Functionalized graphene-based nanocomposites for smart optoelectronic applications

2.1 Optical properties

Graphene with its multilayer structure can absorb radiation over a range of frequencies, and its ability to generate optical transitions in the electric fields is called gate-dependent optical transitions [64–68]. The absorption rate of mono- and bilayer ability of graphene in the presence of light is shown in Figure 4a, and the graphene material can absorb only 2.3% of its incident white light with 97.7% transmittance [69]. The graphene has the thickness of one atom, and the frequency of incident light has no relationship with the intensity of absorption [26]. These exceptional characteristics result from the electrons, conical band, and holes at the Dirac fermions in its sheets [70]. However, the bilayer graphene can absorb white light up to 4.6% and in accordance with the increase of the number of layers, the absorption capabilities of graphene also increase linearly. The individual layer absorption can be obtained as A = 1 − T = πα = 2.3%, where α ∼1/37 represents the fine-structure constant [71]. So, with each layer absorbing 2.3% of light and with a graphene sample of five layers can have a total of 11.5% absorption and 88–88.5% optical transparency. Hence in a study, the ultraviolet radiation spectrum in the wavelength range of 900–300 nm was used to identify graphene, where the results proved that the graphene under these conditions is featureless, and at the same time, the maximum absorption was noted for this material around 270 nm wavelength, thereby confirming its optical properties at specific regions [69].

Figure 4

(a) Absorption rate of mono- and bilayer graphene in the presence of light and (b) the dynamic simulation of graphene-saturated absorption [73].

2.2 Linear absorption

The unique band structure of graphene has novel optical properties, i.e., when incident light falls on the surface of graphene, the electrons are ejected from the valance band and excited to the conduction band by absorbing the photon energy. Thus, the conductivity caused by photons in single-layer graphene depends upon the structural constant as follows [63]:

Also, T is the linear transmittance of single-layer graphene and is represented as follows:

In the visible region, the reflectance of graphene is less than 0.1% and is counted as 2% for graphene with ten layers. Figure 4b represents the dynamic simulation studies of graphene to explain its optical properties, and from that, it can be inferred that graphene can absorb light of any wavelength due to the zero bandgap [72]. Furthermore, the theoretical calculations also confirm that graphene can absorb different bands of light at different configurations of QDs and substrate-based graphene [49,50]. Graphene maintains its absorption rate of light even at weak incident light power when the energy band near the Dirac point of graphene is not filled. It is observed that graphene cannot continue the absorption of light to achieve sutural absorption because of the Pauli blocking effect. It has a nonlinear absorption [51–56], and its saturable absorption (SA) coefficient can be calculated as follows [51]:

where α ^∗ S and α ^∗ N _S represent saturated and unsaturated absorption components, respectively, N is the photo-induced electron–hole density, and N _s is the saturation density.

2.3 SA

SA is a nonlinear occurrence consisting of the quenching of optical absorption beneath high-intensity radiation. Thus, the outcome of SA is a continuous wave broken down into a train of ultrashort pulses, which are the key constituent of passive-mode locking (PML) in laser cavities. Most of the materials need higher optical intensities even closer to their optical-damage threshold, at which they undergo SA [74]. In this phenomenon, the mirrors used to operate in a narrow spectral range need advanced fabrication methods, since they are not very tunable. Recently, carbon-based nanomaterials have proved themselves to be potential, cost-effective, and feasible alternatives for developing next-generation PML lasers. Graphene, as a carbon material, has played a role in overcoming these limitations with its strange conical band structure [75], giving rise to broadband resonant SA even at remarkably low light intensities [76]. Moreover, graphene’s band structure can be further tuned by applying an external voltage to the gate [77]. The PML ultrafast laser action, broadband adjustability, and exchange quality factor are few of the properties that can be accomplished using SA components prepared using graphene to generate large energy pulses. Modern theories have shown that the single-mode function of the random laser is achievable by implanting graphene flakes in the gain medium [78]. For this purpose, the single-particle MDF representation turns out to be a suitable theoretical support [79].

2.4 Luminescence

Graphene has been used for photoluminescence and electromagnetic transport applications because of its extraordinary characteristics and so has created a wide range of research interests [80–83]. The fragmented graphene and graphene-derived QDs (GQDs) present exceptional photoluminescence properties in the preparation process and make graphene a photoluminescent material taking advantage of the coherence between π electrons that can be reduced by tuning its chemical and physical properties. The oxidation plasma treatment of single-layer graphene sheets can introduce bright light into it [80], and in the same way, the bulk graphene and dispersion can exhibit a large uniform area of photoluminescence [84,85]. A routinely prepared photoluminescent graphene material can be used in sandwich form with a conductive layer of graphene to make sandwich-type light-emitting diodes (LEDs), which can operate with a wavelength range of infrared to blue [80]. In general, G-QDs are synthesized using a blue-radiated hydrothermal method [83]. Wang and his colleagues investigated the photoluminescence mechanism in G-QDs [86] and found that the excited-state transfer between electrons and holes and the influence of edge-effect light luminescence (as shown in Figure 5) made in visible photoluminescence devices within the scope of the preparation of a theoretical basis.

Figure 5

Comparison of the UV-Vis absorption spectra of GQDs and surface functionalized GQDs [86].

With a decrease in the size of the QDs, graphene exhibits quantum confinement and unique edge effects [81–86]. A few other reports presented the photoluminescence of GO as recombination of e–h pairs of localized states of the sp² cluster [84]. The size or conjugate length of the sp² cluster is the major part that defines the energy gap between π* and π [87], which is likely to be explained by the edge effect of graphene and the defects related to the oxidation process. Some reports say that photoluminescence in GQDs is the consequence of the radiative recombination of electrons at the graphene edges [88], by a transition in free zigzag sites instead of transition between π* and π as defined previously. However, it doesn’t matter which mechanism is used for photoluminescence of GQDs, but it has the same characteristics; the excitation wavelength is responsible for the changes in emission spectrum wavelength. Therefore, the organic fluorescent material has a great influence in developing cheap optoelectronic devices [89].

Although the exact structure of GO is not well-understood, there is a general agreement that the bandgap is influenced by morphology sizes, oxygen coverage densities O/C ratio, surface functionalities, and other factors. As the size of GO NSs grows larger, the number of conjugated aromatic rings increases, lowering bandgap energy. Upon conjugation with different functional groups, such as atomic vacancy, hydroxyl, epoxy, and carboxyl groups, the gap energy of ideal graphene clusters decreases. Based on Gaussian and time-dependent DFT simulations, Chien et al. found that these functional groups caused distortions in the aromatic rings and that these disorder-induced localized states caused absorption in the lower energy regions (Figure 6) [90].

Figure 6

(a–e)The energy levels of ideal graphene NSs and decorated with various functional groups, such as atomic vacancy, hydroxyl, epoxy, and carboxyl groups [90].

Thorough investigation of the fluorescence difference between GQDs and graphene-derived GOQDs (GOQDs) will help in the in-depth understanding of their luminescence mechanisms. For example, Liu et al. have successfully synthesized highly homogeneous GQDs and GOQDs from the suspension of GO solution after sonication and centrifugation [91]. Both GQDs and GOQDs had circular shapes with a diameter of 4 nm, but GOQDs had different oxygen-containing functional groups, while GQDs had a pure sp² carbon crystalline structure without oxygenous defects. GQDs and GOQDs released transparent blue (420 nm) and green color (480 nm) emissions, respectively, according to the spectral analysis, and the fluorescence intensity of GQDs was about 3.5 times higher than that of GOQDs, despite its concentration, which is about 10 times lower (Figure 7a and b) [91].

Figure 7

(a) PL spectra and the corresponding fluorescent images of GQDs and GOQDs, (b) PL excitation spectra of GQDs and GOQDs with varying detection emission wavelengths [91].

During the fabrication of graphene from GO NSs, reductive treatment is often used to eliminate oxygen-containing groups. Meanwhile, the bandgap structure of GO NSs is evolving, resulting in a slew of new optical properties. For example, Eda et al. observed blue PL based around 390 nm for thin-film samples deposited from thoroughly exfoliated suspensions after exposure to hydrazine vapor [84]. The localization of e–h pairs was facilitated to radiative recombination by adequately regulating the concentration of isolated sp² clusters within the carbon–oxygen sp³ matrix via reduction treatment, and the PL intensity could be increased ten-fold compared to the as-synthesized materials. Furthermore, it was found that the bandgap energy of GO NSs can be continuously tuned by precisely regulating the reductive extent, resulting in a variety of luminescent colors. The bandgap of GO can be in situ and nonvolatile tuned by a redox reaction using a solid electrolyte thin film by simply applying direct current (DC) voltage inside an all-solid-state device (Figure 8) [92]. The PL peak wavelength can be adjusted from 393 to 712 nm by changing the applied DC voltage between −3.5 and 2.5 V (Figure 9) [93], where the polarity of voltage was defined as positive when GO was oxidized and negative when reduced. A wide emission peak at 676 nm was observed for pristine GO.

Figure 8

Schematic illustration of GO-based all-solid-state electric double-layer transistor. G, S, and D denote the gate, source, and drain electrodes [92].

Figure 9

DC bias dependence of normalized PL spectra tuned by application of various bias voltages for 1,200 s [93].

The mechanisms that explain the evolution of GO photoluminescence is shown in Figure 10. Figure 10a shows the original GO, which is made up of various disorder-induced defect states within the p–p* gap and has a large prominent PL spectrum centered at longer wavelengths. As shown in Figure 10b, the number of disorder-induced states within the p–p* gap decreases after deoxygenation, whereas the number of cluster-like states from newly formed small and isolated sp² domains increases. At shorter wavelengths and with narrower bandwidth, electron–hole recombination among these sp² cluster-like states induces blue fluorescence. As a result of modifying the heterogeneous electronic structures of GO and rGO with variable sp² and sp³ hybridizations by reduction, the tunable PL spectra of GO are due to variance of the relative intensity ratios of PL emission from two different forms of electronically excited states [94].

Figure 10

Proposed PL emission mechanisms of (a) the predominant IP1 emission in GO from disorder-induced localized states, (b) the predominant IP2 emission in rGO from confined cluster states [94].

3.1 Top-down approach

As we said, the synthesis of graphene has been widely categorized into “top-down” and “bottom-up” processes. The top-down approach is cost-effective and suffers from the limitations of large-scale production and quality control [104]. This approach generally involves the separation, peeling, cleaving, or exfoliation of graphite or its derivatives (GO and graphite fluoride) [105]. Usually, the formulation of imperfection-free graphene with superior quality involves diverse techniques, such as mechanical shedding of graphite, electrochemical peeling, sonication, surface functionalization, acid corrosion disintegration, alkylation of graphene subsidiaries, chemical methods for aqueous/organically treated GO, thermal methods for shedding graphite, and chemical methods for the reduction of GO [55,105]. The production of graphene and its nanocomposites by the peeling, surface functionalization, and chemical reduction with comprehensible strategies has been extensively examined by Potts et al. [106]. Comparably, Dhand et al. [107] have also investigated the synthesis of graphene, elucidating several analogous approaches. A diagram of the production of graphene by the top-down approach is provided in Figure 11 [108].

Figure 11

Schematic representation of the formation of highly × rGO from graphite using the top-down approach.

A well-thought-out top-down scheme for extricating mono-layered graphene flakes on the preferential substrate is mechanical peeling. This technique is the principally accepted strategy for the synthesis of graphene flakes as it usually necessitates ∼300 nN μm⁻² force to detach a single atomic layer from graphite’s surface. The piling up of sheets in graphite results in some moderately enveloped π orbital, contradictory to the flat surface of the sheet, and the attractive van der Waals force is outstripped with the micromechanical chasm using the scotch tape to separate the graphite sheets [26,103]. A lateral force can also be applied by means of graphite’s self-lubricating ability in the lateral direction to promote the relative motion between two graphite layers (Figure 12). These two mechanical routes are considered to be the prerequisites for the production of graphene by exfoliation techniques. Moreover, the tailoring of these two routes could produce high-quality graphene with high efficiency [109]. Although the exfoliation process is a destructive approach, the force generated by exfoliation can also fragment large graphite particles or graphene layers into smaller ones (Figure 12) [110]. This exfoliation reduces the lateral size of graphene, which is not useful for accomplishing large-area graphene or for facilitating the exfoliation into smaller graphite flakes [109].

Figure 12

Schematic representation of the two kinds of mechanical routes for exfoliating graphite into graphene flakes and the auxiliary route for fragmentation.

3.2 Bottom-up approach

The deciding variables for the control of synthesized graphene’s morphology, crystallinity, and structure are the tiny molecules of chemicals and catalysts within the bottom-up approach, as in prior discussions [111]. In the literature, the employment of hydrocarbons as a source of graphene has also been reported for the generation and use of metal-ion catalysts in the course of CVD practice [95,111–113]. In the CVD technique, the capability and strength to produce graphene layers are probably tremendous, which makes this technique the most acceptable one for the assembly and manufacturing of devices [111]. The assembling of graphite layers on a silicon–carbide substrate has been used since 1893 [114]. Primarily, Van Bommel et al. in 1975 affirmed the growth of graphite on both 6H-SiC (001) substrates [115], and in the recent era, the technique has gained enormous importance because of the high-temperature sublimation escalation deposition of mono-layer epitaxial graphene, grown on the SiC substrate. One group has reported the depositing of mono-layers up to a few layers of graphene deposits on the SiC substrates, mainly by means of high-temperature (1,300–1,800°C) heating of SiC substrate with the use of an ultrahigh vacuum or under an inert atmosphere [114]. As the temperature required for the sublimation of Si is less than that for carbon (∼1,500°C), the SiC surface is given a thin layer of carbon [116]. The number of graphene layers stacked and blended onto the SiC substrate depends on the face orientation of the crystal and layer growth conditions. Even though the most commonly used epitaxial graphene growth surfaces are the hexagonal forms of 4H- and 6H-SiC substrates, the CVD graphene growth has been achieved using the cubic 3C-SiC.

In the modern era, the assembly of SiC wafers has become more aggressive because of the demand for epitaxial graphene in the LED industry and in power electronics research. The epitaxial growth is the deposition of a single crystalline film on monocrystalline substrate to facilitate the epitaxial film. Thus, the fabrication involves the deposition of high-crystalline graphene on single-crystalline SiC substrates, either by means of homo-epitaxial (film deposited on the identical substrate) or hetero-epitaxial (film deposited on a dissimilar substrate) growth. In particular, during 2004, the SiC substrates were primarily intended to measure parameters correlated with the electrical measurements because of a wide-bandgap semiconductor (3 eV) on patterned epitaxial graphene [109]. One can anticipate that the epitaxial graphene growth technique on SiC might be an exceptionally good technique for the large-scale assembling and marketing of graphene-accompanied electronics, such as high-frequency electronics, light-emitting devices, and radiation hard devices [117]. Even though the abovementioned technique is very costly, taking into consideration the novel resistance standards located on the quantum Hall effect [118], top-gated transistors have been made up of graphene on SiC [119]. The high-frequency transistors have furthermore been uncovered with a 100 GHz cut-off frequency [120], superior to the state of the art of Si transistors of similar gate length [121].

During the year 2008–2009, the CVD emerged as an outstanding method for the fabrication and assembly of graphene because of its availability, first-rate quality [122], inexpensiveness, and applications involved in the extensive fabrication of flexible crystal-clear conductors for organic photovoltaic (OPV) cells and effectual transistor production [123]. The graphene prepared by the CVD technique is more extensively intended for the fabrication of n-type graphene as compared to the scotch-tape method and can be fashioned in two uncomplicated steps, carbon atoms formed initially as a result of pyrolysis of the precursor material and the disassociation of carbon atoms to shape the carbon structure of graphene. The process is carried out in a high-temperature furnace (1,000°C) in the company of a catalytic agent, where stress is created on the gas-phase surface of the transition-metal substrate (Ni, Cu, Co, Pd, Ir, Au, or Ru) and by exposure to one of the hydrocarbons (methane, ethylene, acetylene, or benzene) to form carbon layers. Furthermore, the furnace is rapidly cooled to keep the precipitated carbon layer from aggregating in order to form bulk graphite in the course of chemical adsorption, thereby crystallizing into a contiguous graphene layer on the metal substrates. Depending on the substrate quality, precursor characteristics, width, and requisite structure, the various types of CVD processes, such as a thermal, plasma-enhanced, cold wall, hot wall, or reactive type CVD, are usually employed [124].

4.1 Functionalization with molecules

One of the conventional and accepted ways to functionalize graphene is by using small molecules; for instance, surfactants, pyridine, protein, DNA, RNA, peptide, and complex compounds, typically the anticancer drugs, can be functionalized on the graphene surface to upgrade and ameliorate the solution dispensation potential, optical, electronic, and bio-related characteristics. Reconstructing the ability of graphene to dissolve in a wide range of solvents is one of the foremost critical purposes of graphene molecular functionalization. The chemically prepared graphene possesses destitute solvency and is exceptionally troublesome to be used in large-scale productions because of the difficulty of handling and complex procedures. As an illustration, sodium dodecylbenzene sulfonate can be functionalized to modify the graphene surface as well as to attain exceedingly soluble graphene over 1 mg mL⁻¹ in water, which makes possible solution-based fabrications for the conductive film or any conceivable biological analysis [125–127]. An example of pyridine-based functionalized graphene by Gupta et al. is shown in Figure 13 [128].

Figure 13

Schematic representation of the synthesis of pyridine-based functionalized GOs [128].

In order to achieve high solubility, the DNA and protein molecules are used to functionalize graphene as the charges of these biomolecules in the presence of water stabilize the graphene sheets [129,130]. The molecular functionalization improves not only the solubility but also its optical efficiency. A better optical-limiting effect was observed using porphyrin functionalized graphene as compared to the criterion optical-limiting material of C60 [131]. The electron/energy transport that exists between the molecules of porphyrin and graphene was partly responsible for this amazing optical-limiting effect. In another work, the non-covalent bonds and interactions that exist between dendronized perylene bisimides and graphene were accomplished in a homogeneous solution, but the fluorescence of dendronized perylene bisimides was demolished as a result of the above interaction [132]. The characteristics associated with biological molecules tend to be changed by functionalization with graphene. Specifically, the biomolecular functionalization of graphene tends to modify the bio-reactiveness while forming nanohybrids and nanocomposites. For instance, the single-strand DNA forming fusion with graphene has a strong selectivity with the harmonizing DNA down to a single base mismatch [133]. In another work, the non-covalent authoritative and electronic intelligence between dendronized perylene bisimides and graphene was accomplished in a homogeneous arrangement, and the fluorescence of dendronized perylene bisimides was extinguished because of the formation of intelligent-hybrid composite structures [132]. The functionalization of graphene with that of organic molecules and polymers changes the bio-related properties of graphene.

4.2 Functionalization with nanoscale objects

The functionalization of graphene heterostructures and nanocomposites with various nanoobjects, such as nanoparticles (NPs), nanowires, nanorods, and NSs, is another kind of functionalization that is often done. Based on the intrinsic properties of materials, the nanoobjects are endowed with various capabilities in contrast to pristine graphene. The nanoobject-based graphene nanocomposites particularly combine specific optical and electronic properties to advance the possibilities of graphene in numerous electronic and optoelectronic operations. Some outstanding optical characteristics are exhibited by the semiconductor nanoobjects that can overcome the low-absorption behavior of pristine graphene. For instance, the semiconductor CdS QDs/NPs have increased the photo-absorption along with photoelectrical responses appreciably when combined with graphene to form nanocomposites [134,135]. Likewise, the ZnO nanowires/nanorod semiconductors that are UV active possess a wider bandgap that can improve the UV response of graphene when combined with graphene/ZnO heterostructures [136]. Likewise, the TiO₂ NPs confirmed superior photocatalytic and photoelectrical activity when combined with graphene to form nanocomposites [137]. Furthermore, in electrical and electrochemical aspects, the graphene/nanoobjects exhibit outstanding activity; one study observed that the MoS₂ NSs/graphene nanocomposites showcased better electrocatalytic characteristics than did pure MoS₂ in a hydrogen evolution reaction [138]. Also, in studies related to electrochemical capacitors, Ni(OH)₂, the NSs/graphene nanocomposites confirmed superior performance [139]. Similarly, the Co₃O₄ NPs/graphene nanocomposites exhibited much better oxygen reduction than did a C/Pt catalyst, although this activity was scaled down in pure Co₃O₄ NPs [140]. The metal NPs, exclusively Pt-functionalized ones with graphene, exhibit an exceptional electrocatalytic activity during methanol oxidization [141]. Figure 14 shows the functionalization of graphene with nanoscale objects by Shahid et al. [142].

Figure 14

A schematic representation of functionalized graphene with Co₃O₄ nanocubes and Au NPs.

4.3 Functionalization with polymers

An additional functional species extensively used to functionalize graphene is the polymer to produce graphene-based polymer composites. Generally, polymers resembling small molecules have also been functionalized to increase the solubility of graphene. The amphiphilic conjugated triblock copolymer has been functionalized to increase the solubility of graphene in a variety of organic and inorganic solvents [143]. Sulfonated polyaniline and amine-terminated polystyrene analogous to amine-terminated polymers were initiated to be very successful in enriching the solubility equally in organic solvents and water [144,145]. Exclusively in photonic and ultrafast photonic devices, graphene/polymer nanocomposites have worked well [77,146]. Besides, the optical absorption of the poly(vinyl acetate)/graphene nanocomposites was enlarged ten-fold times using only 0.07 wt% inclusion of graphene [146]. Also, some comparable photonic effects were monitored using polyvinyl alcohol/graphene nanocomposites in mode-locked ultrafast lasers [77]. Similarly, poly(3,4-ethylene dioxythiophene) (PEDOT)/graphene nanocomposites have also increased the photoelectrical activity with an appropriate percentage of graphene in the nanocomposite anode used as the LEDs [147]. Figure 15 shows the functionalization of GO by using functionalization of graphene with polymers [148].

Figure 15

Synthetic routes for achieving controllable functionalization of graphene [148].

5.1 Photovoltaic cells

Yin et al. [141] built transparent and conductive electrodes to be used for flexible OPV appliances (schematically represented in Figure 17) by coating the chemically synthesized rGO over polyethylene terephthalate substrates. The efficiency of OPV appliance chiefly relies on the charge-transport effectiveness when the optical transmittance of rGO is higher than 65% [142]. On the other hand, for lesser transmittance, the efficiency of OPV appliances is controlled by the light-transmission efficiency, i.e., the transparency of rGO films [150]. Following the application of about a 2.9% tensile strain on the fabricated OPV device, the device can sustain for thousands of bending cycles [151], thereby demonstrating for the highly flexible properties of rGO films, and thus indicating the potential applications of these materials toward flexible optoelectronics applications.

Figure 17

Schematic representation of (a) layer structure and (b) energy level for the OPV device, which is rGO/PEDOT:PSS/P3HT:PCBM/TiO₂/Al, with rGO as the transparent electrode [141]. PSS: poly (styrene sulfonate).

A study by Şahin et al. [152] synthesized GO using the Tour method and further tailored it with various amine sources (dihexylamine [DHA] and 2-ethylhexylamine [2EHA]) involving various branched alkyl chains. The GOs and modified GOs (mGOs) were used to investigate the efficiency of perovskite solar cells by using them as a buffering layer. The mechanism of formation (represented in Figure 18) shows the customized solar cells with the supplementary coating of mGO derivatives sandwiched between the perovskite and hole-transporting film exhibiting the overall efficiency better than that of the reference cell [152]. This improved performance may have resulted from the improved carrier transport via the mGOs. To be specific, in general, there was a 10% overall performance boost of the solar cells in solar-cell appliances whenever GO customized with 2EHA was used, as compared to the standard cells used devoid of the modified buffer layer [152].

Figure 18

Schematic of the mechanism proposed for the binding of DHA and 2EHA onto the GO.

It can be seen from the investigation that the mGOs, when functionalized as a buffer layer in the mixed halide CH₃NH₃PbI_3−x Cl _x mesoporous perovskite solar cells, exhibited better characteristics than did with the reference appliance [153]. This improvement in characteristics using the mGO coating resulted from the arrangement of a homogeneous porosity observed in mGO film that possibly made way for the holes transport and thereby assisted the reducing charge recombination. Moreover, the hydrophobic behavior exhibited by mGO derivatives proved to be an additional shield to perovskite films that protected them from humidity, air, and therefore escalated the stability of manufactured PSCs. Table 1 summarizes the various parameters that are suitable for photovoltaic applications by making use of functionalized graphene nanocomposites. From the comparison of data, it can be observed that the Fe₃O₄–rGO composite has the superior photocurrent, followed by the CoS/rGO, GO/SnO₂, and so on, and this suggests the importance of incorporating graphene with other metal oxide NPs to enhance the photocurrent in solar cells [154–156].

5.2 Electrochemical sensors

Taking into consideration of the fast-moving ability of graphene-based composites, Eshlaghi et al. [162] generated a sensor by integrating the GO surface [GO-imi-(CH₂)₂-NH₂] with the well-designed imidazole-(CH₂)₂-NH₂ group. This increases the surface area and conductivity of the composite that can be used for reasonable and sensitive recognition of Pb(ii) in aqueous media (the whole synthesis route is provided in Figure 19). The characterization of the composite cleared the favorable synthesis strategy of the GO-imi-(CH₂)₂-NH₂-CPE (conductive polymer electrode) grafting functional surface that can be used as a dependable Pb(ii) sensor. Depending on the ideal conditions, the Pb(ii) calibration diagram recorded linearly (−0.58 V) within the concentration range of 5.0–300.0 nM, and the high surface area and the solid Pb(ii) adsorption capability with the good conductivity of the changed electrode suggested a reasonable Pb(ii) detection limit of 0.30 nM. The electrode provided ease –NH₂ groups incorporating Pb(ii) with good reusability up to five times and reproducibility of approximately 90%, which is satisfactory for electrochemical sensors.

Figure 19

Schematic of synthesis process for the functionalized (GO-imi-(CH₂)₂-NH₂).

Similarly, Wei et al. [163] used an rGO/SnO₂/Au composite for the electrochemical sensing of formaldehyde (HCHO), and its characterization revealed the formation of unique SnO₂ NSs decorated with Au NPs that are homogeneously attached to the rGO’s surface. On testing, the gas-sensing analysis proved that the incorporation of SnO₂ NSs with Au NPs and rGO improved the gas-sensing for HCHO in terms of lower operating temperature, high sensor response, and good selectivity. The improved sensing properties could mainly be attributed to the synergism of the ohmic contact between rGO and SnO₂ NSs, high surface area, and strong gas-adsorption capacity of sheet-on-sheet heterostructure architectures and the catalytic effect of Au NPs. Finally, the work suggests that the rational design of 0D noble-metal NPs, 2D metal oxide NSs, and 2D rGO form ternary composites that provided an opportunity for the achievement of high-performance sensing materials.

5.3 Light-emitting and conducting diodes

For exploring the LED properties of GO-based composites, Wang et al. [164] with the use of spray coating technique fabricated a flexible TCF having the morphologies similar to reinforced concrete, i.e., sandwiched the single-walled carbon nanotubes (SWCNT) between PEDOT:PSS and polydopamine-functionalized rGO (PDA-rGO), and the formed structure is expected to have enriched performance in the LED devices (the whole structure of final composite is schematically represented in Figure 20). Mussel-inspired PDA was popularized as a GO-reducing agent and modifier since the produced PDA–rGO was found to increase the interfacial bonding of sandwiching between the conductive coating layers and the substrate surface, which is a successful post-synthesis technique used for the hybrid film to attain improved conductivity. The electrode prepared by this method reported a low sheet resistance of 52.2 Ω sq⁻¹ along with improved optical transmittance of 88.7% at 550 nm. Moreover, these transparent films displayed long-standing stability with comparatively low roughness (about 2.41 nm), and its architecture maintained its flexibility throughout its bending course.

Figure 20

Schematic illustration of TCFs with ultra-adhesion, relatively low roughness, and excellent electrical conductivity.

In a similar study, the composite film using PDA-rGO/SWCNT/PEDOT:PSS is formed to have applications as organic LEDs, where the device luminance is measured to be 2,032 cd cm⁻² at 15 V, the maximum current efficiency of 2.13 cd A⁻¹ at 14 V, thereby confirming for the presence of strong potentials that are especially applicable in flexible electroluminescent and photoelectric devices. Furthermore, Table 2 provides information about the various parameters and applicability of nanographene and its composites for the LED devices.

5.4 Optical and fiber-based biosensor probes

To test the efficacy of graphene-based composite biosensor probes, Xu et al. [170] promoted a label-free glucose biosensor that was effectively established and planted based on the long-period fiber grating (LPFG) functionalized GO-glucose oxidase (GOD) by means of chemical crosslinking (schematic of the biosensor construction is provided in Figure 21). Thus, the synthesized GO-coated LPFG surface is an ideal candidate for chemical and optical fiber sensing, because of the sensor’s excellent operational environment, which can immobilize GOD by copious binding sites because of the constructive grouping of its remarkably large surface-to-volume ratio. It was found that gluconic acid and H₂O₂ were produced not only because of the major differences in the surrounding refractive index but also because of the reaction between GOD and glucose that leads to an obvious shift in the LPFG transmission spectrum. The optimum conditions of the sensory system include a 4 mg mL⁻¹ GOD concentration and a solution pH of 7, where the sensor has an excellent linear response even in the 0–1.2 mg mL⁻¹ low concentration range with a sensitivity of ∼0.77 nm mg⁻¹ mL⁻¹. At the same time, the sensor showed a faster and shorter response time of 2.16 s. Furthermore, based on the selectivity and sensitivity of the developed detector, it can be confirmed that the developed biosensor can be used to detect practical samples in the pharmaceutical research and medical diagnosis fields.

Figure 21

Schematic illustration of a fiber-optic biosensor based on GO-coated LPFG: FPLG surface with hydroxylation treatment, GO deposition, EDC/NHS-treated, and GOD-immobilized LPFG.

In a different study [171], the GO–ZnS (GOZS) nanocomposite was successfully prepared by a solvothermal method (synthesis route for the composite formation is provided in Figure 22), where the characterization indicated for the even distribution of ZnS particles (20 nm diameter) onto the surface of GO that supported for an increase in the nonlinear absorption and optical limiting (OL) action of GOZS under ultrafast (1,030 nm, 340 fs) excitation. The calculated theoretical values of the nonlinear coefficients (at 20.8 GW cm⁻²) of GO, ZnS, GOZS (4:5), GOZS (2:5), and GOZS (1:5) are 27.6, 25.4, 554.9, 193.3, and 117.1 cm GW⁻¹, respectively. Based on the Z-scan open-aperture investigation, the nonlinear activity elevated nearly 5- to 22-fold in GOZS as compared against pure GO and ZnS and is a result of the three-photon absorption of the sample. Moreover, the homogeneous delivery of ZnS NPs on the GO’s surface increased their surface area as well as their dispersibility and consequently increased its light-absorption capacity. Furthermore, the conjugated structure of GO along with the 2D π-electron permits the composite to generate additional delocalized electrons. An improved absorption of free carriers and nonlinear activity was experienced, because the electron transport between GO layers and ZnS NPs inhibited the recombination of free carriers, as well as all these parameters, reinforcing the ability to organize it as an optical-limiting material.

Figure 22

Schematic of the GOZS synthesis procedure starting from GO with the addition of zinc compound, followed by the heating and filtration to finally produce the composite in black colored powder.

5.5 Optical limiters

To test the efficacy of the charge transfer approach on optical properties, the three kinds of porphyrin–GO nanohybrids developed from the covalent functionalization method by Chen et al. [172] such as TPP-GO, TTP-GO, and TPP-GO-TTP are investigated to possess superior NLO absorption and OL characteristics. The effective charge transfer from the porphyrins to the nanohybrids of GO (schematic of its structural formation is provided in Figure 23) was confirmed by the Raman spectroscopy and other photophysical measurements. Also, within the three GO derivatives tested, the ternary hybrid of TPP-GO-TTP has exhibited superior NLO properties as compared to the other two and thereby emphasizing the importance of fine-tuning the photophysical and optical properties of nanohybrids through the introduction of porphyrins. Furthermore, the results generated from this study are potentially useful for the designing and development of various GO-mediated porphyrin derivatives that have applications in the NLO devices.

Figure 23

Synthetic route for the formation of TPP-GO-TTP nanohybrids.

In a similar study [173], the nanocomposite of (3-aminopropyl)triethoxysilane (APTES) functionalized multilayer GO and its incorporation within the organosiloxane matrices formed by the sol–gel method (sequential synthesis is shown in Figure 24) are observed to maintain better NLO and optical-limiting properties than that of their GO precursor. Such an observed activity of GO–organosiloxane composite than that of pure multilayer GO can be linked to the occurrence of effective crosslinking among the two components that supported the generation of thermal microplasmas.

Figure 24

Schematic diagrams for the preparation of GO functionalized with APTES incorporated in an organosiloxane matrix (f-GO/sol) [173].

Graphene could be used as an EL material in nanoscale field-effect LED with tunable emission color [174]. Wang and co-workers have demonstrated a bright spectrally tunable EL ranging from blue (450 nm) to red (750 nm) at a reduced GO-based field-effect LED (GFLED) in Figure 25 [175]. They explained that the EL was caused by the recombination of Poole–Frenkel emission ionized electrons at the localized energy levels arising from semi-reduced GO and holes from the top of π band (Figure 25).

Figure 25

Scheme for the charge injection process of GFLED.

5.6 Optical frequency converters

To test the influence of polarons and bipolar, and toward the optical frequency of nanocomposites having the grains separated by insulating regions, Dey et al. [176] studied the electrical transport properties of GO and n-PANI (polyaniline) composites (sequence of synthesis is shown in Figure 26). For the formation of GO–PANI composites, oxidative polymerization of aniline monomer with that of different weight percentages of GO was utilized. On testing the as-synthesized nanocomposite’s electron transport properties against the variation of conductivity with those of temperature T and V–I characteristics, the DC conductance at Σ indicated a behavioral change from the insulator-type to low-temperature-dependent behavior (at temperature T _D). The V–I characteristics, in general, are nonlinear, and such nonlinear property seems to be increased with that of decreased temperature. For the studies where the temperatures are greater than T _D (T ≥ T _D), the high voltage values were observed at higher currents, close to the Zener diodes. However, for the studies of lower temperatures (T ≤ T _D), the maximum voltages observed as similar to that of thyristors and so can be utilized for the manufacturing of devices that can exhibit the controlled electrical and insulating properties. From the cumulative analysis, two different phases were noticed for the nanocomposites over the measured temperature range, i.e., an insulating-type of behavior (from room temperature to T _D) and a low-temperature-dependent behavior (below T _D down to 10 K). Also, it was observed that T _D is dependent on GO’s weight percentages, and the formed nanocomposite’s V–I characteristics have exhibited non-ohmic electrical conduction and is characterized by the onset voltage (V ₀), which scales with the ohmic conductance (Σ₀) and nonlinearity exponent (x _T ). The weight percentages of GO are investigated to have negative x _T values and further, all the observed results were explained based on the nanocomposite’s grain structure.

Figure 26

Various steps are involved in the preparation of n-PANI–GO composites along with the representation of both cross-sectional and planar views of the intermediate structures [176].

5.7 Transparent conductors

In another study conducted by Compton and his co-workers [177], several conductive “alkylated” graphene sheets were synthesized by a one‐pot process by modifying the synthesized GO layers with hexylamine (HA) followed by a hydrazine reduction reaction (synthesis for the bonding of alkylated aniline with that of GO is represented in Figure 27). The “alkylation process” with HA not only preserved the stacked-layer arrangement throughout the reduction course but also maintained its well-organized morphology as well as ensuring its consistent conductivity and excellent mechanical properties. The altered amine–alkylate-stacked GO layers produced a stable organization, because of the precise hydrazine-reduction environment. Thus, the resulting “alkylated” graphene sheet had a well-organized morphology along with uniform conductivity, as associated with graphene sheets synthesized without the influence of amines. Therefore, these chemical modification techniques can be used as a smart approach for designing favorable blends and structures with desired characteristics in co-operated graphene sheets. Nevertheless, these “alkylated” graphene sheets can be used as novel materials for various applications. As an example, it is preferred to have the biocompatible material consisting of large pores or integral spaces, which can support the encapsulation of enzymes and the growth of cells. Also, integral functionalization can transform graphene paper into a high-capacity, well-ordered ion-storage medium for supercapacitors and LiBs. Table 3 shows the transparent and electron-conducting efficiency of various kinds of functionalized graphene.

Figure 27

Scheme of amine bonding in HA–GO and HA–graphene papers.

5.8 Terahertz devices

The terahertz electromagnetic interference (EMI) materials were constructed by Lin et al. [183] in an ion-diffusion gelation method with the cross-linking of GO and MXene sheets (2D transition metal carbides and/or nitrides) multivalent metal ions (synthesis scheme and physicochemical characterizations are represented in Figure 28). Thus, formed sheets having porous structures have effectively reduced radiation pollution by serving as a shield. The testing has shown that the MXene foam material having the cross-linked porous structure was promising for the EMI shielding and simultaneously suitable because of its lightweight, high stability, foldable nature in wet environments, and excellent terahertz shielding ability of 51 dB even at a narrow thickness of 85 μm. This study results for the first time have provided an insight into the usage of 3D macroscopic MXenes as high-performance terahertz shielding materials.

Figure 28

(a) Schematic representation of ion-diffusion-induced gelation process, (b) free-standing and flexible Zn²⁺ MXene IV hydrogel and foam, (c) cross-sectional scanning electron microscopy, (d) energy-dispersive X-ray spectroscopic elemental mapping, (e) high-angle annular dark-field scanning transmission electron microscopy images of the Zn²⁺ MXene IV foam, (f) Ti K-edge X-ray absorption near-edge structure, (g) Zn K-edge, and (h) Fourier transform extended X-ray absorption fine-structure spectra of the Zn²⁺ MXene IV foam and references. Scale bars: (c) 50 μm, (d) 5 μm, (e) 2 nm [183].

In a similar study to test the efficacy of organic materials in LiBs, Li et al. [184] identified that the electrode materials made from graphene and CNTs (C₆₀(OH)₁₂) have possessed a high specific capacity that can promise for an increase of energy efficiency. For the studies, the CNTs/GO anode materials were formed by the uniform growth of CNTs onto the surfaces of GO NSs (bonding of CNTs carboxyl and epoxy groups). The synthesis route along with the morphological analysis are presented in Figure 29. On testing, the CNTs/GO anode material is found to have superior battery performance by having the reversible capacity of 1,596 mA h g^–1 at 0.2 A g^–1, which is higher than that of the individual capacities of CNTs and GO, thereby confirming the potential applicability of CNTs/GO nanocomposite for the high-performance LiBs and other conducting applications.

Figure 29

(a) Synthetic route for the active material of the LIB anode: the C₆₀(OH)₁₂/GO composite. High-resolution TEM images of (b) GO, (c) C₆₀(OH)₁₂ NPs, and (d) the C₆₀(OH)₁₂/GO composite; the d-spacing is found by estimating the distance between the two white lines. (e) Schematic diagram showing the C₆₀(OH)₁₂/GO composite with C₆₀ molecules embedded between GO layers [184].

5.9 Photodetectors

Li et al. [185] employed a one-pot hydrothermal method for the synthesis of rGO that was additionally used to prepare a nanocomposite consisting of hemin via π interactions. Following that, the hemin–rGO nanocomposite was incorporated into the Nafion film and then placed on a glassy carbon electrode (GCE). Thus, modified GCE on testing showed exceptional catalytic properties toward H₂O₂, and such activity can be linked to the presence of hemin’s redox-active center [Fe(iii/ii)] and foam’s cross-linked macroporous structure. Furthermore, on testing, the electrode has exhibited a typical working voltage of −0.41 mV (vs SCE), sensitivity (detection limit of H₂O₂) of 2.8 nM, and for the concentration in the range of 5 nM to 5 mM, a sensitivity of 50.5 μA μM⁻¹ cm⁻² was observed. From the analysis, the authors have confirmed that the modified GCE electrode has a fast response and is highly sensitive and thus so can be used for the qualitative and quantitative analysis of H₂O₂ samples.

Similarly, transition metal composites were formed using the sonochemical technique, i.e., Palanisamy et al. [186] prepared CuS nanoplates partially decorated with an rGO (PrGO) nanocomposite (sonication frequency of 37 kHz, power of 150 W). The synthesis scheme and reaction conditions along with the microstructure of the product are shown in Figure 30. On testing the electroanalytical performance of dopamine neurotransmitters, the PrGO–CuS nanocomposite was investigated to have far better activity than that of the corresponding components of GO, CuS, and GO–CuS composites individually. With the use of the PrGO–CuS electrode sensor, dopamine was identified even to a lower detection limit of 0.022 μM with a more extensive linear response range of 0.1–155 μM, and a quantitation limit of 2.6 nM. Thus, from the cumulative analysis of results, the fabricated electrode of PrGO–CuS nanocomposite has superior selectivity with acceptable storage and cyclic stability, and therefore, can be applied for the successful detection of dopamine in any kind of samples. The summary of functionalized graphene nanocomposites as photodetectors is provided in Table 4.

Figure 30

Schematic illustration of the sonochemical preparation of CuS and PrGO–CuS nanocomposite (189).