[Show abstract][Hide abstract]ABSTRACT: Förster resonance energy transfer (FRET), referred to as the transfer of the photon energy absorbed in donor to acceptor, has received much attention as an important physical phenomenon for its potential applications in optoelectronic devices as well as for the understanding of some biological systems. If one-atom-thick graphene is used for donor or acceptor, it can minimize the separation between donor and acceptor, thereby maximizing the FRET efficiency (EFRET). Here, we report first fabrication of a FRET system composed of silica nanoparticles (SNPs) and graphene quantum dots (GQDs) as donors and acceptors, respectively. The FRET from SNPs to GQDs with an EFRET of ∼78% is demonstrated from excitation-dependent photoluminescence spectra and decay curves. The photodetector (PD) responsivity (R) of the FRET system at 532 nm is enhanced by 100∼101/102∼103 times under forward/reverse biases, respectively, compared to the PD containing solely GQDs. This remarkable enhancement is understood by network-like current paths formed by the GQDs on the SNPs and easy transfer of the carriers generated from the SNPs into the GQDs due to their close attachment. The R is 2∼3 times further enhanced at 325 nm by the FRET effect.
[Show abstract][Hide abstract]ABSTRACT: Two kinds of oxide templates, one with and one without undercuts, are employed to study the effect of defects in oxide templates on non-catalytic growth of GaN nanowires (NWs). GaN NWs abnormally grown from the templates containing undercuts exhibit two types of patterns: earlystage growth of premature NWs and abnormally-overgrown (~2 μm) NWs. GaN NWs grown on perfectly-symmetric template patterns are highly crystalline and have high aspect ratios (2 ~ 5), and their tops are shaped as pyramids with semipolar facets, clearly indicating hexagonal symmetry. The internal quantum efficiency of the well-grown NWs is 10% larger than that of the deformed NWs, as estimated by using photoluminescence. These results suggest that our technique is an effective approach for growing large-area-patterned, vertically-aligned, hexagonal GaN NWs without catalysts, in strong contrast to catalytic vapor-liquid-solid growth, and that good formation of the oxide templates is crucial for the growth of high-quality GaN NWs.
Article · Apr 2016 · Journal- Korean Physical Society
[Show abstract][Hide abstract]ABSTRACT: Graphene quantum dots (GQDs) have been prepared by hydrothermal cutting of graphene sheets and subsequently doped with N atoms by using hydrazine treatment to study the effect of doping on the structural and the optical properties of the GQDs. The pyridinic N atoms, which were increased in number by hydrazine treatment, are bonded with C atoms at the edge of the GQDs, thereby making the GQDs not only larger but also n-type conductive with a concomitant reduction of the edge-related defects. The absorption band of GQDs is peaked at ~300 nm, and after doping, they show an additional absorption peak at ~354 nm, which originates from the impurity energy level formed by the N doping. The GQDs emit bright blue photoluminescence (PL) peaked at 418 nm, and after hydrazine treatment, the PL emission is redshifted by ~15 nm and its decay becomes slower, resulting from the N-related impurity level.
Full-text Article · Aug 2015 · Journal- Korean Physical Society
[Show abstract][Hide abstract]ABSTRACT: Graphene quantum dots (GQDs) have received much attention due to their novel phenomena of charge transport and light absorption/emission. The optical transitions are known to be available up to ~6 eV in GQDs, especially useful for ultraviolet (UV) photodetectors (PDs). Thus, the demonstration of photodetection gain with GQDs would be the basis for a plenty of applications not only as a single-function device in detecting optical signals but also a key component in the optoelectronic integrated circuits. Here, we firstly report high-efficient photocurrent (PC) behaviors of PDs consisting of multiple-layer GQDs sandwiched between graphene sheets. High detectivity (>10(11) cm Hz(1/2)/W) and responsivity (0.2 ~ 0.5 A/W) are achieved in the broad spectral range from UV to near infrared. The observed unique PD characteristics prove to be dominated by the tunneling of charge carriers through the energy states in GQDs, based on bias-dependent variations of the band profiles, resulting in novel dark current and PC behaviors.
[Show abstract][Hide abstract]ABSTRACT: Nonvolatile flash-memory capacitors containing graphene quantum dots (GQDs) of 6, 12, and 27 nm average sizes (d) between SiO2 layers for use as charge traps have been prepared by sequential processes: ion-beam sputtering deposition (IBSD) of 10 nm SiO2 on a p-type wafer, spin-coating of GQDs on the SiO2 layer, and IBSD of 20 nm SiO2 on the GQD layer. The presence of almost a single array of GQDs at a distance of ∼13 nm from the SiO2/Si wafer interface is confirmed by transmission electron microscopy and photoluminescence. The memory window estimated by capacitance-voltage curves is proportional to d for sweep voltages wider than ± 3 V, and for d = 27 nm the GQD memories show a maximum memory window of 8 V at a sweep voltage of ± 10 V. The program and erase speeds are largest at d = 12 and 27 nm, respectively, and the endurance and data-retention properties are the best at d = 27 nm. These memory behaviors can be attributed to combined effects of edge state and quantum confinement.
[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.
Full-text Article · Feb 2013 · Applied Physics Letters
[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.
[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 a synthesis approach using a Si-nanocrystal (NC) layer that is formed by annealing a Si-rich oxide with different oxygen contents (stoichiometry, x) for growing vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs). VA-MWCNTs with a largest length and diameter of about 190 mu m and 20 nm, respectively, were grown at x = 1.6. The atomic-resolved transmission electron microscopy image of the tube wall at x = 1.6 revealed almost-straight and well-separated graphitic sheets without defects, possibly resulting from the highest-quality Si NCs at x = 1.6. Active iron catalyst particles were formed on the Si-NC layer, resulting in the formation of highly-aligned MWCNTs. Possible mechanisms are described to explain the experimental results.
Article · Dec 2010 · Journal- Korean Physical Society
[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: ZnO/SiO x films grown on Si (100) wafers by radio-frequency sputtering and ion beam sputtering, respectively were annealed at 1100 °C for 20 min to form hybrid nanostructures of ZnO/Si. As proven by cross-sectional high-resolution transmission electron microscopy (HRTEM) images annealing causes the ZnO/SiO x double layer to transform into a hybrid nanostructure mixed with ZnO nanocrystals (NCs) and Si NCs within SiO 2 Scanning electron microscopy, plan-view HRTEM and scanning-mode TEM images of the sample surfaces exhibit leaf-vein-like nanostructures composed of nanoscale SiO 2 streaks and ZnO NCs. As x increases, the size of the Si NCs in the hybrid nanostructures decreases more steeply than it does in bare SiO x layers The intensities of the O K-edge near-edge X-ray absorption fine-structure features increase as x increases, reflecting an increase in the unoccupied surface states of ZnO NCs.
Article · Jun 2009 · Journal- Korean Physical Society
[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.
Full-text Article · Jun 2009 · Journal of Applied Physics
[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: Si nanowires (NWs) have great potential for future applications in electronics, photonics, biology, and energy owing to their unique structural, electronic, and optical properties based on one-dimensional quantum confinement effects. Intensive research have been performed successfully for understanding the growth mechanism and the electrical properties of Si NWs but only a few studies have been reported on their luminescence properties without a comprehensive understanding of the light-emission mechanism. In this study, crystalline Si/silica (c- Si/SiOx) core/shell NWs are produced by annealing Ni-coated Si-rich SiOx (SRO) films at 1100 degC under a N2 ambient. The room temperature photoluminescence (PL) spectra of the individual NWs have two major emission bands in the near UV (381 nm) and blue (435 nm) ranges at nSi (Si concentration) = 43 at. %, whilst at nsi = 37 at. %, only blue band is observed. The photoluminescence behaviors are discussed based on the possible physical mechanism.