A team of researchers recently published a paper in the journal ACS Applied Nano Materials demonstrating the successful synthesis of copper nanoparticles (Cu NPs) in the organic phase by reduction method for the first time along with the feasibility of using them in practical applications.
Study: Organic-Phase Synthesis of Blue Emission Copper Nanoparticles for Light-Emitting Diodes. Image Credit: Toria/Shutterstock.com
Cu NPs are suitable for different applications such as biomedicine and photoluminescence as they are cost-effective and non-toxic compared to other metal nanoparticles, such as gold (Au) NPs. Copper nanoparticles also have a tunable emission wavelength.
Cu NPs receive less attention than other metal NPs however, as Cu is easily oxidized compared to Au or silver (Ag), making the synthesis of Cu NPs with fewer defects considerably difficult.
Currently, all Cu NPs are synthesized in the aqueous phase owing to a limited number of organic reducing agents. However, aqueous phase Cu NPs are unsuitable in electroluminescent-light emitting diodes (EL-LED)-related applications as aqueous Cu NC films are often damaged during the deposition of the electron transport layer.
Although organic-phase Au NP-based EL-LEDs were fabricated by the phase transfer method to overcome these limitations, the method can potentially cause ligand detachment, leading to low photoluminescence quantum yield (PLQY) and more defects.
In this study, researchers synthesized an organic-phase Cu NPs by the reduction method and evaluated their effectiveness in EL-LEDs.
Copper (II) bromide (CuBr2), indium chloride (InCl3), tris(dimethylamino)phosphine (DMA)3P, trioctylphosphine oxide (TOPO), 1-octadecene (ODE), 1-dodecanethiol (DDT), oleylamine (OLA), and trioctylphosphine (TOP) were used as starting materials for this study.
3 g TOPO, 5 ml OLA, and 0.45 mmol CuBr2 were mixed into a 50 ml three-necked bottle and then degassed at 140oC for 60 min to remove oxygen and water. Subsequently, argon (Ar) was added to the mixture, which was again heated at 200oC. A mixture of 1 ml OLA and 0.35 ml (DMA)3P was quickly added to the above mixture at 200oC to reduce Cu2+.
1.5 ml DDT was then added and the resultant mixture was heated at 200oC for 40 min to obtain Cu NP solution. The obtained Cu NP solution was subjected to centrifugation for 3 min at 11000 rpm to remove impurities, then mixed with methanol and octane, and again centrifuged for 3 min at 11000 rpm. Eventually, the precipitate was dissolved in octane.
Cu NPs passivated by InCl3 were also synthesized following the above method with minor changes. In this synthesis, 7 g TOPO, 3 ml OLA, and 0.45 mmol CuBr2 were used as input materials, and a mixture of 6 ml TOPO and 100 mg InCl3 was used after the reduction of Cu2+ and the resultant mixture was heated for 60 min at 300oC. Eventually, the collected Cu NPs were dissolved in hexane.
Cu NP-based LED was fabricated using 30 nm poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS), 30 nm polyvinyl carbazole (PVK), 40 nm Cu NPs, 100 nm aluminum (Al), and 40 nm zinc-doped magnesium-oxide (ZnxMg1-xO) NPs.
A FEI Tecnai G2 F30 transmission electron microscope, X-ray diffractometer, FluoroSENS-9000 spectrophotometer with a static xenon lamp, Hamamatsu Quantaurus-QY, Auger electron spectroscopy (AES), Fluo Time 300 “Easy Tau” fluorescence lifetime spectrometer, UV-Vis-NIR spectrometer, and Everfine ATA-500 were utilized to characterize the fabricated samples.
Organic-phase Cu NPs were successfully synthesized for the first time using (DMA)3P as the reducing agent. Blue light emission was detected when the samples were excited by an ultraviolet lamp, indicating the successful fabrication of blue-light emitting Cu NPs.
The size of the prepared Cu NPs was 5.24 ± 0.47 nm. The PLQY of Cu NPs increased gradually and eventually attained the highest value of 15% when the reaction temperature was increased from 180 to 200oC. However, the PLQY of the Cu NPs decreased when the temperature was further increased above 200oC.
No considerable influence of the reaction temperature was observed on the Cu NP emission wavelength. The low PLQY value was attributed to the oxidation of Cu(0) to Cu+ on the Cu NP surface.
The In3+ passivation of the Cu NP surface at high temperatures successfully prevented the oxidation of Cu, which increased the maximum PLQY value to 32%. Additionally, a blue emission of 452 nm wavelength was observed.
The fluorescence lifetime of Cu NPs without In3+ passivation and with In3+ passivation was 3.4 ns and 4.9 ns, respectively. The PLQY of the Cu NPs remained almost unchanged even after their exposure to air for 20 days, indicating exceptional stability.
The Cu NP-based EL-LEDs were directly fabricated using organic-phase In3+-passivated Cu NPs without performing the phase transfer process. The maximum current efficiency and external quantum efficiency (EQE) of these fabricated EL-LEDs were 0.3 cd A−1 and 0.14%, respectively. The EQE of these EL-LEDs was comparable to the gold Au NP-based LEDs as the orthogonal ligand of Cu NPs did not interfere with other LED layers.
Significance of the Study
Taken together, the findings of this study demonstrated the successful synthesis of organic-phase Cu NPs with improved emission properties for the first time and their application in the fabrication of the first blue emission Cu NP EL-LED. However, more research is needed to thoroughly understand the In3+ passivation mechanism on the Cu NP surface.
Zhang, W., Xu, W., Liu, H. et al. (2022) Organic-Phase Synthesis of Blue Emission Copper Nanoparticles for Light-Emitting Diodes. ACS Applied Nano Materials https://pubs.acs.org/doi/10.1021/acsanm.1c04541
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