Sung Won Hwang

High-performance graphene-quantum-dot photodetectors

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 (>1011 cm Hz1/2/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.

Size-dependence of Raman scattering from graphene quantum dots: Interplay between shape and thickness

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.

Enhanced ultraviolet emission from hybrid structures of single-walled carbon nanotubes/ZnO films

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.

Size-dependent radiative decay processes in graphene quantum dots

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.

Energy transfer from an individual silica nanoparticle to graphene quantum dots and resulting enhancement of photodetector responsivity

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.

Formation characteristics and photoluminescence of Ge nanocrystals in HfO2

S.H.C. and R.G.E. acknowledge supports from the Korea Research Foundation Grant Grant No. KRF-2007-521- C00094 and from the Australian Research Council Discovery Project, respectively.

Growth and enhanced light emission of hybrid structures of ZnO∕Si nanocrystals

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 20min. High-resolution transmission electron microscopy images demonstrate the coexistence of 4–5nm ZnO NCs and 2–10nm 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.

Strong enhancement of emission efficiency in GaN light-emitting diodes by plasmon-coupled light amplification of graphene

Recently, we have demonstrated that excitation of plasmon-polaritons in a mechanically-derived graphene sheet on the top of a ZnO semiconductor considerably enhances its light emission efficiency. If this scheme is also applied to device structures, it is then expected that the energy efficiency of light-emitting diodes (LEDs) increases substantially and the commercial potential will be enormous. Here, we report that the plasmon-induced light coupling amplifies emitted light by ∼1.6 times in doped large-area chemical-vapor-deposition-grown graphene, which is useful for practical applications. This coupling behavior also appears in GaN-based LEDs. With AuCl3-doped graphene on Ga-doped ZnO films that is used as transparent conducting electrodes for the LEDs, the average electroluminescence intensity is 1.2-1.7 times enhanced depending on the injection current. The chemical doping of graphene may produce the inhomogeneity in charge densities (i.e., electron/hole puddles) or roughness, which can play a role as grating couplers, resulting in such strong plasmon-enhanced light amplification. Based on theoretical calculations, the plasmon-coupled behavior is rigorously explained and a method of controlling its resonance condition is proposed.

Blue-light emission from crystalline Si/silica core/shell nanowires

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.