ZHENG Yong-ping, FENG Qian, TANG Nu-jiang, DU You-wei

(1.Fujian Provincial Key Laboratory of Quantum Manipulation & New Energy Materials, Fujian Normal University, Fuzhou350117, China;2.National Laboratory of Solid State Microstructures & Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing210093, China)

Abstract: Graphdiyne was prepared by a cross coupling reaction and characterized by SEM, TEM, XRD, Raman spectroscopy and XPS. Its photoluminescence (PL) properties were investigated. It was found that the graphdiyne has a thickness of ca. 1 μm, a O/C atomic ratio of ca. 0.174 1 and many oxygen-containing functional groups. It exhibits strong ultraviolet (UV) PL at 397 nm and optical absorption at ca. 280 nm. It is proposed that the oxygen-containing functional groups in the graphdiyne induce charge transfer, leading to the PL properties. As a new kind of artificially synthesized carbon allotrope, graphdiyne with UV PL have many applications.

Key words: Cross coupling reaction; Graphdiyne; Photoluminescence

1 Introduction

Graphene and related materials are considered as promising photoluminescence (PL) materials, which can beused in fluorescent bioimaging stem owing to their biocompatibility[1-2]. The ideal graphene whichconsists of 100% sp2-hybridized carbon atoms and is a zero-band-gap material, is highly unlikely to emit luminescence. For graphene oxide (GO), the bonding structures are much more complicate than that in graphene. Most in-plane carbon atoms in GO sheets preserve sp2-hybridized structures, and can covalently bond with oxygen in the forms of epoxy and hydroxyl groups. Moreover, the edge carbon atoms can be bonded with oxygen in the forms of carboxyl and carbonyl groups. This kind of the hybrid sp2/sp3structure opens up the possibility for PL in graphene-based materials by recombination of electron-hole (e-h) pairs localized within small sp3carbon clusters embedded within a sp2matrix. Thus, by controlling the fraction of sp2/sp3bonds, the PL can be tuned from the visible to near-infrared wavelength range[3-4].

Graphdiyne (GDY), another 2-dimensional periodic carbon allotrope, is greatly different from the case of the hybridized orbital in graphene which only has single or double bonds. GDY can be built from triple- and double-bonded units of two carbon atoms[5-9]. Recently, this kind of new sp-sp2hybrid structure has aroused great interests owing to its potential new physical and chemical properties. In 2010, GDY was first synthesized by Li et al[10-12]. Thereafter, GDY motivated enormous investigations in various areas such as the electronic properties[13-14], Li-battery storage[15-16], metal catalytic[17], solar cell[18]and diode device[19]. Theoretically, GDY is a kind of two self-doped non-equivalent distorted Dirac cone materials, which has been predicted to possess superior electronic properties to graphene[5]. Furthermore, the sp-sp2hybrid bonds construct the structures of rings and chains, which leavesmore complicated π bonds in planar, can be further functionalized through substitution reactions with little damage to the extended π -electron conjugation system. Moreover, this kind of flexible structure can be utilized in composite materials for improving performance. For example, GDY/ZnO composites have been demonstrated to have excellent performance as ultraviolet (UV) photo detectors[20]. However, the PL properties of GDY are still lacking. Therefore, the investigation of the optical properties of GDY is urgent and of great significance.

Herein, we report the PL properties of GDY prepared by cross-coupling reaction. The results show that the as-prepared GDY can emit strong UV PL at 397 nm when excited by UV radiation. Our findings can extend the knowledge of GDY as a new kind of carbon-based PL material.

2 Experimental

GDY was synthesized on the surface of copper foilvia a cross-coupling reaction using hexaethynylbenzene as the precursor[10]. The monomer of hexaethynylbenzene was synthesized by adding tetra- butylammonium fluoride into a hexakis[(trimethylsilyl)- ethynyl]benzene solution in tetrahydrofuran for 10 min in an ice bath. The GDY was grown on the surface of copper foil in the presence of pyridine by a cross-coupling reaction of the monomer of hexaethynylbenzene for 72 h at 80 ℃ under nitrogen atmosphere. In the reaction, the copper foil was used not only as a catalyst for the cross-coupling reaction, but also as a substrate for growing the GDY. After reaction, a black film appeared on the copper foil, and some powder was left after pyridine was evaporated. For purification, both the copper foil and powder was washed sequentially with acetone, hot dimethylformamide and ethanol. Then, the film was peeled off by ultrasonication in deionized water (DI) for 2 h. To remove metallic ion and other inorganic salt, the powder was washed sequentially by 2 mol/L HCl, 4 mol/L NaOH, DI water and ethanol for several times.

The morphology of the sample was investigated by transmission electron microscopy (TEM, JEM-2100, Japan) and scanning electron microscopy (SEM, S-3400N, Japan). Raman spectroscopy (HORIBA, UK) can be used as a finger printing and quality measurement to characterize GDY structures. Raman spectrum of the sample was measured on glass base under 532 nm at room temperature. To avoid damaging the sample, the incident power was reduced to 0.5 W. Chemical structure of the sample was characterized by X-ray photoemission spectroscopy (XPS) on a PHI-5000 Versa Probe using Al Ka radiation. The UV-vis absorption and PL spectra were measured at room temperature on a Shimadzu UV-2450 spectrophotometer and a HORIBA FluoroLog-3 fluorescence spectrophotometer, respectively.

3 Results and discussion

3.1 Morphology and microstructure of GDY

Fig. 1 (a) SEM image of GDY on copper foil surface, (b) Typical TEM image (Insert is the HRTEM image), (c) SAED pattern taken from dash circle area in (b), the ring corresponds to crystal planes and (d) GDY crystal structure with the lattice of 0.944 nm, the dash line indicates a unit cell.

Fig. 2 (a) Raman spectrum, (b) XPS spectrum and (c) High-resolution C 1s spectrum. The circle symbols, line and other symbol lines are the measured, fitting curve and fitted single peaks, respectively and (d) Chemical bond percentages calculated from the high-resolution XPS C 1s spectrum.

3.2 PL properties of GDY

Fig. 3a is the UV-vis spectrum of GDY. It is found that there is an absorption peak at ca. 280 nm (5.14 eV), in consistent with the theoretical prediction of 4.93 eV using the GW+RPA method by considering quasiparticle effects[23]. This peak can be resulted from the transition around the Van Hove singularities at the M and K point. To explore the PL properties of the GDY, a detailed PL study was carried out using different excitation wavelengths ranging from 310 to 370 nm. As shown in Fig. 3b, there is a strong PL emission peak at ca. 397 nm as the excitation wavelength ranged from 310 to 330 nm. However, as the excitation wavelength increases further, the peak is weakened and broadened dramatically along with the red-shift.

Fig. 3 (a) UV-vis absorption spectrum and (b) PL spectra at different excitation wavelengths. Insert is PLE spectrum with the detection wavelength of 397 nm.

To investigate further the PL emission behaviour, we recorded the PL excitation (PLE) spectrum under 397 nm excitation. As shown in the insert in Fig. 3b, the PLE shows one strong peak at 320 nm. One can considerthat the strong peak at 320 nm can be regarded as the transitions betweenσandπorbitals[24]. As known, GDY theoreticallyhas a direct band gap of ca. 0.55 eV. Thus, the band gap transition is tiny and corresponds to near-infrared emission. Thus, it can’t be the source of PL emission observed. Note that some studies have demonstrated that PL emission in graphene quantum dots can attribute to the size because quantum confinement effect can open the band gap. Especially, the effect can be greatly enhanced when the size of quantum dots is reduced to 10 nm[25].However, TEM analysis shows that our GDY sample is thick and has a large size, suggesting that the size effect should be excluded in our GDY sample.It is well known that the possible origin for PL emission in GO is the oxygen-rich functional groups. As reported, the oxygen-rich functional groups on graphyne sheet can lead to significant charge transfer from C to O and, thus can open a band gap in graphyne[26]. Actually, the XPS results clearly show that there are many oxygen-containing functional groups in our GDY sample.Reasonably, one can proposed that the charge transfer from C to oxygen-containing functional groups in GDY sheets is responsible for the UV PL, similar to the PL mechanism in N-doped graphene quantum dots[24,27].

4 Conclusions

In summary, GDY was synthesized by a cross-coupling reaction. The Raman spectrum and morphology show that as-prepared GDY has a 2D crystalline structure. The results show that as-prepared GDY has UV PL at ca. 397 nm. It is proposed that the oxygen-rich functional groups in GDY sheet can lead to charge transfer and thus open the band gap, and which can be the origin of PL emission. The results show that GDY can be a new kind of carbon-based PL material, and can have many potential applications such as UV photodetectors and bioimaging materials.