1. Introduction

In recent decades, organic and complex nanomaterials have attracted more and more attentions in the field of novel optical and nano-optoelectronic devices [1–4], such as organic light-emitting diodes (OLEDs) [5], light-weight panel displays [4], organic field-effect transistors (OFET) [6], spin valves and organic solar cells due to their diversity, tailorability and excellent electronic and optical performances than their inorganic materials counterparts [6–11]. In addition, the crystallinity and molecular packing of organic materials may have a significant influence on the performance of the devices [6]. Generally, the crystalline materials have less defects, boundaries and trap states, which could reduce the scatter of carriers. This can further explore the intrinsic characteristics of crystalline materials [6–9].

The 8-hydroxyquinoline metal complex are known as key materials in the design of OLEDs and spin valves [6,9–11]. Especially, tris(8-hydroxyquinoline) aluminum (Alq₃) as the most well-known member of this family since the pioneering work of first efficient low-voltage-driven OLED was developed based Alq₃ by Tang and VanSlyke in 1987 [12]. Afterwards, a lot of groups invest of more attentions to investigate the basic structural, preparation methods of nanostructures and physicochemical properties both in solid and liquid state of Alq₃ [6,11,13]. Besides, tris(8-hydroxyquinoline) gallium (Gaq₃) is another important member of this family, which was firstly reported by Burrows and his associates [14]. Compared to Alq₃, Gaq₃ exhibit a lower turn-on voltage in OLED devices, and hence a proportionately higher power efficiency because it is an excellent π-conjugated electron transport and light-emitting organic materials. Also, Gaq₃ is a more promising therapeutic material to cancer [15]. This has been demonstrated by the recent successes of Gaq₃ in phase I clinical trials, where it shows a strong activity against renal cell cancer [15,16]. Attracted by such interesting phenomenon concerning Gaq₃, many researchers have paid more interests to this material both in theoretical studies and fabrication methods, especially in crystalline nanostructures [17–22]. For example, it has been demonstrated that Gaq₃ is a stable polar and chiral molecule with two isomers, which is meridional (mer-) isomeric form with C₁ symmetry and facial (fac-) with C₃ symmetry, δ-polymorph formed by annealing of α-phase at 380°C under the vacuum conditions [21,22]. Zhang et al. studied the geometries of ground and first excited states of mer- Gaq₃, they concluded that the HOMO mainly localized on A-quinolate ligand, and LUMO mainly localize on B-ligands [23–25]. Afterwards, Gillin et al. carefully investigated the influence of spectroscopic properties of Gaq₃ under high hydrostatic pressure [24]. Accompanying the dramatic development of nanoscience and nanotechnology, much efforts have been devoted into the controlled preparation of zero and one dimensional (0D and 1D) of Gaq₃ nanostructures, particularly, 1D nanomaterials can be utilized as building-blocks for nanoscale devices [21,22]. For instance, Yu et al. reported thermal properties and spectroscopy performances of crystalline Gaq₃ 1D nanostructures and nanospheres, which were fabricated by thermal evaporation under cold trap [22]. In 2014, Muhammad and Sulaiman reported the enhanced morpho-optical properties of amorphous nanorods, which were obtained by post-thermally in vacuum from Gaq₃ films [20]. Again, the addition of Gaq₃ organometallic material as a secondary acceptor is to boost the performance and reproducibility of the solar cells [21], especially optoelectronics parameters have great relationship with nanostructure evolution [6]. In addition, the isolated Gaq₃ molecules are non-magnetic, but Gaq₃ molecules could show magnetism if existing an H vacancy [25]. However, all of the 1D and 0D nanostructures of Gaq₃ reported to date were prepared by traditional physical vapor deposition or post-annealing method, which cannot accomplish at room temperature. Therefore, an extremely facile solution approach, to fabricate 0D or 1D crystalline Gaq₃ nanostructures, is highly desired.

In this study reported here, we, for the first time, utilizing a facile solution method to controllable synthesis crystalline Gaq₃ nanoparticles and nanorods with regular shape and uniform size without surfactant, anti-solvent and other reagents. All the products were prepared by volatilizing Gaq₃ solution in air directly. The prepared Gaq₃ nanoparticles and nanorods exhibit excellent optical performances. Our method could potentially be extended to other functional organic light-weight materials, and the obtained nanostructures have great potential in nanoscale electronic and optoelectronic applications.

2. Experimental

2.1 Materials

Commercial Gaq₃ powders are provided by the Nichem company (Taiwan, China) while Chloroform (CHCl₃) and ethanol (C₂H₆O) are supplied by the Tianjin Fuyu Fine Chemical Co., Ltd (Tianjin, China). All the reagents were used directly without further purification.

2.2 Synthesis of Gaq₃ microparticles

Initially, Gaq₃ solution was prepared by using only the Gaq₃ powder and organic solvent. Typically, 60 mg Gaq₃ powders and 5 ml CHCl₃ were mixed together, followed by an agitation process for overnight. Then, the yellow and homogeneous Gaq₃ solution can be obtained. Thereafter, a certain amount of Gaq₃ stock solution was dropped slowly onto the silicon substrate, which was placed in a beaker, by a long capillary needle. The beaker was sealed with parafilm to prevent CHCl₃ volatilizing off quickly at room temperature. About 24 h latter, the Gaq₃ nanoparticles formed on a large scale on the substrate.

2.3 Synthesis of Gaq₃ nanorods

The preparation method basically same as that of Gaq₃ nanoparticles. Typically, 150 mg Gaq₃ powders and the component solvent (5 ml CHCl₃ and 1 ml EtOH) were mixed together and with agitation for overnight. Then, a certain amount of Gaq₃ stock solution was dropped slowly onto the silicon substrate. Slowly, it can be found that the Gaq₃ nanorods grew on the substrate on large scale, the evident nanostructures should be measured by high resolution Scanning Electron Microscope. The evaporation process need about 48 h due to the existing of EtOH. The general processes can be illustrated by Fig. 1.

 

Fig. 1 A schematic view of the preparation process of crystalline Gaq₃ nanostructures.

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2.4 Measurement

The surface morphology and the size of the as-prepared products were characterized by a high resolution Scanning Electron Microscope (SEM, S-4800) and a transmission electron microscope (TEM, Tecnai G² F20) with energy dispersive X-ray (EDX). The microstructure analysis were carried out on a Powder X-Ray Diffractometor (XRD, Bruker AXS, D8 Advance) and FTIR by FT-IR & Raman spectrophotometer (Thermo Nicolet, NEXUS 670). The photoluminescence (PL) spectra were dispersed by a Jobin-Yvon iHR320 monochromator excited at 325 nm. In addition, the CIE chromaticity coordinates of the PL spectra have been calculated by using software CIE1931. The color luminescence images were taken from confocal laser scanning microscope (OLYMPUS, FV500).

3. Results and discussion

Figure 2 illustrates SEM and TEM images of the as prepared products at different resolutions, which indicate that many rod or particle-shaped assemblies were successfully fabricated, respectively. From the Fig. 2(a) (low magnification SEM image), we can see that randomly distributed Gaq₃ nanorods can be synthesized on a large scale. The rods length of the longest one can reach 50 ± 5 μm with an aspect ratio of length/diameter (L/D ratio) of about 60. Clearly, the Gaq₃ nanorods have regular hexagonal appearance (Fig. 2(b), high-magnification SEM image), and each rod has six regular facets and smooth surfaces along with their entire length. A TEM picture of the products in Fig. 2(c) further confirms the rod-shape Gaq₃ sample has uniform structure, and few obviously defects can be observed. In addition, the result of EDX microanalysis proves the chemical composition and mass percentage (C, O, N and Ga) of Gaq₃ molecule, see the inset table in Fig. 2(c).

 

Fig. 2 The SEM and TEM photographs of the as-prepared Gaq₃ nanostructures. Figures 2(a) and 2(b) are low and high magnification SEM images of Gaq₃ nanorods, respectively; Fig. 2(c) TEM image and inset table further reveal the morphology and chemical composition of as-prepared Gaq₃ rods; Figs. 2(d) and 2(e) are low and high magnification SEM images of Gaq₃ nanoparticles, respectively, the inset is the higher resolution image of single Gaq₃ particle; The TEM image of Gaq₃ nanoparticles, and the inset table is the chemical composition measured by EDX.

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As previous reports, the shape and the distribution density of Alq₃ nanostructures can be partially controlled by growth conditions such as ambient temperature, volatilization speed of organic solvent, especially the concentration of the Alq₃ solution [6,26–28]. Actually, plenty of coffee-bean-shaped Gaq₃ nanoparticles have been successfully prepared by this facile evaporation-induced route when the concentration of the solution is 12 mg/ml, as shown in Fig. 2(d). The particles lay on the substrate at random directions (low resolution SEM photos), given in Fig. 2(d). The coffee-bean-shaped Gaq₃ nanoparticles have an average length of 1 um and diameter of 500 nm, and L/D ratio is about 2. Figure 2(e), the high-magnification SEM image, shows the nanoparticles have regular geometric shape and uniform size. It is different from the Gaq₃ nanorods, however, the surface of the coffee-bean-shaped Gaq₃ nanoparticles are very rough with many stripes or laminated structure, see Fig. 2(e) and the inset. Further, the morphology and chemical composition of the Gaq₃ particles were ulteriorly characterized by TEM in combination EDX, see Fig. 2(f) and the inset table. The mass percentages of elements C, N, O and Ga are 44.49%, 31. 40%, 7.67% and 16.44%, respectively. Therefore, the result exhibits that the fabricated nanoparticles are composed of Gaq₃ molecules.

The Raman spectra were measured to further reveal the composition and phase of obtained Gaq₃ nanostructures [27–29]. As a reference, the Raman spectrum of Gaq₃ powders was also tested as exhibited in Fig. 3(a). The Raman spectra of Gaq₃ nanorods and particles are highly similar to that of the starting powders, manifesting the high purity of sample and no degeneration or change of chemical bonding. The peaks at 499.4, 528.1 cm⁻¹ are considered as the skeletal in-plane bending of quinoline ligand [13]. The low-energy peaks at 116.9, 178.4 and 214.4 cm⁻¹ are attributed to the characteristic Raman fingerprints of the α polymorph [13]. Since the low-energy modes are highly sensitive to molecular packing, the Raman results thus reveal the prepared Gaq₃ nanostructures are composed of α-Gaq₃. The XRD pattern in Fig. 3(b) matches well with that of the α-phase Gaq₃ 1D nanostructures [13,27], confirming further that the assembled products are α-phase Gaq₃ with a preferred orientation of [001] direction, that is, the crystalline c-axis direction [13]. For reference, the XRD of Gaq₃ powder was also recorded. Evidently, the relative peak intensities and positions in the XRD pattern of Gaq₃ nanorods and nanoparticles are remarkably different from those of the starting material, revealing orientated growth in the Gaq₃ nanostructures.

 

Fig. 3 (a) and (b) the Raman and XRD spectra of of Gaq₃ powder, rods and nanoparticles, respectively.

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It has been reported that the functional performances of nanomaterials have closed relationship with it shapes and structures. Therefore, we can optimize the properties for practical utilization through controllable fabrication of materials with different appearances and nanostructures. To investigate the functional prospects of these newly synthesized products, the photoluminescence and waveguide characteristics were recorded, shown in Fig. 4. The PL spectra, at room temperature, of the Gaq₃ nanorods and nanoparticles were excited at the wavelength of 325 nm, given in Figs. 4(a) and 4(c). Both of them show green mission with the PL peaks at 526 nm (rod-shaped, 2.36 eV) and 539 nm (coffee-bean-shaped, 2.3 eV), respectively. Clearly, all the peaks show an excellent symmetry in the wavelength of 450 nm to 750 nm, which is similar to that of the Gaq₃ crystalline films obtained by post-thermal treatment [29]. The broad peak is due to the transitions from S₁ to S₀ and the tail can attribute to triplet exciton recombination [27,29]. In addition, the peaks position shift (about 13 nm) between Gaq₃ rods and particles due to the lattice fluctuation, the shapes and nanostructures of the samples [27]. These results suggest that the assembles (rod-shaped and coffee-bean-shaped Gaq₃ nanostructures) preserve the natural optical properties of Gaq₃ molecules while having the merits of a greater tunability and potential applications on nano-optoelectronic devices.

 

Fig. 4 (a) and (c) the PL spectra, excited at 325 nm, of Gaq₃ nanorods and nanoparticles, the insets are calculated CIE chromaticity coordinates (0.283, 0.551) and (0.277, 0.553) of Gaq₃ rods and particles, respectively; Figs. 2(b) and 2(d) are the photoluminescence microscopy images of Gaq₃ nanorods and nanoparticles excited with 380-450 nm.

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Up to now, the majority of studies of Gaq₃ nanostructures are concentrated on PL, electroluminescence (EL), and field emission. To investigate the novel applications of Gaq₃ nanomaterials, such as waveguides, is highly desirable. Figures 4(b) and 4(d) exhibit the a color luminescence microscopy images of Gaq₃ samples (rods-shaped and particles-shaped) on glass substrates, which further indicate a characteristic green emission for Gaq₃. Obviously, high luminescence zones could be observed at both ends of Gaq₃ rods and particles. It reveals that the rods and particles can propagate the emission light towards the tips under suitable excitation light. According to the experimental results, the newly synthesized Gaq₃ assembles are typical optical waveguide materials, having the same properties as Alq₃ sub-micro wires [30].

4. Conclusion

In summary, a facile self-assembly route without surfactant, antisolvent, and other solvent has been developed to synthesize Gaq₃ nanorods and coffee-bean particles with regular shape and crystallinity. This approach also would be extended to other organic materials for controlled synthesis of nanostructures. Raman spectra and XRD results show that the prepared products have the α-phase crystal structure. PL analysis show the optical properties have closed relationship with shapes and nanostructures. of Gaq₃. The good crystallinity and excellent waveguide properties make it be a potential building blocks for optical and photoelectron devices.