Synthesis of Quantum Dots
A lot of synthetic methods have been developed for the synthesis of traditional quantum dots, i.e. metal-based quantum dots which come under two classes, i.e. top-down approach and bot- tom-up approach. In top-down methodology, the bulk material is finely grounded to particles with the size range in nanometers that include sonication, physical vapor deposition, ball milling, etc. In the case of bottom-up methodology, the quantum dots are prepared using a chemical reaction of various metal ions using certain types of techniques that include hydrothermal synthesis, solvothermal techniques, microemulsion, thermal decomposition, etc.
In the case of the top-down approach, bulk material is finely grounded to form very small and fine materials that are in the nano range of dimensions, especially quantum dots. Lithography, particularly electron beam lithography, is commonly used in top- down methods to fabricate quantum dots. The lithographic techniques used on semiconductor materials are generally helpful for constructing integrated circuits. In this technique, the semiconductor wafer is treated with photosensitive material which is further protected with a stencil and treated with UV light. The part which remains untreated with the UV light is then washed away. The wafer is further treated with hydrogen fluoride that reacts with the semiconductor with a photosensitive coating. This technique carries various advantages that include control over shape and size with particular packing geometries that are essential for exploring the quantum confinement effect. In addition, there are also some other types of lithographic techniques available for the effective fabrication of quantum dots, i.e. focused laser or ion beam lithography. Schnauber et al. have fabricated indium arsenide quantum dots using electron beam lithography for the development of a multi-node quantum optical circuit (Schnauber et al. 2018). Fatimy et al. have fabricated epitaxial graphene quantum dots using lithographic techniques for bolometric sensors (Fatimy et al. 2016). Instead of carrying so many advantages, there are certain major disadvantages with these techniques such as structural deficiencies by patterning, integration of impurities.
Etching also plays a vital role in the fabrication of quantum dots. In the case of dry etching, the reactive gas molecules are introduced in the etching compartment, followed by applying the radio frequency of desired voltage to generate plasma which disintegrates the molecules (gas) to more responsive fragments. These highly energetic species collide on the surface and fabricate a reactive product to etch the patterned sample. If these energetic fragments are ions, the etching methodology is called reactive ion etching. Gallium arsenide/aluminum gallium arsenide quantum dots are reported to be synthesized by using reactive ion etching with the aid of boron trichloride and argon (Scherer et al. 1987). Zinc telluride quantum dots have been synthesized by the same reaction protocol using methane and hydrogen gas (Tsutsui et al. 1993).
In the case of lithography for high lateral precision of synthesized quantum dots, focused ion beam techniques are used. In this methodology, the semiconductor substrate surface is sputtered by the extremely focused beam form melted metal sources, such as palladium/arsenic/boron, gallium, gold/silicon/barium, or gold/silicon. The size of the ion beam in lithography tailors the size, shape, and interparticle distances in quantum dots (Peng et al. 1997), and is also helpful to selective deposition of material from a reacting gas. To monitor the established patterning lithography as well as other parameters, scanning ion beam images can be established by ion beam nanolithography but due to certain constrains like slow processing, expensive equipment, surface damage etc., limit its use. In this context, one more methodology has been developed to attain patterns with quantum dots, i.e. exposure of electron beam lithography followed by lift off, and is called the ‘etching process’. This methodology gives a low degree of stiffness in the design of nanostructures i.e. wire, rings spheres etc. and is effectively applied for the synthesis of II-VI and III-V types of quantum dots.
There are various methodologies for the preparation of carbonaceous quantum dots under the top-down approach that include laser ablation, arc-discharge, electrochemical oxidation, etc. It was first prepared in 2004 accidentally by Xu et al. while preparing a caron nanotube using the arc-discharge method (Xu et al. 2004). Later on, Sun et al. explored the laser ablation method for the synthesis of fluorescent carbon dots (Sun et al. 2006). Zhou and his coworkers fabricated fluorescent carbon dots with the aid of the electrochemical method in which multi-wall carbon nanotube is used as a working electrode (Zhou et al. 2007). The obtained quantum dots are of 2.8 nm size with the quantum yield of about 6.4%. Also, efforts have been made to fabricate fluorescent quantum dots by replacing multi-wall carbon nanotube with graphite as a working electrode (Zhao et al. 2008; Zheng et al. 2009). To further improve the efficacy of the electrochemical method for the synthesis, ionic liquids have been used as electrolytes and graphite as working electrodes. It has been observed that when ionic liquids have been used as electrolytes, the reaction rate improves to a greater extent. Also, when different mole fractions of ionic liquids and water have been taken as electrolytes, the emission wavelength of the carbon dots can be tailored (Lu et al. 2009). Graphene quantum dots have been prepared by using graphene oxide by using a rapid continuous hydrothermal technique and the particles’ size has been controlled with the aid of a surface directing agent (Kellici et al. 2017).
Similarly, in another report, the graphene quantum dots were synthesized by using laser treatment with a 1064 nm pulsed laser beam with the pulse rate of 10 ns before the hydrothermal treatment of graphene oxide solution (Qin et al. 2015). The detailed methodologies, as well as transmission electron microscope images, are displayed in Figure 9.8. This top-down route from pulsed laser ablation in liquid is used for the fabrication of the graphene quantum dots. In the typical reaction procedure, firstly graphene nanosheets have been prepared and then treated with laser fragmentation for a certain time. Figure 9.8 shows the prepared nanosheets of graphene. When the nanosheets have been exploited using a laser pulse, the sheets are fragmented to a smaller sized graphene, i.e. graphene quantum dots. A large amount of heat has been liberated by the laser pulse resulting in the cutting of long graphene sheets to a smaller size.
In the case of graphitic carbon nitride quantum dots, the top- down approach to size-controlled synthesis has been done by the hydrothermal treatment of an ethanolic solution of bulk carbon nitride in the presence of potassium hydroxide as an exfoliator (Zhan et al. 2017). With the aid of ethanol and potassium hydroxide, the interlayer spacing of bulk carbon nitride material is greatly extended via weakening of the interlayer interaction that helps in the ease of exfoliation of bulk carbon nitride to graphitic carbon nitride quantum dots when both the temperature and pressure is raised. The inter-lamellar spacing of bulk material is lower than potassium and hydroxide ions, ensuring the intercalation of the bulk carbon nitride materials; also, the potassium ion is а я electron acceptor, while bulk graphitic carbon nitride is a к electron donor, due to which it is efficient in intercalation. In addition to the intercalating agent, it oxidizes the surface of the bulk carbon nitride, resulting in increases of functional groups containing oxygen on the surface of the carbon nitride. Tian et al. (2013) have used an exfoliation process to fabricate graphitic carbon nitride quantum dots using an ultrasonication method. In the typical reaction, 1 mg/ml solution of bulk graphitic carbon nitride quantum dots has been prepared and is treated in ultrasound for continuous 10 h. followed by centrifugation to remove
FIGURE 9.8 The photos of the graphene nanosheet solution target (left), after laser fragmentation (middle) and the raw GQDs after hydrothermal treatment (right), (b) SEM image of the graphene nanosheet, (c) SEM image of the raw GQDs. (d) ТЕМ image of the GQDs. (e) High-resolution ТЕМ image of the individual GQDs. (f) Orientation of the hexagonal GQDs network and the relative zigzag directions, (g) Histogram of diameter distribution. (Source: Qin et al. 2015)
bulk material. The prepared graphitic carbon nitride quantum dots show the quantum yield of 14.5% (Tian et al. 2013).