[show abstract][hide abstract] ABSTRACT: Raman-scattering behaviors have been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The peak frequencies of D and 2D bands are almost irrespective of d, and the intensity of the D band is larger than that of the G band over almost full range of d. These results suggest that GQDs are defective, possibly resulting from the dominant contributions from the edge states at the periphery of GQDs. The G band shows a maximum peak frequency at d = ∼17 nm, whilst the full-width half maximum of the G band and the peak-intensity ratio of the D to G bands are minimized at d = ∼17 nm. Since the average thickness of GQDs (t) is proportional to d, t can act as a factor affecting the d-dependent Raman-scattering behaviors, but they cannot be explained solely by the t variation. We propose that the abrupt changes in the Raman-scattering behaviors of GQDs at d = ∼17 nm originate from size-dependent edge-state variation of GQDs at d = ∼17 nm as d increases.
[show abstract][hide abstract] ABSTRACT: Radiative decay processes have been studied in graphene quantum dots (GQDs) by varying their size. The photoluminescence (PL) decay traces are well fitted to a biexponential function with lifetimes of τ1 and τ2, indicating their fast and slow components, respectively. The τ1 is almost constant, irrespective of the average GQD size (da) for two excitation wavelengths of 305 and 356 nm. In contrast, the τ2 decreases as da increases for da ≤ ∼17 nm, but da > ∼17 nm, it increases with increasing da for both the excitation wavelengths, similar to the size-dependent behaviors of the time-integrated PL peak energy. We propose that the τ1 and τ2 originate from size-independent fast band-to-band transition and size-dependent slow transition resulting from the edge-state variation at the periphery of GQDs, respectively.
[show abstract][hide abstract] ABSTRACT: For the application of graphene quantum dots (GQDs) to optoelectronic nanodevices, it is of critical importance to understand the mechanisms which result in novel phenomena of their light absorption/emission. Here, we present size-dependent shape/edge-state variations of GQDs and visible photoluminescence (PL) showing anomalous size dependences. With varying the average size (d(a)) of GQDs from 5 to 35 nm, the peak energy of the absorption spectra monotonically decreases, while that of the visible PL spectra unusually shows nonmonotonic behaviors having a minimum at d(a) = ∼17 nm. The PL behaviors can be attributed to the novel feature of GQDs, that is, the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at d(a) = ∼17 nm as the GQD size increases, as demonstrated by high-resolution transmission electron microscopy.
[show abstract][hide abstract] ABSTRACT: We report substantially enhanced photoluminescence (PL) from hybrid structures of graphene/ZnO films at a band gap energy of ZnO (∼3.3 eV/376 nm). Despite the well-known constant optical conductivity of graphene in the visible-frequency regime, its abnormally strong absorption in the violet-frequency region has recently been reported. In this Letter, we demonstrate that the resonant excitation of graphene plasmon is responsible for such absorption and eventually contributes to enhanced photoemission from structures of graphene/ZnO films when the corrugation of the ZnO surface modulates photons emitted from ZnO to fulfill the dispersion relation of graphene plasmon. These arguments are strongly supported by PL enhancements depending on the spacer thickness, measurement temperature, and annealing temperature, and the micro-PL mapping images obtained from separate graphene layers on ZnO films.
[show abstract][hide abstract] ABSTRACT: Ge nanocrystals (NCs) are shown to form within HfO <sub>2</sub> at relatively low annealing temperatures (600–700 ° C ) and to exhibit characteristic photoluminescence (PL) emission consistent with quantum confinement effects. After annealing at 600 ° C , sample implanted with 8.4×10<sup>15</sup> Ge cm <sup>-2</sup> show two major PL peaks, at 0.94 and 0.88 eV, which are attributed to no-phonon and transverse-optical phonon replica of Ge NCs, respectively. The intensity reaches a maximum for annealing temperatures around 700 ° C and decreases at higher temperatures as the NC size continues to increase. The no-phonon emission also undergoes a significant redshift for temperatures above 800 ° C . For fluences in the range from 8.4×10<sup>15</sup> to 2.5×10<sup>16</sup> cm <sup>-2</sup> , the average NC size increases from ∼13.5±2.6 to ∼20.0±3.7 nm . These NC sizes are much larger than within amorphous SiO <sub>2</sub> . Implanted Ge is shown to form Ge NCs within the matrix of monoclinic (m) -HfO <sub>2</sub> during thermal annealing with the orientation relationship of m -HfO <sub>2</sub>// Ge NC.
Journal of Applied Physics 06/2009; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report interesting observation of strong enhancement of ultraviolet luminescence from hybrid structures of single-walled carbon nanotubes (SWNTs)/ZnO. SWNTs of 3–120 nm thickness (t) were deposited on top of 100 nm ZnO films/n-type Si (100) wafer by spin coating and vacuum filtration to form the hybrid structures. Photoluminescence (PL) intensity of the hybrid structures increases with increasing t up to 10 nm, becomes almost ten times larger at t = 10 nm than that of the bare ZnO film and decreases with increasing t above 10 nm. This strong PL enhancement is also confirmed by PL mapping. These findings are discussed based on the surface-plasmon-mediated emission mechanism.
[show abstract][hide abstract] ABSTRACT: Hybrid nanostructures composed of ZnO nanocrystals (NCs) and Si NCs have been fabricated by annealing double layers of ZnO and SiOx on Si (100) wafer at 1100 °C for 20 min. High-resolution transmission electron microscopy images demonstrate the coexistence of 4-5 nm ZnO NCs and 2-10 nm Si NCs in the range of x from 1.0 to 1.8. The photoluminescence intensity of the hybrid structures is almost 10 times larger at x=1.0 than that of the ZnO single layer and decreases with increasing x above 1.0, exactly consistent with the x-dependent intensity behaviors of the near-edge x-ray absorption fine structure features. These results are very promising in view of the strong enhancement in the luminescence from ZnO by forming hybrid structures of ZnO/Si NCs.
[show abstract][hide abstract] ABSTRACT: P–n junction GaN light-emitting diodes (LEDs) were fabricated using Ga-doped ZnO (GZO) films as electrical contacts and characterized by electroluminescence (EL) and current–voltage (I–V) measurements. GaN p–n epilayers with a total thickness of ~6 μm were grown on c-plane (0001) sapphire substrates by metal–organic chemical vapor deposition. Half region of the p-GaN layer was etched until the n-GaN layer was exposed, and 100-nm-thick GZO contacts were deposited on the p- and n-GaN layers by RF sputtering with varying Ga concentration (n
) from 1 to 5 mol%. Based on the results of Hall effect, photoluminescence (PL), and X-ray diffraction (XRD), the GZO films were expected to act as best electrical contacts for the LEDs at n
= 2 mol%. Under forward-bias conditions, the I–V curves showed diode characteristics except n
= 5 mol%, and the leakage current was minimized at n
= 2 mol%. Two dominant EL peaks of ultraviolet and yellow emissions were observed at ~376 and ~560 nm, and attributed to near-band-edge- and defect-related radiative transitions, respectively. At n
= 2 mol%, the UV EL showed markedly large intensities for all injection currents, consistent with the results of Hall effect, PL, I–V, and XRD.