Chapter 1: Introduction
1.7. Applications of Various Type of GQDs and their Heterostructures
1.7.2. Optoelectronic Applications: Photodetector, Light Emitting Diodes
the nanoscale level, GQDs are expected to be suitable for small and compact optoelectronic devices among the new generation smart inventions. GQDs are widely used for the fabrication as well as the development of the most useful optoelectronic devices such as high-performance photodetectors (PDs), light-emitting diodes (LEDs), solar cell, etc.. A brief discussion on the GQD based optoelectronic devices is presented below.
PD is a photo-sensitive diode with Schottky junction or p-n junction, which converts the incident photons into electric current usually under externally applied bias. PDs are intensively used in optical communications, video imaging, real-time monitoring, and so on.34 Zhang et al. reported
Table 1.1. Summary of the reported sensing performances of undoped and doped GQDs and their HSs with various analytes.
the fabrication of high-performance PD in deep UV region with GQDs.7 However, due to limitation of the absorption of GQDs in the UV region, the performance of pure GQD based PDs cannot be extended into the visible or higher wavelength region. Meanwhile, HSs of GQDs with conventional semiconductor materials, such as Si NWs,105 Si NPs,126 ZnO,106 etc., exhibited the photodetection in the UV-NIR region. Moreover, the high-performance broadband PDs in UV-
Sample Detecting Analyte Detecting Method Detection Mechanism Detection Range (M) LOD (nM) Ref.
GQDs Cu2+
Turn off fluorometric
CT 50–150 5.0×104 116
GQDs Cu2+ Complex formation 0.2–300 2.3×102 117
N-GQDs Cu2+ ET 0.0–35.0 7.2×101 118
S-GQDs Ag+ CT 0.1–130.0 3.0×101 79
S-GQDs Pb2+ - 0.1–140.0 3.0×101 119
DA/GQDs Fe3+ Complexation and
oxidation of DA 0.02–2.0 7.6×100 100
N, S-GQDs Hg2+ CT and complex
formation 0.1–15 1.4×10-1 120
Glycine/GQDs Ce4+ Complex formation 50–700 5.0×104 101 N-GQDs Al3+
Turn on fluorometric
PET and complex
formation 0.25–100 1.3×103 74
RhB
derivative/GQDs Fe3+ Complex formation 0.0–65.0 2.0×101 121 GQDs/Au NP Hg2+ Electrochemical Oxidation 2×10-5–1.5×10-3 2.0×10-2 115
GQDs HQ
Turn off fluorometric
Oxidation of HQ and
CT 0.01–30.0 5.0×100 122
GQDs DA CT 0.01–60.0 8.0×100 23
GQDs UA CT 2–300 5.0×102 123
Polypyrrole/GQDs DA CT 0.005–8 1.0×10-2 124
B-GQDs GL
Turn on fluorometric
Aggregation 100–2×105 6.0×104 80
GQDs/Cu2+ AA Reduction of Cu2+ to
Cu+ 0.3–10.0 9.4×101 116
Glycine/GQDs/Ce4+ AA Reduction of Ce4+ to
Ce3+ 0.03–17.0 2.5×101 101
Au@r-GQDs/Ag+ Cys
Colorimetric
Aggregation 0.0–5 5.6×100 114
GQDs/Ag NP H2O2 Reduction 0.1−100 3.3×101 113
GQDs/Ag NP GL Reduction 0.5−400 1.7×102 113
Au@N-GQDs H2O2 Electrochemical CT and reduction 0.25−1×104 1.2×102 108
GQDs RB6
SERS CT 0.001–1.0 1.0×100 73
Ag@N-GQDs GL CT 1–106 1.0×102 109
GQDs TNT Turn off
Fluorometric FRET 459–2.3×105 2.2×103 125
19 | I n t r o d u c t i o n
vis-NIR region were reported by the 2D HS of GQDs, such as GQDs/MoS2,103 N-GQDs/WSe2,104 GQDs/graphene,110, 111 N-GQDs/boron nitride nanosheets/graphene,112 etc. Nguyen et al. showed that due to the decoration of N-GQDs on monolayer WSe2, the n-type doping effect improves the PL emission by the neutral exciton emission as well as the photo responsivity in WSe2.104 Mihalache et al. reported ~20 time higher photocurrent in GQDs/Si NW core-shell HS based Schottky PD with a remarkable enhancement of the external quantum efficiency as compared to pure Si NWs based PD, where GQDs act as a hole-blocking layer to increase the charge separation.105 Thus, in these HSs, GQDs behave as an active material for the enhancement of the photocurrent. However, the usages of undoped and doped GQDs in the fabrication of PDs are explored in the literature. Tetsuka et al. reported the N-GQDs/graphene-based PD with the buffer layer of boron nitride nanosheets for the enhancement of the PD performance.112 For the improvement of the PD performance, the usage of an additional layer is expensive for commercial applications.
As a clean and renewable energy source, the solar cell is another potential application field of GQDs. A solar cell is a photovoltaic device that converts solar light to the electric voltage under self-bias. Diao et al. demonstrated GQDs/Si heterojunctions solar cell with high power conversion efficiency (PCE) of ~12.35%, where GQDs behave as a hole transporting layer to separate the photo-generated electron-hole pairs as well as an electron blocking layer to reduce the recombination.127 Li et al. reported the usage of GQDs as an electron-accepting layer in the fabrication of P3HT based solar cells with PCE of ~1.28%.128 Moreover, the GQDs are also used with TiO2, ZnO, etc., for solar cell applications.34 Because of the low cost, chemical stability and non-toxicity, GQDs received inclusive attention in solar cell applications as compared to other traditional materials, such as silicon, perovskite, etc.69
LED is another optoelectronic device, where GQDs are used as a charge transporting layer or light convertor. Song et al. fabricated GQDs based LED, where GQD layers provide additional carrier transport to increase the radiative recombination, resulting in the overall enhancement of the current density.129 Tang and co-workers reported the fabrication of white LED by applying a coating of highly fluorescent GQDs on commercial blue LED, where GQDs converted the sharp blue emission of LED to a broad spectrum leading to the white emission.17 Note that due to the coating of GQDs, the agglomeration causes dramatic quenching of the fluorescence intensity of
GQDs, and consequently, the luminance of the LED is affected badly. Due to the ability of the tuning of the emission color as well as intensity, biocompatibility, chemical stability, low cost, etc., GQDs establish themselves as a potential material in organic and inorganic LED applications.10, 34