Metallated Graphynes as a New Class of Photofunctional 2D Organometallic Nanosheets

Article abstract of DOI:10.1002/anie.202014835

Two-dimensional (2D) nanomaterials are attracting much attention due to their excellent electronic and optical properties. Here, we report the first experimental preparation of two free-standing mercurated graphyne nanosheets via the interface-assisted bottom-up method, which integrates both the advantages of metal center and graphyne. The continuous large-area nanosheets derived from the chemical growth show the layered molecular structural arrangement, controllable thickness and enhanced π-conjugation, which result in their stable and outstanding broadband nonlinear saturable absorption (SA) properties (at both 532 and 1064 nm). The passively Q-switched (PQS) performances of these two nanosheets as the saturable absorbers are comparable to or higher than those of the state-of-the-art 2D nanomaterials (such as graphene, black phosphorus, MoS₂, γ-graphyne, etc.). Our results illustrate that the two metallated graphynes could act not only as a new class of 2D carbon-rich materials, but also as inexpensive and easily available optoelectronic materials for device fabrication.

Full text of DOI:10.1002/anie.202014835

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11326–11334
International Edition:
German Edition:
doi.org/10.1002/anie.202014835
doi.org/10.1002/ange.202014835
Metallated Graphynes
Metallated Graphynes as a New Class of Photofunctional 2D Organ-
ometallic Nanosheets
Linli Xu+, Jibin Sun+, Tianhong Tang+, Hongyang Zhang+, Mingzi Sun, Jianqi Zhang, Jiahua Li,
Bolong Huang, Zhengping Wang,* Zheng Xie,* and Wai-Yeung Wong*
Abstract: Two-dimensional (2D) nanomaterials are attracting
much attention due to their excellent electronic and optical
properties. Here, we report the first experimental preparation
of two free-standing mercurated graphyne nanosheets via the
interface-assisted bottom-up method, which integrates both the
advantages of metal center and graphyne. The continuous
large-area nanosheets derived from the chemical growth show
the layered molecular structural arrangement, controllable
thickness and enhanced p-conjugation, which result in their
stable and outstanding broadband nonlinear saturable absorp-
tion (SA) properties (at both 532 and 1064 nm). The passively
Q-switched (PQS) performances of these two nanosheets as the
saturable absorbers are comparable to or higher than those of
the state-of-the-art 2D nanomaterials (such as graphene, black
phosphorus, MoS2, g-graphyne, etc.). Our results illustrate that
the two metallated graphynes could act not only as a new class
of 2D carbon-rich materials, but also as inexpensive and easily
available optoelectronic materials for device fabrication.
ics, catalysis, batteries, supercapacitors and sensing plat-
forms.[1,2d,3]
Graphynes include graphyne, graphdiyne and graphyne-
n (n > 2), depending on the number of acetylenic units. Large-
area graphdiyne films were isolated successfully in 2010.[4]
The use of different multi-substituted acetylenic monomers
can lead to the construction of a variety of micro-structures of
graphynes/graphdiynes.[5] The fascinating structures and
properties of graphynes (the 2D allotrope of graphene) make
them a promising class of 2D carbon materials.[6] In order to
develop new types of graphyne-based materials, metal
elements as a new form of molecular functionality can be
introduced into the framework of graphdiynes to afford
metallated graphynes. The first-principles density functional
theory calculations have been used to explore the excellent
mechanical, electronic and optical characteristics and thermal
stability of N-, B-, P-, Al-, As-, and Ga-graphyne-based 2D
lattices.[7] However, related research work on the structurally
similar metallated graphynes is still in its infancy and bottom-
up large-area and free-standing metallated graphynes were
never synthesized.[8] In a related context, Sun et al. reported
Introduction
À ꢀ À À ꢀ À
Since the discovery of graphene in 2004, the ultra-thin 2D
nanomaterials, such as the elemental analogues of graphene
(viz. silicene, germanene and phosphorene), black phospho-
rus (BP), transition metal dichalcogenides (TMDs), topolog-
ical insulators (TIs), metal oxides, metal hydroxides and
boron nitrides,[1] have attracted intensive research attention
among scientists since they possess unique chemical, optical
and physical properties.[2] These 2D nanomaterials are gen-
erally composed of single or various elements or molecular
frameworks with periodic structures, which can be used for
a wide range of applications in the electronics/optoelectron-
that the formation of 2D C C Au C C organometallic
networks was observed under high-resolution scanning tun-
neling microscope through dehalogenative homocouplings of
alkynyl bromides on an Au(111) surface at room temperature
(RT) and slight annealing to 320 K.[8a] Besides, Yang et al.
reported the formation of 2D organometallic Ag-bis-acetylide
C C Ag C C networks on Ag(111).[8b] However, Au-bis-
À ꢀ À À ꢀ À
acetylide can be converted into graphdiyne upon further
annealing on Au(111), while Ag-bis-acetylide networks keep
their structure even at a rather high temperature on Ag(111).
Zhang et al. recently described the organometallic honey-
[*] Dr. L. Xu,[+] Dr. H. Zhang,[+] M. Sun, Dr. J. Li, Dr. B. Huang,
Prof. Dr. W.-Y. Wong
E-mail: zhengxie@mail.ipc.ac.cn
T. Tang,[+] Prof. Dr. Z. Wang
Department of Applied Biology and Chemical Technology and
Research Institute for Smart Energy, The Hong Kong Polytechnic
University (PolyU)
Hung Hom, Hong Kong (P. R. China)
and
PolyU Shenzhen Research Institute
Shenzhen 518057 (P. R. China)
E-mail: wai-yeung.wong@polyu.edu.hk
Dr. J. Sun,[+] Prof. Dr. Z. Xie
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences
State Key Laboratory of Crystal Materials and Institute of Crystal
Materials, Shandong University
Jinan 250100 (P. R. China)
E-mail: zpwang@sdu.edu.cn
Dr. J. Zhang
National Center for Nanoscience and Technology
Beijing 100190 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202014835.
29 Zhongguancun East Road, Haidian District, Beijing 100190 (P. R.
China)
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comb alkynyl-silver networks with a micrometer domain size
tinuous films on the transparent substrates,[2d,13b,14a,15a,16b]
which were also detected on Ag(111) under ultrahigh vacuum
conditions via a gas-mediated surface reaction protocol.[8c]
The above 2D Au- and Ag-bis-acetylide networks were all
observed on the metal substrates (Au or Ag) with small areas
of single or few layers and they cannot be physically isolated
easily from the substrates and are thus very difficult to be used
in devices. So, one of the present challenges for the
preparation of metallated graphynes is to develop methods
that can afford their large-area and high-quality films. In this
work, we demonstrate two new large-area free-standing
mercurated graphyne organometallic nanosheets (MGONs)
generated at the interface of an aqueous mercuric chloride
solution and a hexane solution of each of the triacetylenic
ligands. The propensity of d10 HgII ion to form a linear two-
which is, however, hard to achieve using these 2D materials.[17]
Here, the interface-assisted approaches were used to
synthesize the MGONs in which the structural and morpho-
logical control can be achieved by the spatial arrangement of
the molecular precursors in a confined 2D space, which
enables the growth of large-area and high-quality continuous
2D structures with low surface roughness, layered molecular
arrangement and enhanced p-conjugation.[8c,18] Interestingly,
we find that MGONs present excellent broadband NLO and
high transmission performance in the solid states, which
would lead to many promising applications in optical fields.
The MGONs can be transferred to the substrates easily and
used as saturable absorbers directly. The SA properties of
these two MGONs have been realized experimentally, which
can be applied as passive Q-switchers at 1064 nm at the
optimal thickness. This is the first literature report on free-
standing organometallic nanosheets showing the SA proper-
ties and realizing the superb PQS performance as compared
to the 2D materials under the same testing conditions. Our
experimental results reveal that the metallated graphynes
with a large area, regular molecular arrangement and unique
chemical structure can be obtained easily via the interface-
assisted bottom-up method and these 2D materials are
promising optical modulators in integrated optoelectronics.
À ꢀ À À ꢀ À
coordinate C C Hg C C moiety with alkynyl units
makes HgII alkynyls an attractive building block for molecular
wires and organometallic functional materials.[9] Besides, the
heavy Hg center was selected based on our previous studies
for 1D HgII-polyynes where their nonlinear optical (NLO)
effects can be enhanced by copolymerization of organic
polyynes with HgII ions.[10] The heavy metal effect also
typically magnifies the NLO response by enhancement of
the intersystem crossing from S1 to T1 states in metal-
ethynylene-based optical limiters.[11] Hence, it is expected
that interesting NLO properties will also be observed in the
2D planar counterparts for the MGONs.
2D materials with extensively delocalized p-conjugated Results and Discussion
architectures are known to provide good building blocks for
new NLO materials with tunable properties.[12] The 2D
materials with periodic structure present NLO responsive
properties which are determined by the crystalline structure,
possibly based on the Neumann principle.[13] They possess the
NLO performances which are highly desirable for the device
applications, such as optical limiting, pulse shaping, mode
locking, signal and image processing, optical switching, data
storage and communication.[2c,d,14] Among them, low-dimen-
sional nanomaterials can show saturable absorption (SA)
properties which are usually used for passively Q-switching or
mode-locking.[2d,3b,12a,14,15] Q-switching is a technology to
generate a pulse with an ultra-short pulse width and a high
peak power. Compared with mode-locking, Q-switching can
give a laser with a much larger pulse energy (generally by 2 or
3 orders of magnitude). This technique is of practical
importance in many areas, such as military, industry, medical
treatment and basic scientific research, where compact,
reliable and cost-effective nanosecond pulsed lasers are
required. The 2D nanomaterials (e.g. graphene, g-graphyne,
BP, TMDs, etc.) have a low saturation intensity which is
favorably used as passively Q-switchers.[13b,14a,15a,16] The PQS
property of graphene was shown to be tunable with variation
of the number of layers.[14a] However, the state-of-the-art 2D
nanomaterials still have their own shortcomings, such as the
dark color (for graphene and its derivatives), the complex
preparation procedures (for TIs), instability under ambient
conditions (for BP) and defects of nanostructures (for
TMDs).[16f] On the other hand, for the application of these
materials in optical devices, it is necessary to dope them into
transparent matrices[2b–d,12a,15b,16a,c] or prepare large-area con-
Synthesis and characterization of mercurated graphyne
organometallic nanosheets
The ligands of tris(4-ethynylphenyl)amine (L1)
(Scheme S1, Figure 1a) and 1,3,5-triethynylbenzene (L2)
(Figure 1a) were used to synthesize MGONs, HgL1 (Fig-
ure 1b) and HgL2 (Figure 1c), by the base-catalyzed dehy-
drohalogenation reactions. Liquid/liquid and gas/liquid inter-
facial reactions were employed for synthesizing the multi- and
few-layer nanosheets, respectively, following slight modifica-
tions of the method used for other coordination-driven metal-
complex nanosheets derived from bis(pyrrinato)zinc(II) com-
plexes as reported previously by our group.[18b,c] A hexane
solution of L1 (1.0 mM) or L2 (1.0 mM) and an aqueous
HgCl2 (2.0 mM) were layered, and the reaction system was
kept undisturbed for 24 h (see Figures 2a and 3a, respective-
ly). Then, the generation of multi-layer MGONs at the
interface was confirmed as a light yellow (HgL1 nanosheets)
or a colorless (HgL2 nanosheets) continuous film that was
deposited onto a substrate such as glass slide, silicon wafer or
quartz (Figure S3a) for subsequent characterization. It was
found that the thickness of multi-layer HgL1 and HgL2
nanosheets can be controlled by changing the concentrations
of the ligands and HgCl2. In view of this, the thickness of
multi-layer nanosheets was mainly adjusted by changing the
concentration of the ligands while the concentration of HgCl2
was kept constant at 2 mM in the following experiments. The
few-layer MGONs were prepared via the gas/liquid interfa-
cial reaction, in which a dichloromethane solution of L1 or L2
(1.0 mM) was layered on the surface of aqueous HgCl2
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(2.0 mM). Their thickness can be adjusted by changing the
volume of the ligand solution in each case. Few-layer HgL1 or
HgL2 nanosheets were then transferred onto Si wafer
substrates by using the Langmuir-Schäfer method (Fig-
ure S3b) for atomic force microscopy (AFM) observation.
Different from the traditional structure of acetylenic nano-
materials, MGONs are a new type of metal-acetylenic
nanomaterials.[5g,19] In order to provide the structural models
for the HgL1 and HgL2 nanosheets, two control samples of
HgL1M and HgL2M complexes (Scheme S3) were synthesized
successfully by using HgCl2 and L1M (Scheme S2 and Fig-
ure S4a) or L2M under the same basic medium. Their chemical
structures were confirmed by nuclear magnetic resonance
(NMR) spectroscopy, Fourier-transform infrared (FTIR)
spectroscopy (see Figure S4c) and elemental analysis (Ta-
ble S1). This indicates the feasibility of the synthesis of HgL1
and HgL2 nanosheets.
The photographs of HgL1 and HgL2 nanosheets on the
glass slides are presented in Figure 2b and Figure 3b, which
display a large film size (> 1 cm). The colors of the nanosheets
are consistent with those of the HgL1 and HgL2 bulks,
respectively (Figure S5). The morphologies of HgL1 and
HgL2 nanosheets were confirmed by scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM) images, in which both 2D materials showed a sheet
and layer structure (Figures 2c, d and 3c, d). AFM images of
the few-layer HgL1 and HgL2 nanosheets on the silicon wafer
revealed their wrinkle and flat film morphologies, respective-
ly, with the respective film thickness of approximately 17.33
and 40.61 nm (Figures 2e, 3e and Figure S6). In the FTIR
spectra of HgL1 and HgL2 nanosheets (Figure S4b), the sharp
Figure 1. Chemical structures of the ligands and mercurated graphyne
organometallic nanosheets. a) Molecular structures of L1 and L2.
b,c) Synthesis of HgL1 and HgL2 nanosheets.
Figure 2. The morphology and structure analysis of HgL1 nanosheets. a) Multi-layer HgL1 nanosheets generated at the liquid/liquid interface.
b) A photo of HgL1 nanosheets on a slide glass. c) SEM image of HgL1 nanosheets on a Si wafer. d) TEM image of HgL1 nanosheets. e) AFM
image and its cross-sectional analysis of the few-layer HgL1 nanosheets on a mica sheet. f) SEM/EDS mapping of HgL1 nanosheets for Si, C, Hg
and N elements on a Si wafer. g) XPS analysis of HgL1 nanosheets focusing on C 1s, Hg 4f and N 1s. h) 13C CP-MAS solid-state NMR spectrum
of HgL1 nanosheets.
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Figure 3. The morphology and structure analysis of HgL2 nanosheets. a) Multi-layer HgL2 nanosheets generated at the liquid/liquid interface.
b) A photo of HgL2 nanosheets on a slide glass. c) SEM image of HgL2 nanosheets on a Si wafer. d) TEM image of HgL2 nanosheets. e) AFM
image and its cross-sectional analysis of the few-layer HgL2 nanosheets on a mica sheet. f) SEM/EDS mapping of HgL2 nanosheets for Si, C, Hg
and N elements on a Si wafer. g) XPS analysis of HgL2 nanosheets focusing on C 1s, Hg 4f and N 1s. h) 13C CP-MAS solid-state NMR spectrum
of HgL2 nanosheets.
peaks at 3310–3070 cmÀ1 for the C H stretching vibrations
disappeared, which were observed in the FTIR spectra of L1
and L2 (Figure S4a). This indicated that the metal alkynyla-
tion reactions occurred and were essentially completed at the
interfaces. The main framework of HgL1 nanosheets con-
sisted of C, N and Hg elements. Energy dispersive X-ray
spectroscopy (EDS) with SEM mapping (Figure 2 f) revealed
these elements to be spread evenly on the nanosheets. The
main framework of HgL2 nanosheets consisted of C and Hg,
which were also spread evenly on the surface of HgL2
nanosheets and detected by EDS with SEM mapping (Fig-
ure 3 f). The elemental composition and chemical bonding of
HgL1 and HgL2 nanosheets were also confirmed by X-ray
photoelectron spectra (XPS). The XPS spectra in Figure S7
display the presence of constitutive C, Hg and N in HgL1
nanosheets and the C and Hg elements in HgL2 nanosheets.
High-resolution XPS focusing on the C 1s, Hg 4f and N 1s
core levels of HgL1 nanosheets afforded one signal for C 1s
and N 1s and two signals for Hg 4f (Figure 2g). No signal of N
1s appeared from the XPS characterization of HgL2 nano-
sheets, which is a clear distinction from the HgL1 nanosheets
(Figure 3g). Both the C 1s curves of HgL1 and HgL2
nanosheets can be deconvoluted into four Gaussian curves
on the surface of materials), which indicate the abundance
ratio of the sp2/sp carbons and the successful introduction of
Hg ion into the frameworks of graphynes. The abundance
ratios of the sp2/sp carbons are 3 and 1 for HgL1 and HgL2
nanosheets, respectively, which is in good agreement with the
chemical structures of HgL1 and HgL2 nanosheets in Fig-
ure 1. The 13C cross-polarization magic-angle spinning (CP-
MAS) NMR spectra of HgL1 (Figure 2h) and HgL2 (Fig-
ure 3h) also confirm the chemical structures of the backbone
ꢀ À
ꢀ À
with a characteristic chemical shift of the C Hg at about
100–110 ppm,[20] along with the characteristic chemical shifts
attributed to the triphenylamine and phenyl groups for HgL1
and HgL2, respectively. These values are in good agreement
with those estimated from the nanosheets and the corre-
sponding elemental analysis (Table S1). Both MGONs on
a quartz substrate appear transparent under ambient light,
because of their weak absorptions in the visible region. The
absorption spectrum of HgL1 nanosheets has two peaks in the
ultra-violet (UV) region (peak maximum at 265 and 352 nm.
Figure S8a), while the spectrum of HgL2 nanosheets exhibits
two UVabsorption peaks (peak maximum at 271 and 376 nm,
Figure S8a). The UV-visible spectra of HgL1M and HgL2M
model complexes (Figure S8b) are consistent with the results
of HgL1 and HgL2 nanosheets. The thermal stability of the
nanosheets was analyzed by thermogravimetry (TG) (Fig-
ure S9). The onset decomposition temperatures of HgL1 and
HgL2 nanosheets are about 260 and 2358C, respectively,
indicating their good thermal stabilities.
À
(Figures 2g and 3 g). HgL1 nanosheets show the curves of C
Hg (sp, 283.8 eV), C C (sp, 285.0 eV), C C (sp , 284.4 eV)
and C N (sp , 285.5 eV), while HgL2 nanosheets reveal the
2
À
À
2
À
2
À
À
À
curves of C Hg (sp, 284.1 eV), C C (sp, 284.8 eV), C C (sp ,
=
284.5 eV) and little C O (288.9 eV, due to the adsorbed CO2
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The geometries of HgL1 and HgL2 nanosheets
which were generated by L1 at a concentration of 0.75 mM
and 1508C annealing for 1 h were named as HgL1-0.75–150
and HgL2 nanosheets which were generated by L2 at
a concentration of 1.0 mM and 1508C annealing for 1 h were
named as HgL2-1.0–150. Figure 4c and d presents the 2D
GISAXS pattern of HgL1-0.75–150 and HgL2-1.0–150 nano-
sheets, respectively. To quantify the scattering data, the
corresponding out of plane and in plane cuts are given in
Figure S11. The 2D GISAXS patterns of HgL1-1.0–150 and
HgL2-0.75–150 nanosheets show the layer structure and the
same stacking pattern in different orientations of the two
nanosheets. From the DFT simulation results, both HgL1 and
HgL2 crystallize in the space group of Cmm2 with the unit cell
parameters of a = b = 33.13 Š, c = 3.45 Š, a = b = 908, g =
1208 for HgL1 and a = b = 21.54 Š, c = 3.28 Š, a = b = 908,
g = 1208 for HgL2. The DFT-calculated structures of the two
nanosheets were investigated and the corresponding param-
eters are shown in Figure 4e and f and Table S2. From
Figure 4e, the apparent wavy structure was observed in HgL1
nanosheets with the evident rotation of benzene groups. The
rotation angles of the benzene groups are 72.58, 46.98 and
17.18, respectively. This periodic wavy structure increases the
thickness of the structure to around 4.30 Š. The pore size of
each unit in HgL1 nanosheets is about 33.13 Š. By contrast,
the backbone of L2 in HgL2 nanosheets possesses a good
coplanarity (Figure 4 f). The diameter of each unit and the
thickness of one layer in HgL2 nanosheets are estimated to be
about 21.54 and 1.50 Š, respectively. The porosity of HgL1
and HgL2 bulks was investigated by argon adsorption
measurement,[23] which revealed a Brunauer-Emmett-Teller
(BET) surface area of 183.9 and 26.0 m2 gÀ1 and an average
pore size of ꢁ 33.17 and 21.58 Š for HgL1 and HgL2 bulks
(Figure S12), respectively, which agree well with the calcu-
lated results of both MGONs (Figure 4e and f).
To gain a deep understanding about the nature of two
MGONs (HgL1 and HgL2) with different ligand framework
structures, their crystal structures were revealed by using
powder X-ray diffraction (PXRD), grazing-incidence small-
angle X-ray scattering (GISAXS), density functional theory
(DFT)-calculated simulation and high resolution TEM
(HRTEM) results. As shown in Figure S10, the diffraction
peaks at the 2q angles of 6.48, 12.88, 20.08 and 25.88 for HgL1
were assigned to (100), (200), (300) and (001) facets and those
at 9.88, 20.08 and 27.28 for HgL2 were assigned to (100), (200)
and (001) facets, respectively, indicating a long-range order
within the ab plane and the p-p stacking between the
individual layers. The simulated and experimental PXRD
data are almost in agreement, confirming the chemical
structures of both nanosheets in Figure 1. The weaker and
broader diffraction peaks at 25.88 and 27.28 indicate the
interlayer distance of ꢁ 3.45 and 3.28 Š for HgL1 and HgL2
nanosheets, respectively. In addition, HRTEM analysis re-
veals that the interlayer distance is ꢁ 3.43 Š for HgL1
(Figure 4a) and ꢁ 3.27 Š for HgL2 (Figure 4b), both of
which are consistent with those of PXRD and almost like that
of graphdiyne (3.65 Š), graphene (3.50 Š) and graphite
(3.35 Š).[21] 2D GISAXS was also used to study the crystal-
linity and packing structure of the organometallic nano-
sheets.[22] Here, at the optimal conditions, HgL1 nanosheets
The electronic and optical properties of the two MGONs
were also examined. The iso-surface plots of the highest
occupied molecular orbitals (HOMOs)/the lowest unoccu-
pied molecular orbitals (LUMOs) of the MGONs are shown
in Figure S13. The driving force of Hg···Hg interactions is
revealed based on the HOMO/LUMO distributions.[9] The
electronic occupation of the LUMO is noted for the Hg atoms
=
ꢀ
while electrons are mostly localized on the C C or C C
bonds in the HOMO, leading to the possible interactions
ꢀ
between Hg and C C to further enhance the mercurophilic
À
attractions. Moreover, the DFT-calculated Hg C bond
lengths are 1.99 Š for HgL1 and 2.02 Š for HgL2 nanosheets,
which are consistent with those in some previously reported
structures of Hg-acetylide complexes.[24] The two nanosheets
show variable band gaps from 2.14 to 3.37 eV (Table S2). In
agreement with the HOMO/LUMO plots, the dominant
contribution of the local p-orbitals to the density of states
(DOS) and absorption behaviors was observed (Figure S14).
These results suggest that the introduction of appropriate p-
conjugated unit will lead to the optimized electronic struc-
tures. The results of DFT-optimized geometries are in
accordance with those of the structures from microscopies
and UV-vis absorption data (Figures 2 and 3, Figure S8a).
Figure 4. HRTEM micrographs, 2D GISAXS patterns and DFT simula-
tion results of MGONs. HRTEM micrographs of a) HgL1 and b) HgL2
nanosheets, 2D GISAXS patterns of c) HgL1-0.75–150 and d) HgL2-
1.0–150 nanosheets and DFT-optimized geometries of e) HgL1 and
f) HgL2.
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Nonlinear optical responses of HgL1 and HgL2 nanosheets
blocking (band-filling effect) due to their molecular frame-
works of HgII-polyynes with periodic structures.[25]
To evaluate the NLO properties of HgL1 and HgL2
nanosheets, an open-aperture Z-scan technology with nano-
second laser under both 532 and 1064 nm was utilized. The as-
prepared film at the interface of the two-phase solution was
transferred on the quartz plate. HgL1-0.75–150 and HgL2-
1.0–150 were chosen to examine the NLO properties. The
experimental setups and detailed measurement procedures
are illustrated in Figure S1 and the selected data are collected
in Table S3.
HgL1 and HgL2 nanosheets as saturable absorbers in passively Q-
switched lasers
PQS laser performance was investigated under different
conditions, including the variation of thickness of the nano-
sheets, the use of annealing process and the transmittance of
laser output coupler (Figures S2, S15, S16 and S17). At the
As demonstrated in Figure 5 and Table S3, HgL1-0.75– optimal conditions, the samples of HgL1-0.75–150 and HgL2-
150 and HgL2-1.0–150 nanosheets displayed remarkable SA
responses under broadband optical wavelengths of 532 nm
and 1064 nm. By exciting the sample at the wavelength of
532 nm for a laser pulse with the input energy (E0) of 11 mJ,
1.0–150 were also chosen to examine the PQS properties.
Being the saturable absorbers, HgL1-0.75–150 or HgL2-1.0–
150 nanosheets were removed from the resonator and the
continuous-wave (CW) Nd:YAG laser at 1064 nm could be
the modulation depths (As) of HgL1-0.75–150 and HgL2-1.0– obtained. By inserting the saturable absorber into the laser
150 are 171% with SA intensity (Is) of 0.40 JcmÀ2 and 142%
with the Is value of 0.34 JcmÀ2, respectively. The higher As
values indicate the stronger SA properties. The corresponding
cavity, the PQS operation was realized based on their NLO
behaviors. The reproducibility of the experimental results was
examined by repeating the same PQS experiments with
nonlinear extinction coefficient (beff) values of HgL1-0.75– different samples and the corresponding data listed in
150 and HgL2-1.0–150 nanosheets are 184 and 134 cmGWÀ1,
respectively. Upon excitation by a laser pulse with the higher
E0 of 184 mJat 1064 nm, the As and Is values of HgL1-0.75–150
are 235% and 0.78 JcmÀ2, respectively, which means that the
SA property of HgL1-0.75–150 nanosheets is better than that
of HgL2-1.0–150 with the As and Is values of 150% and
0.76 JcmÀ2, respectively. The corresponding beff values of
HgL1-0.75–150 and HgL2-1.0–150 are 187 and 72 cmGWÀ1,
respectively. So, the SA performance of HgL1-0.75–150 is
superior to that of HgL2-1.0–150 at the same E0 under both
532 nm and 1064 nm. Due to the excellent SA properties,
HgL1-0.75–150 and HgL2-1.0–150 nanosheets would be
potentially good candidates as passively Q-switchers. The
SA behavior of the MGONs should be caused by Pauli-
Table S4 presented a good reproducibility of the PQS
performances. The best results were demonstrated for
HgL1-0.75–150 and HgL2-1.0–150 nanosheets as shown in
Figures 6 and 7, respectively. For the annealed HgL1-0.75–150
nanosheets and the output coupler at T= 20%, the maximum
CW and Q-switched output powers were 1.39 W and 50 mW,
respectively, at an absorbed pump power of 3.33 W. The
output spectrum was recorded and presented as the inset of
Figure 6a. The central wavelength was located at 1064 nm,
which was in accordance with the strongest emission peak of
the Nd:YAG crystal. The stable Q-switched laser operation
was achieved at the pump region of 2.01–3.33 W. As shown in
Figure 6b and c, with the increase of the absorbed pump
power, the pulse width decreased and the repetition frequen-
cy, pulse energy and peak power generally presented an
increasing trend in each case. The shortest pulse width was
Figure 5. NLO properties of MGONs. Open-aperture Z-scan data and
theoretically fitted curves (solid curves) of HgL1-0.75–150 and HgL2-
1.0–150 nanosheets at a) 532 nm with an input energy of 11 mJ;
c) 1064 nm with an input energy of 184 mJ. Theoretically fitted plots of
normalized transmittance vs. incident intensity of HgL1-0.75–150 and
HgL2-1.0–150 nanosheets at b) 532 nm with an input energy of 11 mJ;
d) 1064 nm with an input energy of 184 mJ.
Figure 6. Passively Q-switched Nd:YAG laser properties with HgL1-
0.75–150 as the saturable absorber. a) Output power. The inset is the
laser emission spectrum. b) Pulse width and repetition frequency.
c) Single pulse energy and peak power. d) Pulse train with a repetition
rate of 380 kHz and the corresponding pulse profile with a width of
145.1 ns.
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145.1 ns, and the largest pulse energy and the highest peak
power were 0.132 mJ and 0.91 W, respectively. The corre-
sponding pulse train and single pulse profile are shown in
Figure 6d. For the annealed HgL2-1.0 nanosheets and the
output coupler at T= 20% (Figure 7), the maximum CW and
Q-switched output powers were 446 and 87 mW, respectively,
at an absorbed pump power of 1.39 W. The pump region for
stable Q-switched laser operation was 0.59–1.39 W. Just as for
the HgL1-0.75–150 nanosheets, with the increase of the
absorbed pump power, the pulse width decreased, and the
repetition frequency, pulse energy and peak power generally
increased. The shortest pulse width was 439.5 ns, the largest
pulse energy was 0.541 mJ and the highest peak power was
1.23 W. The corresponding pulse train and single pulse profile
are shown in Figure 7d.
orbital and p orbitals of alkyne ligands, the p-electrons are
delocalized over the whole framework, which results in the
extension of p-conjugation and gives rise to an intramolecular
D-p-A charge-transfer process. The presence of mercuro-
philic interactions in the solid state of the model complexes
and the related polyynes[9] would promote stronger p-p
stacking of the frameworks, which facilitates charge trans-
port.[27] This result is consistent with that of the DFT
calculations. Therefore, the mercurated graphynes can pres-
ent better NLO properties than that of the g-graphyne. Thus,
the SA effects are improved and the PQS performance of the
mercurated graphynes are comparable or higher than those of
the representative 2D materials like graphene, g-graphyne,
BP and TMDs.
Nevertheless, the pulse laser performance can be further
improved by optimizing the thickness of nanosheets, the type
of metal, the nanostructure of ligand as well as the laser
design strategy. For the laser design, the pump absorption
efficiency of the present Nd:YAG laser crystal is only 50% or
so, and a large amount of residual, unabsorbed pump power
will cause thermal induced loss in the SA component. So,
a Nd:YAG crystal with a high pump absorption efficiency will
significantly relieve the thermal effect in the SA component.
Besides, other available methods include the anti-reflection
(AR) coated on the substrate to decrease the insertion loss of
SA, using a pump source with a large fiber core diameter to
increase the energy storage ability.
It is worth mentioning that no thermal damage during the
PQS examination was observed in the two nanosheets and the
output was stable during the laser operation at the highest
pump power, which indicates that the MGONs were very
stable under strong laser irradiation that is suitable for
practical applications. With a flash lamp pumping the
Nd:YAG laser, the laser damage thresholds of HgL1-0.75–
150 and HgL2-1.0–150 nanosheets at 1064 nm were measured
to be 657 and 329 MWcmÀ2, respectively. In the PQS laser
experiments, the highest intracavity power density was
estimated to be 21.2 and 28.6 kWcmÀ2 for HgL1-0.75–150
and HgL2-1.0–150 nanosheets, respectively. Both are much
lower than their respective laser damage threshold.
Figure 7. Passively Q-switched Nd:YAG laser properties with HgL2-
1.0–150 as the saturable absorber. a) Output power. b) Pulse width
and repetition frequency. c) Single pulse energy and peak power.
d) Pulse train with a repetition rate of 160.9 kHz and the correspond-
ing pulse profile with a width of 439.5 ns.
The main experimental data from Figures 6 and 7 are
summarized in Table S5. For comparison, the PQS perfor-
mance parameters of 1064 nm solid-state laser of several
famous 2D nanomaterials like g-graphyne, graphene, BP,
MoS2, WS2 and ReS2 are also listed in Table S5.[13b,15a,16b,d,e] It Conclusion
should be noted that the experimental conditions of the data
for the nanosheets listed in Table S5 are the same, so that such
a comparison has more reference value. It could be seen that
the performance of HgL1 nanosheets is comparable to those
of graphene, BP and ReS2. Among the listed materials, HgL2
nanosheets exhibited the best pulse properties, including the
largest single pulse energy and the highest peak power. From
the data in Tables S4 and S5, the PQS performance of two
MGONs (especially HgL2) showed obvious advantages over
the g-graphyne. Recent computational investigations have
suggested that the doping of alkali metal atoms into the
graphdiyne surface can greatly increase the NLO response
which is caused by the intramolecular donor-p-acceptor (D-p-
A) charge transfer mechanism.[26] It is conceived that when
the mercury ion is polymerized with the multi-substituted
alkynyl monomers via dp-pp interactions between the Hg d
Two MGONs were synthesized as a new kind of metal-
lated acetylenic materials, which were isolated as multi- and
few-layers via liquid/liquid and gas/liquid interfacial reac-
tions, respectively. The results of cytotoxicity experiments
indicate that the MGONs have relatively low cytotoxicity and
virtually no strong toxic effect to human body (Figure S18),
adding a weight to their practical value. The NLO properties
of the nanosheets were investigated under excitation wave-
lengths at both 532 and 1064 nm using the Z-scan technique.
Interestingly, both nanosheets showed optical switching
behavior of SA. Based on the adjustable SA behavior and
structure by controlling the concentrations and the types of
the ligands (L1 and L2), the PQS properties at the wavelength
of 1064 nm were realized with the two nanosheets as saturable
absorbers. With the use of the optimal thickness of the
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nanosheets, annealing process and output transmittance of
[1] a) C. L. Tan, X. H. Cao, X.-J. Wu, Q. Y. He, J. Yang, X. Zhang,
a concave mirror, the PQS performance parameters of HgL1
nanosheets are comparable to those of traditional 2D nano-
materials (such as graphene, BP and MoS2, etc.) whereas
HgL2 nanosheets even exhibit better pulse properties, such as
the larger single pulse energy (0.541 mJ) and the higher peak
power (1.23 W). This work reveals that the metallated
graphyne materials can go beyond graphyne and could not
only be a new family of stable 2D carbon-rich materials under
ambient conditions, but also possess unique properties and
application prospects. This may be helpful in the future design
of various structures for optoelectronic materials and devices.
The chemical nanostructure and macroscopic morphology of
the free-standing and high-quality nanosheets can be adjusted
by changing the molecular structures (such as the metal
chromophores as well as the nanostructure and functional
groups in ligands), the concentration of precursors and the
preparation conditions in the future. Preliminary study
reveals that the work can be readily extended to other
transition metals such as the square-planar platinum(II)
groups and work along this line is currently underway.
Further works are still required to develop other potential
applications of these novel large-area metallated graphyne
films. These large-area continuous nanosheets with control-
lable thickness and high transparency can be transferred on
the optical glass and other substrates, and can be directly
applied as free-standing films in optoelectronic devices. The
combination of such outstanding optical properties with good
mechanical processability and control of molecular dimen-
sions are beneficial for the design of optoelectronic materials
and fabrication of their devices in applications such as
nonlinear optics, optical limiting, optical communications
and others.
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Acknowledgements
This work was supported by the Science, Technology and
Innovation
Committee
of
Shenzhen
Municipality
(JCYJ20180507183413211), the National Natural Science
Foundation of China (NSFC, Grant No. 21905241,
21702214, 61975096 and 21875267), the Hong Kong Research
Grants Council (PolyU 153051/17P), the RGC Senior Re-
search Fellowship Scheme (SRFS2021-5S01), the Hong Kong
Polytechnic University (1-ZE1C), Guangdong-Hong Kong-
Macao Joint Laboratory of Optoelectronic and Magnetic
Functional Materials (2019B121205002), Research Institute
for Smart Energy (RISE) and the Endowed Professorship in
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Conflict of interest
The authors declare no conflict of interest.
Keywords: metallated graphynes ·nanosheets ·
optical properties ·passively Q-switched laser ·
saturable absorption
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Accepted manuscript online: February 24, 2021
Version of record online: April 7, 2021
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