Nanostructured graphene
Lu, Liqiang
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2018
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Lu, L. (2018). Nanostructured graphene: Forms, synthesis, properties and applications. Rijksuniversiteit Groningen.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Chapter 1
Introduction
1.1 Graphene
Graphene is an allotrope of carbon with two-dimensional (2D) single layer of carbon atoms ordered in a hexagonal honeycomb structure. Even it was presumed that the strictly 2D crystals were thermodynamically unstable and could not exist, because the melting temperature of thin films fast reduces when reducing thickness, resulting in an unstable film, typically, dozens of atomic layers (segregate into islands or decompose).1 A groundbreaking experimentally research work done by
Geim and Novoselov, who were accordingly awarded the Nobel Prize in Physics 2010, was mechanically exfoliated single-layer graphene from graphite, proving the existence of 2D atomic layers.2 The authors presumption, that freestanding
graphene should be expected to be chemically unstable, means they were unaware at the time that in 1962 Boehm et al. had already produced and identified freestanding graphene sheets.3 Boehm also coined the name “graphene” in 1986.4,5
Historically, the first examples of patterned epitaxial graphene for graphene-based electronics were published in 2004, in a paper titled: “Ultrathin epitaxial graphite and a route to graphene based electronics”.6
The extraordinary mechanical, thermal and electrical properties of graphene with long-range π-conjugation have been also disclosed subsequently. The thermal conductivity of a single-layer graphene at room temperature is up to 5,300 W mK−1,
a record thermal conductivity 10 times as high as copper.7 A suspended single layer
of graphene is one of the stiffest known materials characterized by a remarkably high Young’s modulus of about 1 TPa and an intrinsic strength of 130±10 GPa.8 It
has a very special electronic structure, the π and π* bands touch in a single point at the Fermi energy (EF) at the corner of the Brillouin zone, and close to this so-called Dirac point the bands display a linear dispersion. The mobility of graphene can exceed 15,000 cm2 V–1 s–1 even under ambient conditions.2 Room-temperature
quantum hall effect observed in graphene further indicates its extreme electronic quality. In addition, the excellent optical transparency as high as 97 % (at 550 nm) of monolayer graphene make it rather promising as new generation of transparent conductive electrodes.9Despite of all these exciting properties so far graphene
devices, based on ‘Scotch tape method’ also called ‘transferred graphene’, are not competitive with conventional nanoelectronics. Epitaxial graphene (epigraphene),
grown on various crystalline surfaces, is an exception, and is currently the only type of graphene that is suitable for graphene nanoelectronics.6 Noteworthy, epitaxial
graphene on silicon carbide (epigraphene) was first identified in 1962 by Badami,10,11 followed by van Bommel et al. in 1975.12
The extraordinary properties of graphene are attracting worldwide attentions. Beyond the research on a lab-scale, many companies are also involved in the R&D of graphene products, such as transparent electrodes, batteries, supercapacitors, solar cells, composites, anticorrosion film and so forth. As is stated, “in the last five years, the worldwide production of graphene increased dramatically from only few companies in USA to dozens of manufactures all over the world”. For example, the annual production capacity of small graphene sheets and graphene films has exceeded 400 tones and 110,000 m2 in China within a decade since a first piece of
graphene was mechanically exfoliated from graphite in 2004.13
1.2 Dimensions and foams
The discovery of graphene makes all members of graphitic carbon connected from 0-dimensional (0D) buckminsterfullerene to one-dimensional (1D) single-walled carbon nanotube and three-dimensional (3D) bulk graphite (as seen in Figure 1.1). In addition, graphene can be regarded as the basic building block for these graphitic carbon “stars”. For instance, fullerenes can be obtained from graphene with the introduction of pentagons, and hence, fullerenes can be thought as wrapped-up graphene. Carbon nanotubes can be regarded as a rolling graphene along a given direction and reconnecting the carbon bonds. Graphite is a stacking block of graphene layers that are weakly coupled by van der Waals forces.
Figure 1.1 Graphene can be considered as the 2D building block material for carbon materials of various dimensionalities. It can be wrapped up into 0D buckyballs, rolled into 1D nanotubes or stacked into 3D graphite.1
Strictly speaking, graphene is a single-atom-thick carbon sheet, normally with long-range π-conjugation not being a constitutional part of a carbon material, but freely suspended or supported on a substrate. To date, the graphene members also embrace bilayer graphene, a few layer graphene and multilayer graphene, which are distinguished with the isolated monolayer graphene, e.g. from aspects of electronic properties which critically depend on the number of layers.1Obviously
other properties of graphene such as thermal conductivity, mechanical stiffness, strength and flexibility and optical transparency may also change upon increasing number of layers of graphene. In addition to the longitudinal dimension, the chemical and physical properties could significantly change with the lateral dimension of graphene.
For example, the high electrical conductivity, transparency and good flexibility make large-area single-layer graphene to be a promising alternative material of indium tin oxide (ITO) for transparent conductive electrodes. The defect-free large-area graphene films coated on steel can isolate oxygen and water molecular from the steel and prevent the corrosion. The large-area holey graphene, formed by removing a large number of atoms from the graphitic plane to produce holes distributed on and through the atomic thickness of the graphene sheets, can be used for sensing, sieving, or even electrical DNA sequencing.14 In addition, the
flexible, bendable, stretchable electronics have been fabricated with graphene sheets, which are difficult to achieve in modern Si electronics. The long-range graphene film is also a good precursor for synthesizing nanostructured graphene with other dimensional by photolithography, etching, folding etc.15
In contrast to large-area graphene, graphene quantum dot (GQD), a zero-dimensional derivative of graphene, is defined as graphene dot with interplanar crystal dimension less than 10 layers and basal plane smaller than 100 nm. However, usually the GQDs only have few atomic layers and the lateral dimensions are less than 10 nm as shown in Figure 1.2.16 Owing to the quantum confinement
effect and edge effect, GQDs display extraordinary electronic, spin, photochemical, photoelectric and magnetic properties, which are distinct with the large-size graphene.17,18 Also, in comparison with other semiconductor quantum dots, the
graphene quantum dots have low costs and low toxicity. These unique properties and advantages make graphene quantum dot being a multifunctional carbon material in various applications. For example, those of most promising applications of GQDs are being as biomaterials for bioimaging, drug delivery and cancer therapeutics. In addition, heteroatom (such as nitrogen) doped graphene quantum dots (GQDs) can be used as good catalysts for oxygen reduction reaction to replace the noble Pt material.
Figure 1.2 A high-resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction pattern (inset) of a graphene quantum dot.19
Three-dimensional porous graphene is an architecture that built up by 2D graphene and pores, resulting in a foam-like cross-linked 3D network with π conjugation.20 Different from 2D holey graphene, which has holes on 2D planar
layers, 3D porous graphene could consist of defects-free 2D layers of graphene. 3D porous graphene could also contain holey graphene layers, leading to 3D defective porous graphene. Thus, the 3D graphene also inherits intrinsic physical properties of graphene. The 3D spongy form of graphene has a higher surface area than aggregates of 2D sheets. In particular, the 3D defective porous graphene could have a higher surface area than that of perfect 2D graphene (2,630 m2 g-1). The pore size
of 3D porous graphene can vary from few nanometres to hundreds of micrometres, but usually from tens of nanometres to several microns. Owing to these unique properties and porous channels, 3D porous graphene exhibits promising applications in sensors, energy harvesting, steam generators, water purification, oil–water separation, photodetectors, filters, and energy storage (batteries and supercapacitors) in particular.21,22
1.3 Lithium-sulfur batteries
Rechargeable electrochemical energy storage (EES) devices such as batteries and supercapacitors have been widely used in our life.23,24For example, lithium-ion
batteries (LIBs) are popularly commercialized not only for mobile phones, laptops, cameras and toys, but also for electric transportations. The global market of LIBs increases annually. Moreover, high-energy-density batteries are needed to make the future batteries lighter and smaller and without sacrificing energies. However, the current LIBs meet many bottlenecks. Firstly, the present state-of-the-art LIBs are based on graphite anodes coupled with layered oxides (such as LiCoO2,
layered oxides severely limit the gravimetric capacity (the charge stored per gram of a cell weight), gravimetric and volumetric energy density of the cells. For example, the LiCoO2-graphite system only has a theoretical gravimetric energy
density of only 387 Wh kg-1 and a practical energy density of 120 to ~150 Wh kg-1,
which are much smaller than that of combustion of commercial fuels such as coal of ~6.7 kW h kg-1 and gasoline of ~12 kW h kg-1.25 Another problem is the short span
life of current LIBs. The electrical cars particularly require much longer span life than current LIBs for portable electronics. The high cost of materials such as lithium and transition metals also influences the spreading of LIBs for electrical transportations. Moreover, the safety issue is another plague for further promoting electrical cars.25
The lithium-sulfur (Li-S) battery is a rather new emerging rechargeable device by using sulfur as cathode and lithium as anode. During discharge of the cell, the S-S bonds split to open the S-S8 ring, and subsequently shorten chain length until
become Li2S as shown in Figure 3a.26The overall redox couple of a Li-S battery is
described by the reaction 16Li + S ↔ 8Li S and occurs at a potential of 2.15 V vs Li/Li+.27 The Li-S battery can reach a theoretical specific energy and volumetric
energy density of approximately 2,600 Wh kg-1 and 2,800 Wh L-1, respectively. The
remarkable storage capacity permits electric vehicles to drive >500 km after a single charge. Moreover, sulfur is an attractive electroactive material for cathodes because it is naturally abundant, low cost, and environmentally friendly. Thus, sulfur can be one of the most promising next generation cathode materials.
Figure 1.3 (a) a typical charge/discharge voltage profile of lithium–sulfur batteries in ether-based electrolytes, (b) schematic of the problems such as electrical isolation of sulfur, dissolution and “shuttle effect” of polysulfides, continuous deposition of solid electrolyte interface (SEI) and lithium dendrite formation in a Li-S battery.26
The current Li-S batteries are seriously hampered by various problems as shown in Figure 3b: the low practical capacity, fast capacity fading, poor discharge and charge performances at high-current densities, as well as safety issues. These problems are mainly caused by the following reasons: (a) poor electrically and ionical conductivity of both sulfur and lithium sulfide (product after fully
discharge); (b) “shuttle effect” caused by high solubility of intermediate polysulfides (Li2S4 to Li2S8) in the electrolyte. (c) a large volumetric expansion of
~80% upon full lithiation to lithium sulfide, which can cause pulverization and structural damage at the electrode level, and (d) lithium dendrites formation. As a result, current lithium–sulfur batteries can only work well for a few hundred charging cycles.
Previous work reported many approaches to solve the abovementioned problems of Li-S batteries: (i) to overcome the insulating of sulfur, conductive additives such as carbon (carbon black, carbon nanotube, porous carbon, graphene, etc.), conductive polymers or metals (such as Cu nanoparticles) with high surface areas are commonly used to augment the electrical conductivity by embedding sulfur into them. Wetting sulfur by organic electrolytes can enhance its ionic transport. Through reducing the sulfur particle size, the diffusion path for electrons and lithium ions can also be reduced to increase the utilization of the active sulfur mass; (ii) to decrease the “shuttle effect”, the most common method is encapsulation of sulfur in porous carbon materials. On the other hand, employing chemical binding between polymers, or graphene oxides, or metal oxides/sulfides and lithium polysulfides species was used.
Besides these, implementing an interlayer or modified separators to the sulfur cathode side not only decreases the charge transfer resistivity, but also helps to suppress the lithium polysulfides diffusion to the lithium anode. The interlayer can be an upper current collector to improve active material utilization. (iii) The third way is to protect the lithium metal surface against dissolved polysulfide species by using lithium nitrate (LiNO3) additives to form a passivating solid electrolyte
interface (SEI) film.28
In the last few years, although considerable efforts were devoted, there are still many unsolved issues and challenges on the sulfur cathodes and Li-S batteries: (a) Low practical gravimetric energy density and capacity. The gravimetric energy density of current Li-S batteries is usually ~350 Wh kg-1 and a few reaches ~500
Wh kg-1, which is still much lower than the theoretical specific energy of 2,600 Wh
kg-1. (b) Short service life. For applications such as in electric vehicles, a longer life
(above 2,000 cycles) of battery service is required. However, as aforementioned the current lithium-batteries still severely suffer from the dissolution and “shuttle effect” of polysulfide species. For most Li-S cells, it only serves for few hundred cycles. (c) Low sulfur content and areal loading. There is a competition between the sulfur content, areal loading (mg cm-2) with the capacity. Normally, with increasing
the sulfur loading (e.g. >80 wt.%), more isolated sulfur cannot be utilized. In addition, with increasing the areal loading of sulfur, the battery performances also become worse. Most of the current electrodes sacrifice the sulfur content (mostly 40~70 wt.%) or sulfur loading (only 1-4 mg cm-2) to achieve higher capacity (based
on the mass of sulfur) and longer life. However, the areal capacity based on the mass of whole electrodes is still low.
1.4 Fluorescence bioimaging
Fluorescence bioimaging is a powerful method for investigating living cells, both in vivo and in vitro. Previous usual used probes are semiconductor quantum dots, which appear good stability.29 However, these quantum dots have high
toxicity and high cost. Due to concerns on safety, searching for alternative stable luminescent probes is becoming attractive. Graphene quantum dot may become one of such materials the attractive alternatives.
By controlling the synthetic methods or conditions or post-treatment, the surface and edges of GQDs can be functionalized by hydrophilic groups such as carboxylic, hydroxy, and amino. Owing to these groups, the GQDs normally display good solubility in water. Importantly, by adjusting the groups, the GQDs can also present good biocompatibilities. Many in vitro toxicity tests displayed the cell viability above 80% for numerous kinds of cells even using a high concentration up to 400 μg mL−1 of the GQDs.30 In addition, the in vivo toxicity evaluations
demonstrate the living organisms have no obvious cytotoxicity after 21 days by uptaking 10 mg kg−1 dosages of GQDs. Liu et al. detected imaging penetration
depth in tissue specimens by using amine-functionalized GQDs, which have high two-photon absorption cross section (48,000 Goeppert-Mayer units), and found long-term deep-tissue (1,800 µm) imaging.31
1.5 Scope of this thesis
Dimensionality and microstructures of graphene affect firmly its physical and chemical property and applications. Investigations of graphene from different aspects such as sizes, microstructures, properties could help us to understand, design and use of graphene. This thesis covers a wide range of the synthesis, properties and applications of graphene with various microstructures, and forms from high dimensions to low dimensions. On-basis of the state-of-the-art progresses, and remained questions or challenges, this thesis project will provide some possible solutions to them, and new insights of them.
In chapter 2, the synthesis of the state-of-the-art forms and dimensions of graphene will be reviewed. It covers the following sections: (1) the synthesis methods and properties of graphene flakes, domains and long-range films derived from exfoliation of graphite, chemical vapor deposition (CVD) growth and low temperature synthesis; (2) the synthesis and properties of 3D graphene foams by using different approaches such as cross-linking of reduced graphene oxides and template-assisted CVD growth; (3) the synthesis and properties of graphene quantum dots including top-down approaches and bottom-up approaches.
As porous metallic templates are important for growth of porous graphene, in
chapter 3, a new versatile template-free method for the synthesis of nanoporous
salts. The approach involves thermal decomposition, reduction followed with metal growth into a 3D structure. Topological disordered porous architectures of metals with a controllable distribution of pore size and ligaments size ranging from tens nanometers to micrometers are synthesized. The reduction processes are scrutinized through XRD, SEM and TEM. The formation mechanism of the nanoporous metal is qualitatively explained. Different forms of nanoporous metals are synthesized.
In an application, the as-prepared nanoporous Ni is employed as binder-free current collectors of lithium-ion batteries. The nanoporous Ni electrodes deliver enhanced reversible capacities and cyclic performances compared with commercial Ni foam.
After synthesis of nanoporous metals, in chapter 4 a solid-state-growth approach for the synthesis of 3D interconnected nanoporous graphene at low temperatures (600 - 800 °C) is developed by using nanoporous Ni as templates and catalysts. The microstructures and wall thicknesses are controlled. The porosity and tubular pore sizes of nanoporous graphene are adjusted from tens of nanometres to hundreds of nanometers by using different nanoporous Ni templates. Different forms of nanoporous graphene are prepared.
For application, a novel design of sulfur cathodes by encapsulation of sulfur in the tubular pores of nanoporous graphene (NPG) is created. The electrochemical performances of NPG-sulfur composites are studied. The influences of surface area, porosity, and pore size on the electrochemical performances are investigated.
To further improve the capacity-density of sulfur cathodes, in chapter 5 a three-dimensional stochastic interconnected macroporous graphene foam (3D-MPGF) is developed as lightweight binder-free current collectors of sulfur cathodes of lithium-sulfur batteries. A new one-round heating process, which combines the synthesis of porous metal and CVD growth of graphene together, is explored for synthesis of 3D-MPGF. The features of the as-developed methods, microstructures, and influencing factors during growth are demonstrated by SEM, TEM and Raman microscopy. As host materials of sulfur cathodes, different loadings of sulfur are filled in 3D-MPGF. The electrochemical performances of theses 3D-MPGF/S electrodes are analyzed by galvanostatic charge and discharge measurements.
To understand the diffusion assisted growth of graphene at low temperatures, in chapter 6, large-area graphene-based carbon films were synthesized through a fast and low-temperature (below 350 °C) method. The processing route is illustrated on a free surface of Ni catalyst film by vacuum thermal processing of amorphous carbon. The nucleation and growth of graphene on the free surface of nickel and along the interface between Ni film and SiO2 substrate are investigated
by using a thin film Ni-C-Ni sandwiched structure on a SiO2/Si substrate. Raman
spectroscopy is used to investigate the quality of films. HR-TEM is used to detect the number of layers and growth processes. Growth parameters such as growth
time, growth temperature and carbon/Ni ratio are investigated for a control of graphene growth kinetics.
Chapter 7 presents a novel ultrasonic-assisted liquid-phase exfoliation
technique for synthesizing GQDs in large scale. The new method is environmentally friendly, fast and industrial promising. The production yield of GQDs in N-methyl-2-pyrrolidone (NMP) can reach 3.8 mg mL-1. The GQDs with different sizes,
structures and defect contents were synthesized by using different graphitic carbon precursors for exfoliation. Two types of high-defects GQDs (HD-GQDs) and low-defects GQDs (LD-GQDs) are derived from acetylene black and nano-graphite respectively. Their microstructures, luminescent and absorbance properties are investigated. The as-synthesized GQDs are used as fluorescence nanoprobes for bioimaging. The biocompatibility and fluorescent performances are investigated.
Chapter 8 summaries the content of this thesis, and gives an outlook of
possible future directions related with the scope of this thesis and the advanced materials developed in this thesis.
References
[1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183.
[2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (2004) 666.
[3] H.P. Boehm, A. Clauss, U. Hofmann, G.O. Fischer, Dünnste kohlenstoff-folien, Z. Naturforsch. B Chem. Sci 17b (1962) 150−153.
[4] H.P. Boehm, R. Setton, E. Stumpp, Nomenclature and terminology of graphite-intercalation compounds, Carbon 24 (1986) 241−245
[5] H.P. Boehm, Graphene-how a laboratory curiosity suddenly became extremely interesting, Angew. Chem. Int. Edit. 49 (2010) 9332−9335
[6] C. Berger, Z.M. Song, T.B. Li, X.B. Li, A.Y. Ogbazghi, R. Feng, Z.T. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. De Heer, Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics, J. Phys. Chem. B 108 (2004) 19912−19916. [7] A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior
thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902.
[8] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385.
[9] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, et al., Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574.
[10] D.V. Badami, Graphitization of alpha-silicon carbide, Nature 193 (1962) 569.
[11] D.V. Badami, X-ray studies of graphite formed by decomposing silicon carbide, Carbon 3 (1965) 53−57.
[12] A.J. Van Bommel, J.E. Crobeen, A. Van Tooren, LEED and auger electron observations of the SiC(0001) surface, Surf. Sci. 48 (1975) 463−472.
[13] W. Ren, H.M. Cheng, The global growth of graphene, Nat. Nanotechnol. 9 (2014) 726. [14] W.J. Yuan, J. Chen, G.Q. Shi, Nanoporous graphene materials, Mater Today 17 (2014) 77−85. [15] V.P. Pham, H.S. Jang, D. Whang, J.Y. Choi, Direct growth of graphene on rigid and flexible
[16] Z. Zhang, J. Zhang, N. Chen, L.T. Qu, Graphene quantum dots: an emerging material for energy-related applications and beyond, Energy Environ. Sci. 5 (2012) 8869−8890.
[17] J.C. Ge, M.H. Lan, B.J. Zhou, W.M. Liu, L. Guo, H. Wang, et al., A graphene quantum dot photodynamic therapy agent with high singlet oxygen generation, Nat. Commun. 5 (2014) 4596. [18] F. Liu, M.H. Jang, H.D. Ha, J.H. Kim, Y.H. Cho, T.S. Seo, Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence, Adv. Mater. 25 (2013) 3657.
[19] L. Wang, Y.L. Wang, T. Xu, H.B. Liao, C.J. Yao, Y. Liu, et al., Gram-scale synthesis of single-crystalline graphene quantum dots with superior optical properties, Nat. Commun. 5 (2014) 5357.
[20] A, Bianco, Y. Chen, Y. Chen, D. Ghoshal, R.H. Hurt, Y.A. Kim, et al., A carbon science perspective in 2018: Current achievements and future challenges, Carbon 132 (2018) 785. [21] X. Yao, Y.L. Zhao, Three-dimensional porous graphene networks and hybrids for lithium-ion
batteries and supercapacitors, Chem 2 (2017) 171.
[22] Y.F. Ma and Y. Chen, Three-dimensional graphene networks: synthesis, properties and applications, Nat. Sci. Rev. 2 (2015) 40.
[23] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19.
[24] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, et al., Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage, Science 347 (2015) 1246501.
[25] J. Park, S.H. Yu, Y.E. Sung, Design of structural and functional nanomaterials for lithium-sulfur batteries, Nano Today 18 (2018) 35.
[26] H.J. Peng, J.Q. Huang, X.B. Cheng, Q. Zhang, Review on High-Loading and High-Energy Lithium–Sulfur Batteries, Adv. Energy Mater. 7 (2017) 1700260.
[27] Z W. Seh, Y.M. Sun, Q.F. Zhang, Y. Cui, Designing high-energy lithium–sulfur batteries, Chem. Soc. Rev. 45 (2016) 5605−5634.
[28] S.S. Zhang, Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions, J. Power Sources 231 (2013) 153−162.
[29] M. Montalti, A. Cantelli, G. Battistelli, Nanodiamonds and silicon quantum dots: ultrastable and biocompatible luminescent nanoprobes for long-term bioimaging, Chem. Soc. Rev. 44 (2015) 4853.
[30] D. Wang, J.F. Chen, L.M. Dai, Recent advances in graphene quantum dots for fluorescence bioimaging from cells through tissues to animals, Part. Part. Syst. Charact. 32 (2015) 515–523. [31] Q. Liu, B.D. Guo, Z.Y. Rao, B.H. Zhang, J.R. Gong, Strong two-photon-induced fluorescence
from photostable, biocompatible nitrogen-doped graphene quantum dots for cellular and deep-tissue imaging, Nano Lett. 13 (2013) 2436−2441.
