Materials for Frequency Down-Conversion Light-Emitting Diodes: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Mauro Mosca.

Luminescent carbon dots (CDs) are a new form of nanocarbon quantum dot (QD) that have gained a huge amount of interest in recent years for their properties; in particular, their optical properties make them suitable for light-emitting diode (LED) manufacturing. One of the most frequently used methods for obtaining different emission spectra is color conversion, typically used for high CRI and white LEDs and displays, where the lowest emission wavelength source pumps other layers of materials, commonly called phosphors, that induce a conversion to the highest wavelengths.

  • carbon dots
  • color conversion
  • organic materials
  • LEDs

1. Phosphors

Briefly, phosphors are particular materials or compounds that present an energy gap between the valence and the conduction band, in a similar way to semiconductors. To achieve light emission, the transition between these two bands must be “direct” or without any change in the electron momentum. The color conversion ability of these materials lies in the different energy levels existing between the valence and the conduction band. These levels, or traps, are generated by lattice defects or impurities present in the crystal lattice, and introduce non-radiative paths that dissipate energy without the emission of photons. The color conversion process is due to the distribution of these trap levels and the energy gap of the material: when a phosphor is exposed to a photon with higher energy than its energy gap, an electron from the valence band can absorb this energy and be excited to a higher energy level. This level could be the edge of the conduction band or a trap level. Since trap levels typically dissipate energy in a non-radiative way, transitions between them cause a decrease in electron energy, and when the direct transition occurs the relative photon will be emitted with the remaining energy, therefore with a higher wavelength than the absorbed photon. This process is known as phosphor-based frequency down-conversion, and represents the base of color conversion.
Usually, phosphors consist of an inorganic host material, such as transition metal compounds, where defects in the crystal lattice are introduced by dopants, dislocations, or impurities, which are called activators. The most frequently used materials for phosphors are sulfides, selenides, and cadmium, and different rare earth materials, such as cerium, with added activators. Commercial white LEDs are typically attained by an InGaN/GaN blue LEDs pumping Ce:YAG phosphor powder dispersed in a polymeric matrix material. These materials could cause different problems in the foreseeable future because of their toxicity, low biocompatibility and, in some cases, availability difficulties. In other words, they may be non-sustainable materials.

2. Quantum Dots (QDs)

Another way to make color conversion possible is the use of quantum confinement effects. Quantum confinement occurs when the charge carriers of a material are confined in a space region smaller than its Bohr exciton radius [24][1]. In other words, looking at the charge carriers as waves, the region of space must be smaller than the de Broglie wavelength of the electron [25][2], which can be quite different from material to material, and is determined by the quantum mechanical nature of the electrons and holes in the materials [26][3]. In such cases, electrical and optical properties of the material change, leading to the formation of discrete energy levels instead of bands, and their position and energy gaps depend on the dimension of the confinement region [27][4]. The quantum confinement could be attempted along the three dimensions, giving rise to three types of confinement structures: mono-dimensional confinement or quantum well, bi-dimensional confinement or quantum wire, and three-dimensional confinement or quantum box/dot [26][3]. These structures are typically called 2D, 1D, and 0D, respectively, according to the remaining degrees of freedom.
Color conversion in QDs acts in a similar way of phosphor, but in this case the different levels and gaps are given by the quantum confinement effects instead of lattice defects or impurities, and could be tuned simply by varying both the dot dimensions and the materials used to synthesize QDs. The bandgap energy, which determines the color of the fluorescent light, is inversely proportional to the square of the size of the quantum dot [29,30][5][6]. Larger QDs have more energy levels that are more closely spaced, allowing the QD to emit (or absorb) photons of lower energy. In other words, the emitted photon energy increases as the dot size decreases, because greater energy is required to confine the semiconductor excitation to a smaller volume [31][7]. Although QDs seem to be the future of illumination, display, and optoelectronics, the typical materials used to create QDs are binary compounds, such as lead sulfide [32][8], lead selenide [33][9], cadmium selenide [34][10], cadmium sulfide [35][11], cadmium telluride [36][12], indium arsenide [37][13], and indium phosphide [38][14]. Dots may also be made from ternary compounds, such as cadmium selenide sulfide [39][15]. Furthermore, recent advances have been made concerning the synthesis of colloidal perovskite quantum dots [40][16]. Although QDs are very popular due to their promising optical performances, they are problematic because of their content of heavy metals and toxic materials, which poses a serious health risk to most living beings due to cytotoxicity [41][17]. Additionally, heavy metals are well-known environmental pollutants due to their toxicity, persistence in the environment, and bioaccumulative nature [42][18]. In recent years, research has focused on different and less troublesome materials, and much interest has been aroused by CDs, which are non-hazardous and biocompatible QDs based on carbon.

3. Carbon Dots (CDs)

CDs are very promising zero-dimensional structures because they are based on carbon, which makes them environmentally friendly. Additionally, they exhibit low toxicity, good water solubility, and chemical stability [43][19]. CDs also present very good optical properties, such as strong photoluminescence, optical tunability, luminous stability, and a very high photoluminescence QY [5[20][21][22][23],44,45,46], comparable with those of traditional inorganic QDs. Today, a photoluminescence QY of over 80% has been reached [47,48][24][25].
CDs consist of two parts: one is a spherical-like core, formed by the stacking of multiple graphene fragments in an ordered or disordered manner; the other is rich in functional groups distributed on the surface of CDs [43][19]. In general, the fluorescence behavior of CDs is controlled by the relationship between the carbon core and surrounding chemical groups [49][26]. The color of the fluorescence is determined by several features, such as the electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores, and the doping element present in the dot structure [50][27]. Moreover, the light emission intensity depends on different mechanisms, mainly π-domains, surface states, and molecule states [50][27]. For CDs with large conjugated π-domains and few surface chemical groups, the light emission is mostly due to conjugated π-domains, which are considered the carbon core state fluorescence centers. The bandgap of conjugated electrons derives from quantum confinement effects; thus, the emission color of CDs can be adjusted by tuning the size of the conjugated π-domains [51][28].
Surface defects contribute to light emission mechanisms, the surface chemical groups of CDs having various energy levels that cause different emission wavelengths by radiation relaxation. Furthermore, sp3 and sp2 hybrid carbon on the surface of CDs, and other surface defects, can lead to multicolor emissions from their local electronic states [50][27]. When CDs are excited by light, electrons accumulate in adjacent surface defect traps, and return to the ground state emitting visible light at different wavelengths. In this case, the emission color depends on the number of surface defects. Increasing the degree of surface oxidation also increases the number of surface defects, leading to a red shift in emission wavelengths [50][27]. Another contribution to light emission derives from small organic molecules that through the carbonization process become fluorophores. These could be attached to the surface or the interior of the carbon skeleton, and emit light independently [50][27]. In general, the QY of fluorescent molecules is higher than the QY of the core fluorescence, but the molecular luminescence stability is lower.

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