Crystalline Gaq31-D nanostructures and nanospheres could be fabricated by thermal evaporation under cold trap. The influences of the key process parameters on formation of the nanostructures were also investigated. It has been demonstrated that the morphology and dimension of the nanostructures were mainly controlled by working temperature and working pressure. One-dimensional nanostructures were fabricated at a lower working temperature, whereas nanospheres were formed at a higher working temperature. Larger nanospheres could be obtained when a higher working pressure was applied. The XRD, FTIR, and NMR analyses evidenced that the nanostructures mainly consisted of δ-phase Gaq3. Their DSC trace revealed two small exothermic peaks in addition to the melting endotherm. The one in lower temperature region was ascribed to a transition from δ to β phase, while another in higher temperature region could be identified as a transition from β to δ phase. All the crystalline nanostructures show similar PL spectra due to absence of quantum confinement effect. They also exhibited a spectral blue shift because of a looser interligand spacing and reduced orbital overlap in their δ-phase molecular structures.
KeywordsGaq3 1-D nanostructures Nanospheres Thermal evaporation Crystallization Phase transition
In the last decade, nanoscale materials have drawn considerable attention because they present an extremely high surface area to volume ratio which makes a certain number of optical, electrical, mechanical, and physical properties apparently different from those of their counterpart bulk solids [1–3]. Among the nanoscale materials, one-dimensional (1D) form is particularly attractive because it may provide access to three different contact regions, inner and outer surfaces as well as both ends. One-dimensional nanomaterials can also be used as the building blocks for nanoscale devices. A number of studies have been devoted to generate 1-D nanomaterials from most kinds of materials, which clearly indicate that solid materials can be prepared as 1-D nanostructures by properly selected preparation methods . However, the efforts were mostly focused on inorganic or metallic nanomaterials. Only few studies concerning organic nanomaterials have been reported [5–8]. Until recently, it has been demonstrated that some 1-D organic nanostructures exhibit promising applications for optoelectronic devices due to their unique characteristics such as flexibility, high photoconductivity, nonlinear optical effects, good field-effect mobilities, and remarkable chemical and thermal stabilities [9–11]. Therefore, more exploration of 1-D organic nanostructures is certainly required, and precise morphological control of the organic nanostructures has to be obtained before practical applications. Previously it has been reported that single-crystalline copper phthalocyanine (CuPc) nanoribbons with a good controlled diameter ranging from 50 to 125 nm could be formed by physical vapor transport technique. Various architectures of organic field-effect transistors (OFETs) based on patterned CuPc nanoribbons were also achieved [12–14].
8-Hydroxyquinoline metal chelate complexes (Mq3), one type of the organic semiconducting materials, are attracting increasing interests because they can be employed in organic light-emitting diodes (OLEDs) as an electron transport and emitting material [15–17]. They not only contribute to lower operational voltages and high efficiency of the devices, but also provide the capability for color tuning which can be achieved by grafting different substituents . Among the Mq3, tris(8-hydroxyquinolinato)aluminium(III) (Alq3) is most well known and has been frequently used in OLEDs due to its stability and good charge transport ability. Its fundamental characteristics, such as molecular geometry and molecular orbitals, have also been explicitly reported [18, 19]. More recently, it was demonstrated that Alq3 nanostructures could be prepared by means of physical thermal evaporation [20–23]. The amorphous Alq3 nanoparticles could grow into α-phase crystalline nanowires by a one-step heat treatment process. A complete structural transformation to crystalline nanowires would lead to a blue shift and enhanced intensity of the photoluminescence (PL) spectrum [20, 21]. Some inorganic semiconductor quantum dots also exhibited outstanding optical properties due to the large oscillator strengths, narrow spectral linewidths, and high stability, so that they could be easily integrated inside devices [24, 25]. Unfortunately, the rigidity and bio-uncompatibility of most inorganic nanomaterials will be bottlenecks limiting their applications to flexible and biological devices. Thus for long-term development tendency, organic semiconductor nanostructures reveal more potential and advantages, as compared to inorganic nanomaterials.
Tris(8-hydroxyquinoline)gallium(III) (Gaq3), another Mq3 first reported by Burrows et al., could provide a higher electroluminescence yield than Alq3 when it was used in OLEDs. This suggested that it could be a more promising candidate as an electron transport and emitting material. [26–28]. Therefore, the preparation method, optical, physical, and crystallographic characteristics of Gaq3 nanostructures are worthy of further investigation. In this work, a similar thermal evaporation approach for fabrication of Gaq3 nanowires and nanospheres was disclosed. The key process parameters such as working gas, working temperature, and working pressure were varied to achieve various morphologies and dimensions. It was demonstrated that the nanostructures mainly consisted of δ-phase Gaq3. The DSC analysis of crystalline nanospheres revealed a transition from δ to β phase in the lower temperature region and another transition from β to δ phase in the higher temperature region. All the nanostructures showed similar PL spectra and a spectral blue shift due to a looser interligand spacing and reduced orbital overlap in the crystalline nanostructures.
Gaq3 nanowires and nanospheres could be fabricated by thermal evaporation. The schematic thermal evaporation system had been presented elsewhere . This system mainly consists of four parts: a process chamber, a pumping system, a gauge system, and a heating system. Two graphite electrodes are installed in the middle of the process chamber. A graphite boat spanning across the two electrodes is used as a resistive heater. The DC current applied to the graphite boat is converted by a power supply transformer. A K-type thermocouple in contact with the boat is employed to control the working temperature. The conjunctional circuits of the power supply, thermocouple, and cooling water are arranged below outside the process chamber. A movable shutter is utilized to control evaporation time. The pumping system including a rotary vane pump and a turbo pump is able to evacuate the process chamber down to a pressure lower than 1 × 10−6 torr. The top of the process chamber is a liftable cap with a hollow cavity inside. Liquid nitrogen can be poured into and fill the cavity for rapid uniform cooling of the n-type (100) silicon substrates. The substrates were repeatedly ultrasonically rinsed in acetone followed by dry purge of N2 gas before use. They were then adhered to the underside of the cap for growth of Gaq3 nanostructures. A stainless steel ring was put on the graphite boat, and commercial Gaq3 powder was placed into the ring. The distance between the graphite boat and the substrate was fixed at 10 cm.
The working gases used in this study are He and Ar. After the process chamber was evacuated to 1 × 10−6 torr, the working gas was introduced into the chamber. Once the graphite boat was heated to the working temperature, the shutter was moved away and thermal evaporation started. Meanwhile, liquid nitrogen was poured into the hollow cavity for cold trap of sublimed Gaq3molecules on the substrate. After the condensation was complete, the process chamber was evacuated again, and the whole system returned to room temperature. The key process parameters in the thermal evaporation process are working gas, working pressure, and working temperature, etc. Various parameters cause dissimilar nanostructures. The working pressures of 10 and 50 torr and the working temperatures ranging from 310 to 400 °C were adopted to investigate their influences on the morphology and dimension of nanostructures by a field emission scanning electron microscope (FESEM, JEOL-JSM6500F). An X-ray diffraction (XRD) spectrometer (Shimazu-Mode-XRD-6000) with Cu Kα radiation (λ = 1.545Å) and a scanning rate of 1 deg/min was employed to examine the crystallinity of Gaq3powder and nanostructures. A differential scanning calorimeter (DSC, Seiko 220C) with a heating rate of 20 °C/min was used to analyze their thermal properties. The infrared (IR) spectra were achieved by a fourier transform infrared (FTIR) spectrometer (HORIBA FT-730) with a scanning rate of 5 mm/s and a resolution of 4 cm−1to identify their isomorphism. The nuclear magnetic resonance (NMR) spectra were obtained by the spectrometers of Bruker DSX400WB and Varian Unityinova 500. Their PL spectra ranging from 400 to 700 nm were measured using a fluorescence spectrometer (Perkin Elmer LS55) with an excitation wavelength of 390 nm and a scanning rate of 500 nm/min.
Results and Discussion
Preparation of Gaq3nanostructures
Working gas type
Working temperature and working pressure
Structural characterization and spectroscopic analysis
The major difference between Gaq3 and Alq3 nanostructures is that Gaq3 nanostructures are crystalline whereas Alq3 nanostructures are amorphous [20–22]. Because the molecular weight of Alq3 is lower than that of Gaq3 and the working temperatures for evaporation of Alq3 nanostructures are higher than those of Gaq3 nanostructures; the energy loss and nucleation of sublimed Alq3 molecules are rapid, resulting in faster growth of Alq3 nanostructures on the substrate. The Alq3 molecules can thereby stack in a more disordered way and generate the amorphous state. The formation of crystalline Gaq3 nanostructures can be attributed to slower sublimation and growth so that Gaq3 molecules are able to stack in a more ordered way. The thermal properties of Gaq3 and Alq3 nanostructures are also similar [20–22]. Both Gaq3 and Alq3 nanospheres exhibited two peaks on their DSC curves, implying two phase transitions occurred in their heating processes. One was at around 120–150 °C and the other was at around 350–390 °C. The one in the lower temperature region of amorphous Alq3 nanospheres has been identified as a transition to α phase . Since the melting point of Gaq3 is around 10 °C lower than that of Alq3, the intermolecular interaction of Gaq3 is comparatively weaker. Thus, it is reasonable to deduce that the two-phase transition temperatures of Gaq3 nanostructures are lower than those of Alq3 nanostructures.
This study has disclosed a physical thermal evaporation approach for fabrication of crystalline Gaq3nanospheres and 1-D nanostructures under cold trap. The influences of working gas, working temperature, and working pressure on the formation of the nanostructures were explored as well. It was demonstrated that their morphology and dimension were mainly controlled by working temperature and could be modulated by varying working pressure. A lower working temperature caused growth of 1-D nanostructures, whereas a higher working temperature resulted in formation of nanospheres. When working pressure increased, larger nanospheres were obtained. To summarize, 1-D crystalline nanostructures could be fabricated in He gas at 310–330 °C, and crystalline nanospheres could be formed in He gas at 390–400 °C. According to XRD, FTIR and NMR analyses, Gaq3raw powder was identified as β phase and the crystalline nanostructures mainly consisted of δ-phase Gaq3. The DSC trace of crystalline nanospheres revealed two small exotherms in addition to the large melting endotherm, implying two phase transitions occurred during the heating process. The one in lower temperature region was ascribed to a transition from δ to β phase, and another in higher temperature region could represent a transition from β to δ phase. Due to absence of quantum confinement effect, all crystalline nanostructures show similar PL spectra with an emission maximum at around 508 nm regardless of their morphology and dimension. Compared with the β-phase powder, the δ-phase nanostructures had a loose molecular packing and interligand spacing, leading to decreased orbital overlap and a spectral blue shift.
This work was supported by the National Science Council of Taiwan under Contract No. NSC 93-2216-E-007-034 and NSC 94-2216-E-007-029.