Carbon Dots

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This entry is adapted from the peer-reviewed paper 10.3390/coatings11010005

Carbon dots (CDs) are part of the nanocarbon family including quasi-spherical nanoparticles with sizes around 10 nm. They consist of amorphous and crystalline parts, mainly composed of carbon with a fringe spacing of 0.34 nm, which corresponds to the (002) interlayer spacing of graphite. Since their first discovery in 2006, CDs have gained ever-increasing attention due to their fascinating properties like distinctive optical behaviour, tunable emission, different functional groups, good biocompatibility, chemical and photo-stability, low toxicity, and low-cost production. More importantly, CDs properties can be changed by controlling their size, shape, and heteroatom doping and by modifying the surfaces. They are considered promising Green alternatives to traditional fluorescent dyes and have been proposed for different optoelectronic applications such as sensing, bioimaging, fingerprint detection, gene delivery, solar cells, or printing inks

carbon dots organic light-emitting diode photoluminescence sustainable materials green chemistry optical properties

1. Introduction

The growing demand for electronic devices for an ever-increasing number of applications means that green and sustainable electronics are no longer just a dream but a pressing need ^[1].

In this context, the electronics and optoelectronics based on organic semiconductors showed, in the last few years, significant growth in many areas dominated by traditional electronics ^[2]. The foremost advantage of organic materials is that they are cheap, lightweight, easy to be processed, and flexible ^[3]. However, the synthetic techniques currently used for production of organic semiconductors ^[4] suffer from toxicity and environmental problems that can seriously prevent their large-scale production. In this regard, application of the principles of Green Chemistry for development of synthetic sustainable methods for the synthesis of semiconductors is essential to support the development of organic electronics, thus moving towards increasingly sustainable electronic devices ^[5].

In fact, the use of organic materials to build electronic devices ^[6] holds the promise that future electronic manufacturing methods will rely on safer and more abundant raw materials ^[7]. The vision is for resource-efficient synthetic methodologies, whereby both devices themselves and manufacturing of those devices use less and more safe materials.

The first pillar on which the new sustainable industrial revolution is based on is inevitably the development of new materials that are possibly safe and sustainable by design ^[8].

Among the emerging classes of materials able to meet these needs, carbon dots (CDs) are attracting considerable interest. They are part of the nanocarbon family, and differently from the best known carbon nanotube ^[9], they include quasi-spherical nanoparticles with sizes around 10 nm. Since their first discovery in 2006, CDs ^[10]^[11] have gained ever-increasing attention due to their fascinating properties like distinctive optical behaviour, tunable emission, different functional groups, good biocompatibility, chemical and photo-stability, low toxicity, and low-cost production. More importantly, CDs properties can be changed by controlling their size, shape, and heteroatom doping and by modifying the surfaces, thanks to the quantum confinement effect (QCE) ^[12] (Figure 1). They are considered promising green alternatives to fluorescent dyes ^[13]^[14]^[15] and generally to toxic metallic colloidal semiconductor nanocrystals and have been proposed for optoelectronic applications in general ^[12]^[16]^[17]^[18]^[19] (Figure 1), such as sensing, bioimaging, fingerprint detection, gene delivery, solar cells, or printing inks.

Figure 1. Carbon dots general overview.

In general, CD preparation methods can be grouped into two main approaches: top-down and bottom-up. The first strategy ^[20]^[21]^[22]^[23]^[24]^[25]^[26]^[27] involves the use of techniques such as arc discharge, laser ablation, chemical oxidation in strong acid, and electrochemical synthesis to break down carbon sources such as graphite, carbon nanotubes, and nanodiamonds to form fluorescent CDs. In contrast, in the bottom-up approach ^[28]^[29]^[30]^[31]^[32]^[33]^[34], CDs are synthesized from organic molecules by applying hydrothermal solvothermal methods, ultrasonic or microwave treatments, or simple thermal combustion.

Although CDs have been synthesized from different starting materials with a great variety of techniques, there is a huge effort to develop sustainable synthetic paths which adhere to the principles of Green Chemistry. It has been demonstrated that CDs can be obtained from any carbon-based materials. This, in particular, guarantees that most by-products of the food supply chain can be reused to produce CDs. Agriculture products contain a myriad of natural molecules that can make up a diverse source of surface functional groups in CD formation.

A relevant feature for CD sustainability has to do with synthetic methodologies for their fabrication. CDs can be produced hydrothermally, that is, by heating the starting materials in water under atmosphere or elevated pressure ^[28]^[29]^[30]^[31]^[34]^[35]. Consequently, the cost of production is low, and the operation is easy, relatively safe, and free from organic solvents (Figure 2). Furthermore, shorter processing time and lower energy consumption for CD manufacture can be obtained when microwaves are used as the heating source ^[36]^[37].

Toxicity studies of the CDs were performed with both plants and animals (mice), revealing good biocompatibility ^[38]^[39]^[40], and opened the way not only to their bio-application but also to biodegradable electronics ^[19]^[40].

Figure 2. Schematic representation of carbon dots’ main characteristics: adapted with permission from ^[14], copyright 2019 American Chemical Society, and adapted with permission from ^[41], copyright 2017 Elsevier.

With this perspective, in the present review, we will discuss CDs obtained with a bottom-up approach as it is the one that best adheres to the principles of green chemistry. They can be obtained from renewable sources or waste such as citric acid or agricultural waste ^[37]. The synthetic methodologies are simple and inexpensive, and they do not require the use of metal catalysts or chlorinated solvents. We will focalize on CDs as a sustainable new platform for organic light-emitting device (OLED) technology ^[42]^[43] without deepening the methods of synthesis on which relevant works have already been written ^[28]^[29]^[30]^[31]^[32]^[33]^[34]^[35]^[36]^[37]^[44]. In fact, among the various interesting applications of organic electronics, OLEDs are certainly those that have already carved out a slice of the market ^[42], and for this reason, the development of sustainable active materials and green technologies can already help the economy make a green turn.

2. Carbon Dots and Optical Properties

Since their fortuitous discovery in 2004 by Xu et al. ^[10] and subsequently by Sun et al. in 2006 ^[11], CDs attracted a great deal of attention. Generally, CDs are 0-dimension nanocarbons with a typical size of less than 10 nm, although approximately 60-nm-size CDs have also been reported. CDs are quasispherical nanoparticles consisting of amorphous and crystalline parts, mainly composed of carbon with a fringe spacing of 0.34 nm, which corresponds to the (002) interlayer spacing of graphite ^[12]^[13]^[14]. CDs are often confused, or associated, with graphene quantum dots (GQDs) ^[15], which are always part of the family of carbon-based nanomaterials but have different characteristics and origins. In fact, GQDs are nanofragments of graphene exhibiting a graphene structure inside the dots with a typical fringe spacing of 0.24 nm associated with the (100) in-plane lattice spacing of graphene. They have only one or a few layers of graphene with variable thickness between 2 and 10 nm and with usually 100 nm in lateral dimension, and they are generally produced by converting graphene or graphene oxide via top-down approaches ^[14].

Over the last 15 years, CDs have been synthesized with different approaches (i.e., top-down and bottom-up). However, only recently, sustainable precursors and methodologies have been deeply investigated for their production ^[45]^[46]. These approaches look for sustainable materials which are low-cost, scalable, industrially and economically attractive, and based on renewable and highly abundant resources. This means that CD synthesis can meet the requirements of circular chemistry. Interestingly, CDs after synthesis can be further functionalized with various surface groups. In particular, oxygen-based functional groups, such as carboxyl and hydroxyl, give excellent solubility in water and are suitable for surface passivation and derivatization with various organic materials. Surface functionalization modifies both the physical properties of CDs such as their solubility in aqueous and non-aqueous solvents and their optical properties. For example, after surface passivation, the fluorescence properties of CDs can be improved ^[47]^[48]. In addition, the large conjugated structure endows CDs with some important characteristics, like good photostability, high surface area, and robust surface grafting ^[49]. Their electronic structures can be tuned by their size, shape, surface functional groups, and heteroatom doping, as theoretically investigated and experimentally confirmed by several groups ^[50]^[51]^[52]^[53]^[54]. The tunability of optoelectronic properties by modifying synthetic parameters and precursor strictly resembled the conjugated polymer features ^[55].

As mentioned above, the CD emission properties are their most amazing characteristics ^[41] and significant advances have been made in the last years, reaching photoluminescence (PL) quantum yields (PLQYs) up to 80% in CDs produced from citric acid as a renewable precursor or bright and stable PLQYs of 26% converting toxic cigarette butts ^[28]^[54].

Mostly, CDs are blue emitters, but emissions from ultraviolet to near-infrared ^[55]^[56]^[57]^[58] as well as white light emission ^[59] were reported. In general, their PL spectra are symmetrical and broad, with large Stokes shifts (mainly due to the CD size distribution), and usually have an excitation-dependent behaviour, with the emission peak varying with the excitation wavelength ^[22]^[58].

The emission mechanism of CDs is a longstanding debate, and several hypotheses have been proposed ^[41] (see Figure 3) such as (i) size-dependent emission, (ii) surface state-derived luminescence, and (iii) embedded molecular luminophore ^[60]. Regardless of the type of mechanism, it has been shown that CD emissions could be regulated by controlling their size (mainly referring to sp₂ carbon domains), their surface passivation and/or functionalization, and their doping due to the presence of heteroatom ^[61]^[62]^[63].

The first hypothesis proposed is based on the size of CDs ^[60]. Yuan and coworkers synthetized multicolour-emitting CDs with different dimensions from citric acid (CA) and diaminonaphthalene (DAN) by controlling the process parameters. CDs showed average sizes of about 1.95, 2.41, 3.78, 4.90, and 6.68 nm (Figure 4A), with corresponding tunable absorption (Figure 4B) blue (430 nm), green (513 nm), yellow (535 nm), orange (565 nm), and red (565 nm) emissions, respectively (Figure 4C). In accordance with the results, they deduced that, by increasing the size of CDs and consequently the conjugated π-domain, the bandgap decreases (Figure 4D,E) ^[65].

Figure 4. Preparation of bright multicolor bandgap fluorescent (BF) CDs by solvothermal treatment of citric acid (CA) and diaminonaphthalene (DAN) (A) from blue to red (B–E): (A) reprinted with permission from ^[14], copyright 2019 American Chemical Society, and (B–E) reprinted with permission from ^[65], copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A second hypothesis is related to the surface states of CDs. Ding et al. ^[66] synthetized tunable photoluminescent CDs by one-pot hydrothermal synthesis (Figure 5A). Noteworthy, these CDs had comparable particle size and carbon core but variable degree of oxidation of the surface state. Therefore, a gradual reduction in their band gaps and a red shift in their emission peaks from 440 to 625 nm (Figure 5B,C) was observed by increasing the incorporation of oxygen species into their surface structures (Figure 5D) ^[66]. Also, Miao et al. ^[67] hypothesized a similar mechanism. They modulated the CD emission from 430 to 630 nm by controlling the degree of graphitization and the number of surface –COOH groups by changing the molar ratios of CA to urea at different temperatures (Figure 6). The increasing number of –COOH groups on the surface increases the electronic delocalization, and the emission wavelength is consequently red-shifted ^[64].

Another relevant hypothesis to explain CD emission is molecular luminophore-derived emission or molecular state emission. Small molecules or oligomeric luminophores could be produced during CD synthesis, and these luminophores could be attached to the surface of CD backbones, allowing CDs to have bright emission properties ^[68]. Song et al. ^[69] studied the chemical structure and PL mechanism of CDs from CA and ethylenediamine (EDA). They proved the presence of a type of bright blue fluorophore and that CD emission was a result of small molecules, polymer clusters, and carbon cores. Indeed, the fluorophore may be attached to the carbon core, that may strongly affect the PL properties of the CDs.

Figure 5. (A) One-pot synthesis and purification route for CDs with distinct photoluminescence (PL) characteristics, (B) eight CD samples under 365 nm UV light, (C) corresponding PL emission spectra, and (D) model for the tunable PL of CDs with different degrees of oxidation: reprinted with permission from ^[70], copyright 2016 American chemical society.

Figure 6. Multicolor-emitting CDs (called as CDs3) by using different molar ratios of CA to urea at different temperatures: the emission of CDs3 can be adjusted from 430 to 630 nm. The photoluminescence quantum yields (PLQYs) of the three CDs3 in blue, green, and red were 52.6%, 35.1%, and 12.9%, respectively. Reprinted with permission from ^[64], copyright (2019) Springer-Verlag.

3. OLED-based Carbon Dots

CDs with amazing properties such as optical characteristics, intrinsic high stability, low-cost, and environment-friendliness, find natural and practical applications as crucial components in OLED technology. In the last decade, the interest in OLED based on CDs (hereafter CD-OLEDs) has been growth, and an increasing number of research groups have started to investigate in this field.

It is important to point out the dual employment of CDs as emitter and as a charge regulating interlayer (Figure 7) in the typical OLED architecture glass/indium tin Oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/emitting layer/interlayer/cathode. Besides, many groups used CDs also as a remote emitter, endowing blue commercial LEDs with a color converting filter based on CDs embedded in poly(methyl-methacrylate) (PMMA) or other matrices. The blue LED emission was tuned from blue to red by altering the film thickness of the filter or the doping concentration of CDs ^[68]^[69]^[70]^[71]^[72].

Figure 7. Scheme of the possible device architectures that incorporate CD as an emitter or as an interlayer.

Table 1 gathers the main characteristics of the OLED in which the CDs are used as emitter or charge regulating interlayer. Starting materials, dimension and shape, electroluminescence peak (EL_PEAK), maximum luminance (L_MAX), current efficiency (η_c) and publication year are reported (where available).

CDs as neat emitter are reported in the first 8 entries of the table, while entries 9-15 reported the main OLED performance with CDs as guest emitters in blend dispersion. In the last four entries the main parameters of devices featuring CDs as charge regulating interlayers are shown.

Table 1. Summary of performance ^[42] of Organic Light-Emitting Devices (OLEDs) incorporating a CD layer (2020 updated).

Entry	Starting Materials	Dimension (nm)/Shape	EL_PEAK (nm)	L_MAX (cd/m²)	η_c (Cd/A)	[Ref] (Year)
	CDs as neat emitter
1	Citric acid with 2,3-diaminonaphthalene	1.95 nm	455	136	0.084	^[65] (2017)
		2.41 nm	536	93	0.045
		3.78 nm	555	60	0.02
		4.9 nm	585	65	0.027
		6.68 nm	628	12	0.0028
2	ethylenediamine and phthalic acid	5.53 nm	455	4.97	-	^[66] (2017)
3	citric acid + octadecene + 1-hexadecylamine	Spherical 5 nm	550-670	35	0.022	^[73] (2011)
4	1-octadecene 1-hexadecylamine	6 ± 1.9 nm	460	21	0.06	^[74] (2019)
5	1-hexadecylamine and anhydrous citric acid	3.3 nm	426	24	0.018	^[75] (2013)
6	1-hexadecylamine and anhydrous citric acid		426, 452, 588	61
7	1-hexadecylamine and anhydrous citric acid + ZnO nps		426, 452, 588	90
8	anhydrous citric acid and hexadecylamine	Spherical 2.0–2.5 nm lattice spacing 0.22	554	5.7		^[76] (2018)
	CDs as guest emitter
9	citric acid and diaminonaphthalene.	quasi-spherical 2.4 lattice spacing 0.21 nm	450	5240	2.6	^[77] (2020)
10	Phloroglucinol	triangular 1.9 nm	476	1882	1.22	^[78] (2018)
		2.4 nm	510	4762	5.11
		3 nm	540	2784	2.31
		3.9 nm	602	2344	1.73
11	Citric acid with 2,3-diaminonaphthalene	2.41 nm	536	2050	1.1	^[65] (2017)
12	human hair	2D array of CDs 2–6 nm	498	350	0.22	^[79] (2020)
12	human hair	2D array of CDs 2–6 nm	498	700	0.2	^[79] (2020)
13	N,N-dimethyl-, N,N-diethyl-, and N,N-dipropyl-p-phenylenediamine	quasi-spherical 2.2 ± 0.31. 2.3 ± 0.28. 2.3 ± 0.26 nm lattice spacing 0.21 nm	605/434 612/435 616/435	5248–5909	3.65 3.85	^[80] (2019)
14	anhydrous citric acid and hexadecylamine	spherical_2.0–2.5 nm lattice spacing 0.22	558-550	339.5–455.2	-	^[73] (2011)
15	anhydrous citric acid and hexadecylamine	-	474	569.8	-	^[81] (2018)
	CDs as interlayer
16	ethylenediamine and citric acid	-	532	30 730	93.8	^[82] (2020)
17	banana leaves	4–6 nm (quasi-spherical)	486	-	-	^[83] (2019)
18	Ethanolamine	-	622	3500	0.63	^[84] (2017)
19	Citric acid and p-phenylendiamine			13	9x10^-4	^[85] (2020)
	Citric acid and ethylenediamine			70	2x10^-3
	Citric acid and urea			146	2x10^-3
	Citric acid and 1- hexadecylamine			174	8x10^-4

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Subjects: Materials Science, Coatings & Films

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Mariacecilia Pasini

Umberto Giovanella

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1. Introduction

2. Carbon Dots and Optical Properties

3. OLED-based Carbon Dots

References