Abstract
Introduction
Since the discovery of graphene by Novoselov et al.
MAX phases are layered ternary carbides, nitrides, or carbonitrides with a hexagonal crystal structure with space group P63/mmc. Unlike other bulk 3D layered materials such as graphite and TMDs, which could be mechanically exfoliated with ease because of the weak van der Waals interactions that hold the structure together, the bonds between the layers in the MAX phase are too strong to be broken with similar means. However, by employing the relatively weaker M-A bonds compared to the M-X bonds, it is possible to selectively etch out the A layer by chemical means, leaving us with M-X layers of MXenes with a general formula Mn+1XnTx, where Tx is the notation for a surface-terminating functional group (O, OH, F, H, etc.)
Quantum dots derived from 2D materials (2D-QDs) have shown promising prospects for applications in nanomaterial-based devices. They not only inherit the merits of their 2D counterparts but also exhibit improved properties such as better dispersibility, higher chemical stability, easier functionalization, larger surface-to-volume ratio, and stronger photoluminescence (PL) after a size reduction (typically <10 nm) resulting from the strong quantum confinement and edge effect
More and more studies on MQDs are published in recent years; however, comprehensive reviews on MQDs are rarely found, and only one review has been published recently
Synthesis of MAX phase and MXene
Even though there are numerous techniques and methods available to synthesize MQDs, the basis of synthesizing light-emitting MQDs is the same which is reducing the size of MXene (typically <10 nm) to induce bandgap expansion by quantum confinement effect
Synthesis of MAX phase
The Ti3AlC2 MAX phase was first discovered by Pietzka et al.
The synthesis of the MAX phase is usually performed at very high temperatures. Gogotsi et al.
The Nb2AlC phase was synthesized for the first time in 1980 by Schuster and Nowotny
Zhou et al.
Synthesis of MXene
For the synthesis of MXene, the MAX phase is etched using 10–50 wt.% HF as reported in most synthesis protocols
Owing to the hazardous nature of HF, a fluoride salt was employed to make a mild etchant. A mild etchant solution was prepared by mixing HCl and LiF under stirring at room temperature. This resulted in a clear HF solution with a low concentration, normally 3%–5%
Synthesis of MXene quantum dots
Multiple methods and approaches have been used to prepare MQDs. These methods can be categorized into top-down and bottom-up approaches
Top-down approaches usually involve the cleavage of bulk MXene precursors by employing physical
Figure 1.
Hydrothermal/solvothermal method
Hydrothermal/solvothermal is the most common approach that uses MXene as a precursor to synthesize MQDs
Figure 2.
The solvothermal synthesis method employs organic solvents as the reaction medium instead of water; ethanol, DMSO, and dimethylformamide (DMF) are commonly used solvents. The solvothermal method is advantageous over the hydrothermal synthesis in that the morphology
Hydrothermal/solvothermal-ultrasound method
Combining ultrasonication with the solvothermal/hydrothermal method is an efficient strategy to synthesize MQDs rather than using solvothermal or hydrothermal methods alone
Figure 3.
Ultrasonic, ball milling, and intercalation methods
Several other top-down approaches to prepare MQDs include intercalation
Figure 4.
Top-down synthesis methods are efficient for obtaining MQDs, but they possess drawbacks such as long synthesis time and low yield, as discussed above. Thus, advanced methods such as microwave synthesis or electrochemical synthesis have been used. These methods exhibit good reproducibility, involve simple operations, and are cost-effective and thus yield good results.
MQDs were also synthesized using the following bottom-up synthesis approach. The bottom-up method uses organic or inorganic molecular materials as precursors, which enables the precise manipulation of size distribution, morphology, or surface functionalization
Molten salt synthesis
In the molten salt synthesis method, a salt with a low melting point is added to the reactants. After the salt addition, the precursors are heated above the melting point of the salt. This causes the salt to melt and act as a solvent. Cheng et al.
Figure 5.
Pyrolysis method
Wang et al.
Currently, only top-down method has successfully yielded light-emitting MQDs. This may be due to the precursor selection for MQDs synthesis. Mo2C is known to have metallic electronic properties
MQDs | Synthesis methods | PLQY | Ref (Year) |
Ti3C2 | Hydrothermal (pH = 9): 100 °C, 120 °C, 150 °C, 6 h | 10% (Blue) | ref. |
Ultrasound (25% TMAOH): 24 h | 8.9% (Blue) | ref. | |
Hydrothermal (pH = 7, N-doped): 160 °C, 12 h | 18.7% (Blue) | ref. | |
Acid Reflux: 100 °C, 12 h, Hydrothermal: pH = 7, N-, P-doped120 °C, 12 h | 20.1% (Green) | ref. | |
Ultrasound: 600 W, 6 h, Solvothermal: 80 °C, 48 h | 9.36% (White) | ref. | |
Hydrothermal (pH = 7, S-,N-, SN-doped); 150 °C, 12 h | 28.12% (Blue), 8.33% (Yellow), 7.78% (Orange) | ref. | |
V2C | Ultrasound: 1 h, Hydrothermal (alkaline): 120 °C, 6 h | 15.88% (Blue) | ref. |
Nb2C | Ultrasound (pH = 6, 60 ml TPAOH): 10 h | 8.4% (Bluish Green) | ref. |
Table 1.
Light-emitting properties of MXene quantum dots
PL is the emission of light from matter after the absorption of incident light or photons. The reaction mechanism behind PL from MQDs is not yet completely clear. Factors such as functional groups, surface defects, degree of passivation, and quantum confinement have previously been proposed to be the origin of PL in MQDs
Origin of photoluminescence, absorption, and quantum yield
It was theoretically (density functional theory, DFT) predicted that Ti3C2 MXene has a small bandgap of ~0.1 eV, which could be further expanded by quantum effects, and light emission can be induced
Figure 6.(
Figure 7.DFT calculation of total and projected density of states of (
The presence of large heterogeneity during the synthesis of MQDs results in the PL properties of MQDs being affected by the size, defects, shape, functional groups, edge configuration, and heterogeneous hybridization of the carbon network
Figure 8.(
Modulation of photoluminescence by surface defects, functionalization, and passivation
Surface modification and engineering are employed to overcome the drawbacks of MQDs such as oxidation and aggregation. Surface engineering methods include functionalization of MQDs such as composite construction and hetero-atomic doping
Yang et al. synthesized Nb2C QDs by employing a pulsed ultrasound method, followed by physicochemical exfoliation in TPAOH when the pH reached above 6
Surface passivation is performed by coating the surface of QDs with another material to protect the QD core. It plays a vital role in improving the fluorescence of QDs by reducing surface defects
Applications
Optoelectronic applications
Currently, approximately 20% of the world’s electricity has been reported to be consumed for lighting purposes. As the world population has increased over time, low-cost and efficient artificial lights have been in high demand
However, most inorganic QDs are fabricated using heavy metals that are harmful to humans and the environment
MQDs have proven to be advantageous for LEDs because their convenient functionalization enables us to tune the emission wavelength and strong PL emission. Xu et al.
Figure 9.(
Photoluminescence-based sensors
The detection of metal ions and biological components by monitoring the biological system at the cellular level is beneficial for a healthy life. Furthermore, the release of pollutants from industrial waste has caused serious environmental problems. Therefore, designing a sensitive and selective sensor for specific targets is important for maintaining the biological and environmental systems
MQDs were tested for their high sensitivity and selectivity by using different metal ions (Fe3+, Fe2+, Ca2+, Cd2+, Mg2+, Na2+, Sn2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Al3+, Cr3+)
Figure 10.(
It was observed that cysteine, serine, arginine, ascorbic acid, dopamine, H2O2, and various other metal ions have little or no effect on PL quenching of MQDs (Fig. 10(a)). The presence of both H2O2 and Fe2+ was found to reduce the PL intensity of nitrogen-doped Ti3C2 MQDs. However, there was no observable behavior in the presence of either H2O2 or Fe2+
Bioimaging
Bioimaging is a powerful technique that can effectively provide clear biological information
Figure 11.(
Cao et al.
Cellular imaging of N- and P-functionalized Ti3C2 MXene quantum dots (N,P-MQDs) was carried out by Quan et al.
Conclusion and perspective
The synthesis of MQDs has raised considerable interest because they not only retain the properties of MXene but also demonstrate light-emitting properties. Currently, studies on light-emitting MQDs have shown progress in terms of synthesis methods to fabricate multicolor-PL-emitting MQDs, and the highest reported QY has been 28.12%. Despite its excellent properties, MXene does not possess PL emission, which limits its applications. However, MQDs overcome this limitation and have thus found applications in optoelectronic devices, PL-based sensors, and bioimaging. Regardless of the great progress, research on MQDs is still in its early stages, and the PL mechanism has not yet been fully comprehended. Comprehensive studies are required for better understanding considering the challenges for their potential applications.
Synthesis of nitride and carbon nitride MQDs
Currently, over 100 types of MAX phases have been reported. Among these, more than 30 kinds of MXenes have been experimentally obtained since 2011
With the current advances in research on 2D-derived QDs (2D-QDs), various methods can be applied to synthesize different types of MQDs from different groups of MXenes, including carbon, nitride, and carbon nitride. It is expected that with the large number of currently available MXenes, more interesting properties or improved properties could be observed, which would help to advance the research on MQDs.
Synthesis methods of light-emitting MQDs
There are two main approaches for synthesizing MQDs: top-down and bottom-up. Among these, only the top-down method has successfully yielded light-emitting MQDs
By controlling these three variables and understanding the surface chemistry, it is possible to design MQDs with desirable properties. Currently, only Ti3C2, Nb2C, and V2C MQDs have been reported to show light-emitting properties. By varying the synthesis procedure, light-emitting MQDs with more interesting properties could be fabricated.
Optoelectronic applications
We focused on light-emitting MQDs in this review because MQDs have promising future prospects in optoelectronic applications. The increase in energy consumed every year is worrisome because the world still relies on fossil fuels as the main energy source, which is scarce and sometimes hazardous. Thus, finding a cheap and clean energy source that can provide high power conversion efficiency is in high demand. MQDs not only have shown potential as a material to be used in efficient light-emitting devices but also exhibit low toxicity, making them potential material for bio-applications. However, the QY of MQDs is low and needs improvement. Furthermore, the emission and absorption of MQDs are tuned mostly by functionalization, while the tunability of semiconductors can be simply achieved by changing the reaction temperature and/or time. Understanding the origin of the PL of MQDs is the main key to fully explore the potential of MQDs for optoelectronic applications.
MXene is known to be a suitable material for solar cells. Currently, researchers have used MXene as one of the electron transport layers in perovskite solar cells (PSCs) in order to increase the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), resulting in a much higher power conversion efficiency (PCE) compared with that of PSCs without MXene layers
Currently, there exist challenging issues related to device compatibility because most semiconductor QDs show a significantly low quantum efficiency when used for devices
Medical applications
For medical applications, the strongest advantage of MQDs over conventional inorganic semiconductor QDs is their low toxicity. MQDs have been used in the medical field, for example, in photothermal therapy for cancer
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