Kar Seng Teng

Deep Ultraviolet to Near-Infrared Emission and Photoresponse in Layered N-Doped Graphene Quantum Dots

Material that can emit broad spectral wavelengths covering deep ultraviolet, visible, and near-infrared is highly desirable. It can lead to important applications such as broadband modulators, photodetectors, solar cells, bioimaging, and fiber communications. However, there is currently no material that meets such desirable requirement. Here, we report the layered structure of nitrogen-doped graphene quantum dots (N-GQDs) which possess broadband emission ranging from 300 to >1000 nm. The broadband emission is attributed to the layered structure of the N-GQDs that contains a large conjugated system and provides extensive delocalized π electrons. In addition, a broadband photodetector with responsivity as high as 325 V/W is demonstrated by coating N-GQDs onto interdigital gold electrodes. The unusual negative photocurrent is observed which is attributed to the trapping sites induced by the self-passivated surface states in the N-GQDs.

Energy-level structure of nitrogen-doped graphene quantum dots

The doping of carbon-based materials is of great importance due to its ability to modulate their optical, electrical and optoelectronic properties. Nitrogen-doped graphene quantum dots (N-GQDs) have received significant attention due to their superior electrocatalytic activity, optical properties and biocompatibility. The energy-level structure of N-GQDs remains unknown, which hinders the development of N-GQDs for various applications. Here, we report a one-pot synthesis method to prepare large-quantity N-GQDs at room temperature and atmospheric pressure under a prolonged reaction time. Using this approach, we can effectively dope N into the N-GQDs. As revealed by electron energy loss spectroscopy, N-doping introduces a new energy level into the electronic structure, which is responsible for tuning the optical properties of the N-GQDs.

An efficient and stable fluorescent graphene quantum dot–agar composite as a converting material in white light emitting diodes

Graphene quantum dots (GQDs) have attracted great attention due to their unique optoelectronic properties. There remains a critical challenge to utilize the water-soluble GQDs for device applications. Here we report a facile method to fabricate a GQD–agar composite. The composite exhibits excellent optical stability and no luminescence quenching is observed. The composite is successfully applied as a colour converting material in blue light-emitting diodes (LEDs) to achieve white light emission. The luminous efficiency and light conversion efficiency of the white LED are 42.2 lm W−1 and 61.1% respectively. The light conversion efficiency of the WLED is stable for over 100 hours of continuous operation.

Magnetotransport properties of p-type carbon-doped ZnO thin films

Carbon-doped ZnO (ZnO:C) thin films exhibiting Curie temperature above room temperature were fabricated using ion beam technique. The magnetic moment of the ZnO:C films was found to be around 1.35 μB per carbon atom. The ZnO:C films showed p-type conduction with a hole concentration of ∼5×1017 cm−3. In addition, the anomalous Hall effect and negative magnetoresistance can be detected in the ZnO:C films. The magnetotransport properties of the ZnO:C suggested that the films possessed charge carrier spin polarization.

Ultraviolet photoluminescence from ferromagnetic Fe-doped AlN nanorods

Fe-doped AlN (AlN:Fe) nanorods on silicon substrates were fabricated using a catalysis-free vapor phase method. The AlN:Fe nanorods exhibited high crystalline quality and preferred c-axis orientation. The spontaneous saturated magnetization of the AlN:Fe nanorods was determined to be ∼0.64μB∕Fe at room temperature. Room temperature photoluminescence measurement of the AlN:Fe nanorods revealed two strong ultraviolet emissions at 3.69 and 6.02eV which could be attributed to Fe3+-related and band edge emissions, respectively. The Fe-doped AlN nanorods not only exhibited ferromagnetism but also significantly enhanced the band edge emission as compared to the undoped AlN nanorods.

Coupling gold nanoparticles to silica nanoparticles through disulfide bonds for?glutathione detection

Advances in the controlled assembly of nanoscale building blocks have resulted in functional devices which can find applications in electronics, biomedical imaging, drug delivery etc. In this study, novel covalent nanohybrid materials based upon [Ru(bpy)3](2+)-doped silica nanoparticles (SiNPs) and gold nanoparticles (AuNPs), which could be conditioned as OFF-ON probes for glutathione (GSH) detection, were designed and assembled in sequence, with the disulfide bonds as the bridging elements. The structural and optical properties of the nanohybrid architectures were characterized using transmission electron microscopy, UV-vis spectroscopy and fluorescence spectroscopy, respectively. Zeta potential measurements, x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were employed to monitor the reaction processes of the SiNPs-S-S-COOH and SiNPs-S-S-AuNPs synthesis. It was found that the covalent nanohybrid architectures were fluorescently dark (OFF state), indicating that SiNPs were effectively quenched by AuNPs. The fluorescence of the OFF-ON probe was resumed (ON state) when the bridge of the disulfide bond was cleaved by reducing reagents such as GSH. This work provides a new platform and strategy for GSH detection using covalent nanohybrid materials.

Ferromagnetic Cu doped ZnO as an electron injector in heterojunction light emitting diodes

Ferromagnetic and highly conductive copper doped ZnO (ZnO:Cu) films were prepared by filtered cathodic vacuum arc technique. By employing a biasing technique during growth, the electron concentration and resistivity of the ZnO:Cu films can be as high as 1020 cm−3 and 5.2×10−3 Ω cm, respectively. The ferromagnetic behavior is observed in all the conductive films, but its magnetization is quenched with an increment in carrier concentration, suggesting that carrier induced exchange is not directly responsible for the ferromagnetism. Heterojunction light emitting diodes have been fabricated using the conductive ZnO:Cu layer as an electron injector and a p-type GaN as hole injector. Electroluminescence can be detected from the devices.

Ultra-high resolution imaging of DNA and nucleosomes using non-contact atomic force microscopy

Visualisation of nano‐scale biomolecules aids understanding and development in molecular biology and nanotechnology. Detailed structure of nucleosomes adsorbed to mica has been captured in the absence of chemical‐anchoring techniques, demonstrating the usefulness of non‐contact atomic force microscopy (NC‐AFM) for ultra‐high resolution biomolecular imaging. NC‐AFM offers significant advantages in terms of resolution, speed and ease of sample preparation when compared to techniques such as cryo‐electron microscopy and X‐ray crystallography. In the absence of chemical modification, detailed structure of DNA deposited on a gold substrate was observed for the first time using NC‐AFM, opening up possibilities for investigating the electrical properties of unmodified DNA.

Photoresponse of polyaniline-functionalized graphene quantum dots.

Polyaniline-functionalized graphene quantum dots (PANI-GQD) and pristine graphene quantum dots (GQDs) were utilized for optoelectronic devices. The PANI-GQD based photodetector exhibited higher responsivity which is about an order of magnitude at 405 nm and 7 folds at 532 nm as compared to GQD-based photodetectors. The improved photoresponse is attributed to the enhanced interconnection of GQD by island-like polymer matrices, which facilitate carrier transport within the polymer matrices. The optically tunable current-voltage (I-V) hysteresis of PANI-GQD was also demonstrated. The hysteresis magnifies progressively with light intensity at a scan range of ±1 V. Both GQD and PANI-GQD devices change from positive to negative photocurrent when the bias reaches 4 V. Photogenerated carriers are excited to the trapping states in GQDs with increased bias. The trapped charges interact with charges injected from the electrodes which results in a net decrease of free charge carriers and a negative photocurrent. The photocurrent switching phenomenon in GQD and PANI-GQD devices may open up novel applications in optoelectronics.

Wavelength-tunable and high-temperature lasing in ZnMgO nanoneedles

Zn1−xMgxO nanoneedles were prepared by an ion-beam technique on Zn1−xMgxO thin films with Mg contents of up to 21at.%. The photoluminescence emission energies of the Zn1−xMgxO nanoneedles measured at room temperature increased monotonically with Mg contents and it reached 3.6eV when x=0.21. Random laser action was observed in the Zn1−xMgxO nanoneedles with x⩽0.1 at temperature ranging from 300to470K under 355nm optical excitation. The characteristic temperature of the Zn1−xMgxO nanoneedles was determined to be 84K. The high-temperature lasing of the Zn1−xMgxO nanoneedles are attributed to the high crystal quality of the nanoneedles, enhancement of oscillator strength in nanostructures, and a self-compensation mechanism in random laser cavities.