Woosung Kwon

Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite

Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%.

Size‐Controlled Soft‐Template Synthesis of Carbon Nanodots toward Versatile Photoactive Materials

Size-controlled soft-template synthesis of carbon nanodots (CNDs) as novel photoactive materials is reported. The size of the CNDs can be controlled by regulating the amount of an emulsifier. As the size increases, the CNDs exhibit blue-shifted photoluminescence (PL) or so-called an inverse PL shift. Using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, it is revealed that the CNDs are composed of sp² clusters with certain energy gaps and their oleylamine ligands act as auxochromes to reduce the energy gaps. This insight can provide a plausible explanation on the origin of the inverse PL shift which has been debatable over a past decade. To explore the potential of the CNDs as photoactive materials, several prototypes of CND-based optoelectronic devices, including multicolored light-emitting diodes and air-stable organic solar cells, are demonstrated. This study could shed light on future applications of the CNDs and further expedite the development of other related fields.

Control of Photoluminescence of Carbon Nanodots via Surface Functionalization using Para-substituted Anilines

Carbon nanodots (C-dots) are a kind of fluorescent carbon nanomaterials, composed of polyaromatic carbon domains surrounded by amorphous carbon frames, and have attracted a great deal of attention because of their interesting properties. There are still, however, challenges ahead such as blue-biased photoluminescence, spectral broadness, undefined energy gaps and etc. In this report, we chemically modify the surface of C-dots with a series of para-substituted anilines to control their photoluminescence. Our surface functionalization endows our C-dots with new energy levels, exhibiting long-wavelength (up to 650 nm) photoluminescence of very narrow spectral widths. The roles of para-substituted anilines and their substituents in developing such energy levels are thoroughly studied by using transient absorption spectroscopy. We finally demonstrate light-emitting devices exploiting our C-dots as a phosphor, converting UV light to a variety of colors with internal quantum yields of ca. 20%.

Improving the functionality of carbon nanodots: doping and surface functionalization

Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthesis routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Electrocatalytic carbonaceous materials for counter electrodes in dye-sensitized solar cells

Carbonaceous materials have received much attention as alternative catalysts for platinum in dye-sensitized solar cells. Recently, various forms of carbon materials have been intensely investigated due to their low cost, excellent electrochemical stability, reasonable catalytic activity, and process adaptability. In this review, we introduce the current research issues in carbon counter electrodes including the equivalent circuit analysis, the electrochemical properties, the photovoltaic performances, and the research outlook. In this regard, the electrochemical properties of selected carbon materials are compared with each other by means of the impedance spectroscopy and equivalent circuit analysis. Also, fabrication methods and related photovoltaic performance are discussed. This knowledge will offer valuable insight to inspire investigation of carbon materials and encourage a multitude of applications ranging from all-carbon electrodes to flexible devices.

Sulfur-incorporated carbon quantum dots with a strong long-wavelength absorption band

In this work, we synthesize sulfur-incorporated carbon quantum dots (S-CQDs) and report the effect of sulfur on their electronic structure. Sulfur provides the density of states or emissive trap states for photoexcited electrons, and hence improves absorbance and photoluminescence intensity in the long-wavelength (∼500 nm) regime. The formation of the emissive trap states in the band-gap is confirmed by time-resolved emission decay spectroscopy. It is revealed that the emissive trap states prolong the fluorescence lifetime of low energy (∼2.5 eV) photoexcited electrons. To explore further the change in the band-gap energy, we demonstrate charge transport in S-CQD films that serve as the channels of field-effect transistors (FETs). The turn-on voltage of the S-CQD-based FETs decreases with the increase of the sulfur concentration, which is consistent with the optical changes. Our results establish a technical basis to incorporate heterogeneous atoms into CQDs and examine the related changes made to their optoelectronic properties. This method would open up new prospects to control the band-gap energy of CQDs in mild conditions, and hence promote their applications in imaging agents and optoelectronic devices.

Highly Efficient Light-Emitting Diodes of Colloidal Metal-Halide Perovskite Nanocrystals Beyond Quantum Size

Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.

Formation of highly luminescent nearly monodisperse carbon quantum dots via emulsion-templated carbonization of carbohydrates

Highly luminescent nearly monodisperse carbon quantum dots (CQDs) are synthesized by facile emulsion-templated carbonization of low cost and non toxic carbohydrates excluding the size selection procedure. The present method is further combined with in situ nitric acid treatment to offer high quantum yields up to 53% which, to our best knowledge, is unprecedented in the past.

Carbon quantum dot-based field-effect transistors and their ligand length-dependent carrier mobility.

We report electrical measurements of films of carbon quantum dots (CQDs) that serve as the channels of field-effects transistors (FETs). To investigate the dependence of the field-effect mobility on ligand length, colloidal CQDs are synthesized and ligand-exchanged with several primary amines of different ligand lengths. We measure current as a function of gate voltage and find that the devices show ambipolar conductivity, with electron and hole mobilities as high as 8.49 × 10(-5) and 3.88 × 10(-5) cm(2) V(-1) s(-1), respectively. The electron mobilities are consistently 2-4 times larger than the hole mobilities. Furthermore, the mobilities decrease exponentially with the increase of the ligand length, which is well-described by the Miller-Abrahams model for nearest-neighbor hopping. Our results provide a theoretical basis to examine charge transport in CQD films and offer new prospects for the fabrication of high-mobility CQD-based optoelectronic devices, including solar cells, light-emitting devices, and optical sensors.

Soft-template synthesis of nitrogen-doped carbon nanodots: tunable visible-light photoluminescence and phosphor-based light-emitting diodes

Carbon nanodots (CND) have recently attracted tremendous attention for a variety of optical and optoelectronic applications. Although many attempts have been made to obtain high-quality CND in high yield, there have been few endeavors to regulate its energy structure. In this work, we have synthesized nitrogen-doped carbon nanodots (N-CND) through soft-template synthesis using citric acid and ethylenediamine (EDA) as carbon and nitrogen sources, respectively. The energy structure of N-CND has been modulated by regulating the concentration of EDA. A series of spectroscopic measurements showed that the absorption peak is developed near 370 nm and the photoluminescence peak is red-shifted with increasing nitrogen content. To utilize N-CNDs strong near-UV absorption, broad-range visible-light emission, high quantum yield, and good organic compatibility, we have demonstrated phosphor-based light-emitting diodes employing N-CND as a phosphor.