microscopy both revealed the smooth
and flat nature of the nanosheets. Selected area diffraction
was used to elucidate the hexagonal crystal structure of the
framework. Electronic devices were fabricated on thin
samples of the Ni analogue and they were found to be
mildly conducting and also showed back gate dependent
conductance.
raphene, an atomically thin π-conjugated 2D organic
polymer, is the strongest material to date and also an
extraordinary conductor of heat and electricity. The extraordinary physical properties of graphene have been utilized in
various applications such as electronics,1−3 spintronics,4,5 solar
cells,6,7 and batteries.8 However, graphene is just one member
of a wider class of 2D materials, some of which possess
additional functionalities such as semiconducting band gap,
superconductivity, magnetism, and topological order. Thus far,
the major graphene alternatives are 2D transition-metal
chalcogenides, which are locked by their constituent elements
and are difficult to modify by design. A “bottom-up” approach
to 2D materials, on the other hand, allows one to tailor material
properties by design. In this context, 2D coordination polymers
(CPs), which can be synthesized via judicious selection of metal
centers and coordinating ligands, are of particular interest due
to their high tunability.
Currently, only a limited number of syntheses of twodimensional CPs have been reported.9−17 Moreover, the
resultant 2D CPs are usually either multilayered with thickness
greater than 300 nm or are polycrystalline with submicrometer
sized crystallites. Such characteristics make them unsuitable for
advanced device fabrication which requires single (or few)
layered π-conjugated CPs that have large lateral dimensions
(μm2 or mm2). As such, the synthesis of few-layered (<10
layers) 2D CPs still remains to be a formidable challenge.12−14
Herein we report the synthesis of a family of 2D CPs that are
not only large in lateral dimension (μm2) but also ultrathin
(<10 nm) and crystalline. Using hexaaminobenzene as the
coordinating ligand and metal ions such as Ni2+, Cu2+, and Co2+
G
© 2016 American Chemical Society
Figure 1. (Top) General synthesis of CPs. (Bottom) Schematic
illustration of the structure of the nanosheet (tube model with
hydrogens omitted for clarity: Gray, carbon; blue, nitrogen; red, metal
ion) and photograph of the reaction vessel highlighting the liquid/
liquid (EtOAc−water) interfacial synthesis of Ni-HAB complex
showing the polymer as a black solid at the interface.
allow for the ethyl acetate to evaporate. The 2D (hexaiminobenzosemiquinonato)-κN-metal complex (1) could be seen to
form spontaneously as a film on the surface. The thin films
were then transferred to suitable substrates (e.g., SiO2 (300
nm)/Si, polydimethylsiloxane (PDMS) gel, etc.) using a
Langmuir−Schafer type of extraction that involves the gentle
lowering of the substrates face-down onto the surface of the
reaction mixture and then lifting it up. After washing with water
and isopropyl alcohol, the substrate adhered CPs were
Received: September 20, 2016
Published: December 12, 2016
19
DOI: 10.1021/jacs.6b09889
J. Am. Chem. Soc. 2017, 139, 19−22
Communication
Journal of the American Chemical Society
Figure 2. Characterization of 2D-CPs. (a) (Top) AFM height profile image of 1-Ni on SiO2 (300 nm)/Si substrate with OM image in inset;
(bottom) SEM image of 1-Ni. (b) (Top) AFM height profile image of 1-Cu on SiO2 (300 nm)/Si substrate with OM image in inset; (bottom) SEM
image of 1-Cu. (c) (Top) AFM height profile image of 1-Co on SiO2 (300 nm)/Si substrate with OM image in inset; (bottom) SEM image of 1-Co.
All OM image scale bars are equivalent to 10 μm.
Figure 3. Transmission electron microscope images of 2-Ni. (a) Selected-area diffraction (SAED) on the nanosheet 2-Ni showing a hexagonal lattice
(scale bar: 2 nm−1). (b) Secondary electron image of 2-Ni using TEM, brighter regions indicate the film, whereas darker regions the carbon film
support and elemental mapping images of carbon, nitrogen, nickel, and sodium using S/TEM-EDS. (c) EDS of the corresponding area of 2-Ni where
SAED was performed showing the elemental composition.
subjected to sonication in EtOAc for 10 s followed by baking at
100 °C under a dynamic vacuum for 12 h.
Analysis of the 2D CPs on 300 nm SiO2/Si with optical
microscopy (OM) and scanning electron microscopy (SEM)
confirmed the CPs were homogeneous and crystalline films that
generally lacked cracks over fairly large areas (Figure 2).
Furthermore, atomic-force microscopy (AFM) profilometry of
Ni-HAB (1-Ni), Cu-HAB (1-Cu), and Co-HAB (1-Co)
showed that the family of 2D-CPs had similar thicknesses of
∼10 nm (10, 11, and 13 nm, respectively, Figure 2), which
corresponds to about 8−10 monolayers of the respective 2DCPs stacked on one another. A single layer of fairly large
dimensions, however, could not be obtained possibly due to
strong π-stacking interactions between layers.
The composition of the respective 2D CPs films was
examined by X-ray photoelectron spectroscopy (XPS).
Importantly, XPS of each 2D CP confirmed only presence of
the carbon, nitrogen, and expected transition metal (i.e., Ni, Cu,
or Co) as well as sodium, the counterion used during syntheses
(Figures S1, S8, and S14). In the Ni-HAB complex (1-Ni), a
high-resolution scan of the Ni (2p) region showed the presence
of Ni2+ in the complex (Figure S2). Furthermore, deconvolution of the N (1s) region in 1-Ni reveals the presence of three
distinct N (1s) peaks (Figure S4). The peak at 400 eV
corresponds to adventitious nitrogen that is physically adsorbed
on the surface. The peaks at ∼399 and ∼398 eV were attributed
to nitrogen at “0” and “−1” oxidation states respectively which
were present in a 42:58 ratio (Table T1). In other words, the
Ni-HAB complex (1-Ni) had a net negative charge that was
balanced by the presence of Na+ counterions. Similar results
were obtained for the 1-Cu and 1-Co analogues. The Cu-HAB
complex was found to only have peaks corresponding to
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DOI: 10.1021/jacs.6b09889
J. Am. Chem. Soc. 2017, 139, 19−22
Communication
Journal of the American Chemical Society
copper, carbon, and nitrogen (Figure S8). A high resolution
scan in the Cu (2p) region, however, revealed the presence of a
mixture of Cu2+ and Cu+ species. Such mixed valencies have
been previously observed in copper complexes and can be
attributed to the redox-active nature of the ligand.9 Similarly,
the Co (2p) region of the Co-HAB complex was found to
consist of two sets of peaks (Figure S15). The first set was at
780 and 796 eV; the second set was at 785 and 802 eV. These
were attributed to Co2+ and Co3+ species, respectively.
Interestingly, the Cu-HAB (1-Cu) and the Co-HAB (1-Co)
were found to be neutral as supported by the absence of Na+
ions in the XPS spectra of the complexes.
To investigate further the internal structure and crystallinity
of these 2D CPs, selected area diffraction (SAED) was
performed on 2-Ni using TEM. Ultrathin films (<10 nm) of
1-Ni could not be used for TEM studies due to extensive
radiolysis damage. Analysis of a thick film (2-Ni, ca. 1 μm) that
was synthesized using a liquid/liquid interfacial reaction (see SI
for details) displayed the expected hexagonal diffraction
pattern, thus confirming the Ni-HAB complex was crystalline
in nature with a hexagonal lattice (Figure 3). The lattice spacing
was determined to be 0.5 nm. Similar hexagonal diffraction
patterns were also observed for both Cu-HAB (2-Cu) and CoHAB complexes (2-Co), which had d-spacings of 0.40 and 0.43
nm, respectively (Figures S12 and S18). Additionally, the atomratio of nickel to nitrogen obtained from S/TEM-EDS (EDS,
energy-dispersive spectroscopy) of 2-Ni spectra was close to
1:4 and supports the proposed structure. Elemental mapping
on the complexes also revealed a homogeneous distribution of
carbon, nitrogen, and the respective metal on the nanosheet
(Figures 3b, S12, and S18).
Nanosheets were electrically characterized by fabricating
devices using either direct transfer on prefabricated leads,
shadow mask lithography, or electron-beam lithography. Using
the first procedure results in bottom contacts (Figure 4a) and
yields top contacts (Figure 4a) with the other two procedures.
In all three cases, devices were similarly resistive, in the GΩ
range, and showed slight modulation of conductance G with
respect to variation of a back gate voltage Vbg. Figure 4c shows a
typical gate-dependent conductance of a Ni nanosheet device.
Finally, Figure 4d shows the temperature dependent conductance of a different Ni nanosheet device down to 4 K from
room temperature. In general, devices tended to decrease in
conductance as temperature was lowered indicating insulating
behavior in charge transport.
In conclusion, we synthesized a family of hexaaminobenzene
based Coordination Polymers (CPs) having a hexagonal
honeycomb crystal lattice. The morphology, composition, and
crystal structure of the complexes were confirmed by OM, FESEM, AFM, XPS, and S/TEM. Both top and bottom contacts
used for electrical characterizations yielded similar values of
resistances. We attribute the low conductivity of the samples to
the low crystallinity of the framework resulting in crystal defects
and grain boundaries. Efforts toward the synthesis of samples
with lower defects by optimizing reaction conditions and
fabrication techniques are currently underway.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b09889.
Experimental details, FE-SEM, HR-TEM, SAED, XPS, S/
TEM-EDS data, and electrical characterization data
(PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*J.L. louie@chem.utah.edu
*V.V.D. vikramvd@gmail.com
ORCID
Janis Louie: 0000-0003-3569-1967
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
MRSEC program at the University of Utah under grant #DMR
1121252. We gratefully acknowledge Brian van Devener and
Paulo Perez for help with TEM data. We also gratefully
acknowledge Jonathan David Ogle for assistance with Raman
data and Emily Fullwood for help with acquiring TGA data.
■
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