A Perspective on the state-of-the-art functionalized 2D materials Free

Functionalization is a technique of modifying the surface chemistry of a material, which offers various functions, abilities, and new properties to the material. The technique has been used throughout many fields, including chemistry, physics, materials science, molecular biology, and nanotechnology. Functionalization enables either chemical reaction or adsorption of the functional chemicals onto the surface of a material.

Functionalization techniques have been performed in ancient times in various areas and at different eras to decorate materials, either by plating or by applying surface treatments.¹ The process contains chemical baths, the use of different powders and lacquer combined with minerals, surface coating, inventive processes, and materials to create the appearance of higher-quality materials and adapted to specific requirements.¹ For instance, covering silver was already employed via diffusing bonding to hide the rivets on minor luxury items of individual usage, including cosmetic scrapers and tweezers (third and second millennium BC).² The Mycenaean long swords (1400–1300 BC) are another good example.¹ They were made of gold-plated steel and used in ancient battles. In addition, functionalization can be thought of as any physical and behavioral characteristic of an animal that aids in its survival ecosystem. Stripped fur is a kind of unique adaptation. In most circumstances, it facilitates animals accommodating to their surroundings. This benefits the animal in a variety of ways that involve concealing it from predators and creeping up on prey. Considering such things, functionalization techniques are widely used in a wide area of nature and in different disciplines.

Consequently, following the industrial revolution in a short time, first, micro-technology and then nanotechnology are developing rapidly nowadays. To keep up with this development, materials research at the nanoscale requires the exploration of new materials. It has been known to mankind since ancient times. Functionalizing techniques can be used by adapting them for nanotechnology.

Since the mechanical exfoliation of graphene from its bulk form, graphite, in 2004, 2D materials have attracted a great deal of interest owing to advances in experimental techniques of their demonstrations and their use in innovative nanodevices.³ A wide variety of 2D materials, such as monoatomic buckled crystals of black phosphorus (bP) and transition metal dichalcogenides (TMDs) with various possible phases, have been considered in materials science and physics. Such atomically thin 2D materials exhibit strong in-plane covalent bonding and weak van der Waals interlayer interactions that have a crucial impact on their properties as the dimensional reduction from few-layers to ultra-thin form is achieved.⁴ The quantum confinement of electrons in the out-of-plane direction has the effect in tailoring the optical and electronic properties of the materials.⁴ In addition, 2D materials provide a good platform through their large surfaces by means of the chemical functionalization, which plays a crucial role in tuning their properties. Such fascinating characteristics combined with the experimental functionalization techniques open up new avenues involving the seeking for novel 2D materials, the improvement of new techniques to grow 2D materials with high quality, and the investigation of effective methods to increase the long-term durability of the materials as well as their use in a wide variety of state-of-the-art device applications.

Since its successful synthesis, graphene has been paid a great deal of attention due to its extraordinary electrical and thermal conductivity, elasticity and high strength, and optical transparency.^3,5 Graphene is still known to be the thinnest (0.34 nm of thickness) and the strongest material ever known.⁶ Its combined fascinating properties make graphene a potential candidate for various application areas, such as desalination,⁷ healthcare,^8,9 energy storage,¹⁰ drug delivery,¹¹ solar cells,¹² and flexible electronics.¹³ The experimental observations and theoretical predictions on the properties of graphene revealed a room-temperature electron mobility of $2.5× 10 5$ cm $2$ V $− 1$ s $− 1$¹⁴ and a thermal conductivity of 3000 W mK $− 1$⁠,¹⁵ which are considerably high. Furthermore, 1 TPa of Young’s modulus and 130 GPa of intrinsic strength indicate the significant mechanical properties of graphene.¹⁶ In addition, the optical absorption of $πα=2.3$%,¹⁷ being impermeable to gases,¹⁸ exhibiting the capacity of carrying high electric current densities,¹⁹ and easy functionalization,^20–22 makes graphene to be still attractive for future applications.

Although recent experimental advances allowed researchers to demonstrate high-quality large-area graphene sheets, some of those techniques end up with relatively defective graphene samples. Such defect areas are suitable for the grabbing of either single atoms or small-size molecules. Graphene oxide (GO) is an important and widely used example of such graphene sheets, which contains different chemical groups, such as H, H $2$⁠, OH, and O attached to the defect sites of the graphene layer. The oxidation degree and the thickness are the two main factors determining the properties of GO.^23,24 For example, GO exhibits mechanically tunable Young’s modulus (varying from 380 to 740 GPa) depending on the coverage of oxygen groups.²³ In addition, upon bending, GO was reported to show good flexibility, which is almost $∼100$ times higher than that of graphene.²⁵ Contrary to semimetallicity of graphene, GOs usually exhibit large electronic bandgaps, which makes them to be classified as insulators. The main reason for the tuning of the electronic features of graphene is having a high sheet resistance (⁠ $> 1010Ω$ per square) and is the existence of a large number of oxidized sp $3$ hybridized C atoms. When utilizing a graphene derivative, one should mind the distinction between Ohmic and Schottky contacts. Ohmic contacts provide benefits in applications such as sensors and transistors, since they supply the system with rapid charge transport. On the other hand, Schottky contacts usually utilized in diode or photovoltaic applications for their current rectifying properties. Engineering the contacts carefully leads to a much optimized system in an appropriate application. Electronically, the properties of GOs are significantly determined by both the oxidation degree and the types of functional groups that are attached to the GO surface.²⁶ Theoretical predictions reported that, based on the coverage amount, positioning, and proportion of hydroxyls and epoxies, the bandgap of GO varies from 0 to 4 eV.^27,28 Moreover, GO exhibits intrinsic fluorescence as well as other non-linear optical properties, which make it more useful for optical applications as compared to graphene.^29,30 Furthermore, GO shows decent optical transmittance, which heavily relies on the thickness of its structure.³¹ Since oxygen groups reduce the thermal conductivity of GOs, the reduced thermal conductivity makes GOs as suitable candidates for thermoelectric applications.^32,33 Such extraordinary properties of GOs can be further improved since having oxygen-bearing functional groups on the surface increases the functionalization ability of GO.^34–36 Below, various application areas of both graphene and GO structures are discussed.

As the leading ultra-thin material of 2D family, graphene has been widely considered in sensor applications. As graphene and graphene oxide exhibit high electrical conductivity, high surface/volume ratio, and good sensitivity for polar functional groups, it is inferred that these materials are fit for sensor and device applications. Specific receptors can be incorporated onto GO, by functionalizing chemically by utilizing its polar functional groups, making it more selective and effective as the sensor surface. Many experimental procedures have been considered for the chemical functionalization of the graphene surface in order to improve its sensing mechanisms against the various chemicals. For example, González et al. used lithography and a self-limiting variant of diazonium chemistry to covalently modify the basal plane of the graphene layer by using three different chemicals, namely, carboxyphenyl, nitrophenyl, and bromophenyl (see Fig. 1). Such an easy and effective method can be used to produce chemically patterned graphene sheets with versatile properties, making it highly desirable for sensing or device applications by selectively increasing the active sites of the graphene’s basal plane. Such methodology can be useful for designing variable sensors based on functionalized graphene.³⁷ 

Since heavy metal ion pollution has become a vital problem, their detection is important. For such a purpose, an electrochemical sensor was created from hydrosulphonyl functional covalent organic framework (COF-SH) and graphene. Graphene was used as a conductive material, while COF was used to attach heavy metals. The prepared sensor was demonstrated to detect heavy metal ions in coastal water samples at concentrations ranging from 1 to 1000  $μ$g l $− 1$⁠. Moreover, the sensor displayed good recovery and stability even after multiple analysis.³⁸ Interdigitated electrodes (IDEs) are commonly used in gas sensing mechanisms; however, their fabrication is quite complex. Yang et al. proposed a gas sensing mechanism using a highly porous laser-induced graphene (LIG) structure. A sensing region and a serpentine interconnect region were combined to create the LIG sensing platform. Self-heating was provided by a thin film of Ag coated in the serpentine interconnect region as localized Joule healing occurring as coating reduced the resistance of the structure. The production of LIG is shown in Fig. 2. The mechanical roboustness of the sensor was protected over a tensile strain of 20%, thanks to the stretchable nature of serpentine interconnect region. The demonstrated gas sensor showed the fast response and recovery process while having significant selectivity and low detection limit (below 1 ppb) toward NO $2$⁠.³⁹ 

Ultrasensitive detection of DNA and RNA molecules was achieved with a deformed graphene-based FET biosensor.⁴⁰ The cancer-related miRNA let-7b sequence was used in buffer and in human serum as analytes to test the nucleic acid adsorption on an electrode. The detection limit was found to be 600 zM, which corresponds to 18 nucleic acid molecules. Molecular dynamics simulations were also performed to show the deformations created electrical hot spots, thus enhancing sensitivity.⁴⁰ In another study, in order to detect non-structural 1 (NS1) protein, the surface of graphene oxide was functionalized with opto-electrochemically active ruthenium bipyridine complex [Ru(II)].⁴¹ According to ultraviolet photoelectron spectral readouts, intermolecular bonding between GO and Ru(II) changed work function and ionization energy of GO, which tunes the success of functionalization. The structure that resulted was shown to exhibit a linear response in both chronoamperometric and fluorescence quenching-based immunoassays, with detection limits of 0.38 and 0.48 ng ml $− 1$⁠, respectively. In addition, interferents did not affect the sensitivity of the GO-Ru(II) immunosensor toward NS1.⁴¹ 

Biocompatibility and high surface/volume ratio of both graphene and graphene oxide provide availability for their use in drug delivery, imaging, and biosensor applications. By functionalizing graphene and GO with biomolecules (e.g., proteins, DNA), their nature can be tailored into interacting more readily with targeted drugs and enabling better imaging in cells. As a promising material for sensor applications, graphene has also been considered for biosensing applications. Hajian et al. developed a graphene-based field-effect transistor (FET) in order to detect the desired nucleic acid sequence using clustered regularly interspaced short palindromic repeats (CRISPR) technology.⁴² The deactivated CaS9-CRISPR was used to functionalize graphene in order to combine its sensitivity with the gene targeting capacity of CRISPR-CaS9, resulting in an inexpensive, simple, and selective detection of target genes.⁴² In another study, boronic acid-functionalized graphene sheets (see Fig. 3) were synthesized in order to investigate the interaction of the sheets with bacteria and nematodes.⁴³ Boronic acid was chosen due to its affinity toward biosystems containing cis-diol. A 70% decrease in the viability of nematodes was observed when nematodes were incubated with functionalized graphene sheets for a day. It was also reported that demonstrated sheets have the ability to heal diabetic wounds relatively better than commercially available drugs. Lou et al. synthesized magnetic graphene nanocomposites by aniline polymerization as an initiator.⁴⁴ After that, laccase was then immobilized with these composites to create biosensors. In order to test the electrical properties, cyclic voltammetry and chronoamperometry were applied and it was found that the polyaniline/magnetic graphene composite has high sensitivity, detection limit, and linear range toward hydroquinone. Mohanraj et al. developed a low-cost paper-based double-stranded DNA sensor.⁴⁵ The electrochemical exfoliation was used to obtain high-quality graphene nanosheets from corn cob. Then, the sheets were used to prepare conductive ink to print a paper-based graphene electrode. Electrochemical analysis of the analyte reveals two irreversible oxide peaks obtained from paper-based printed graphene electrode, and the observed peaks correspond to the oxidation of guanine and adenine in dsDNA. A 3D printed graphene/polylactic acid (G-PLA) electrode was proposed by Cardoso et al. for the analysis of biological fluid.⁴⁶ The oxygenated group from polylactic acid enables enzyme immobilization by cross-linking with glutaraldehyde for a biosensor construction to detect glucose in blood plasma. A limit of detection of 15  $μ$mol l $− 1$ was achieved by the biosensor. Also, surface treatment resulted in enhanced electrical properties, thus making the direct detection of nitrite and uric acid possible. Another study demonstrated the modification of the porous laser-induced graphene (LIG) surface with acetic acid using the dipping technique.⁴⁷ Such facile treatment increased the carbon–carbon bond ratio and conductivity while decreasing the sheet resistance. Pt nanoparticles (PtNPs) and chitosan-glucose oxidase composites were placed on the sheets to produce a sweat glucose biosensor. The resulting biosensor showed not only high sensitivity (4.6  $μ$A/mM) but also ultra-low limit of detection (less than 300 nM). The results are summarized in Fig. 4. Drug release by remote stimulation [for example, light or microwave (MW) radiation] of drug delivery systems is currently in demand. However, MW-responsive materials lying in hydrogels generally overheats quickly with the applied radiation. To overcome the overheating problems of MW-responsive hydrogels, a graphene-diaminotriazine (G-DAT) hybrid material was proposed by Castillo et al.⁴⁸ The hybrid hydrogels were prepared with mechano-chemical exfoliation. The resulting G-DAT hydrogel was responsive to MW irradiation of $ν=915$ MHz. Graphene was used as a heat-sink, which prevents overheating of the system; thus, this hybrid hydrogel can be used as a soft scaffold in hydrophobic drug delivery systems. On top of that, under acidic conditions, improved drug release was observed when it was triggered remotely by microwave irradiation. Since ion transportation is vital for biological systems and membrane-based technology, the surface charge of graphene oxide membranes was modified to control ion transportation. To achieve this, ionizable functional groups with several protonation/deprotonation were attached to the surface of graphene oxide sheets. GO membrane having highly charged surface ions with high-valent co-ions attracts ions of low-valent counterions, which showed noticeable ion rejection with high water permeance.⁴⁹ 

The integration of nanotechnology into daily life has become an important issue in order to utilize the advantages of ultrathin materials. Mechanical flexibility, high surface area, and safety of the graphene and GO in contact with human skin provide these materials to be used in wearable device technology as well. Zhou et al. prepared graphene sheets attached with octadecane loaded titanium dioxide nanocapsules (OTNs) and multi-branched polyurethane (PU) to obtain a self-healing flexible multifunctional film. The prepared film exhibited piezoresistive sensing, ultraviolet protection, and thermal insulating properties. Such properties make the OTN–graphene–PU film suitable for wearable electronics, artificial intelligence devices, and human–machine interaction.⁵⁰ Due to their sustainable power without an external power supply, self-powered stretchable triboelectric nanogenerators (S-TENGs) are becoming very popular. Lee et al. created a S-TENG touch sensor with the ability to fit the motion of the skin (see Fig. 5). Atomically thin graphene, polyethylene terephthalate, and polydimethylsiloxane were used as the electrode, substrate, and electrification layer, respectively. The structure has an auxetic mesh design in order to obtain the required stretchability. Advanced functions, i.e., the detection of touch sliding velocity and information input through the trajectory mode, were detected by the S-TENG touch sensor.⁵¹ To create lightweight and flexible material to be used in wearable electronics, graphene oxide was coated onto conductive silk fabrics. The thin-film coating of GO on silk fabrics was proven by the SEM results. The $β$-sheet nature of silk fabrics was preserved during the process, providing good mechanical properties to the structure. Sufficient resistivity (3.28 k $Ω$ cm $− 1$⁠) and conductivity (⁠ $3.06× 10 4$ S cm $− 1$⁠) make the silk fabric suitable for wearable electronics.⁵² The solgel method was applied to prepare graphene oxide/isobutyltriethoxysilane to create a waterproof composite emulsion. FTIR, XPS, SEM, and EDS were used to characterize the composite, while capillary water absorption and water contact angle experiments were performed to test the waterproof performance. A hydrophobic layer formation on the concrete surface was shown by SEM and EDS results, which indicates the waterproof effect.⁵³ 

By high carrier mobility and transparent conductivity, graphene has very high availability for optoelectronic devices. With GO, broadband absorption or even fluorescence can be achieved by introducing new band edges by functionalizing the surface of the material. To optimize the performance of these materials, the most convenient way of doing so would be forming new heterostructures or functionalizing with new functional groups that provide advantageous properties to the device. Selecting tetraphenylethylene monomers through photoelectric activity, carefully designed photosensitive 2D Carbon Organic Frameworks (COFs) having highly ordered donor–acceptor topologies have been in situ synthesized on graphene, forming COF-graphene heterostructures. Ultrasensitive photodetectors were fabricated with the COF-graphene heterostructure, and it was shown that the structure exhibited an excellent performance with a photoresponsivity of $3.21× 10 7$ AW $− 1$ at 473 nm and a time response of 1.14 ms. Moreover, due to the high surface area and the polarity selectivity of COFs, the photosensing properties of the photodetectors can be reversibly regulated by specific target molecules.⁵⁴ A novel flexible transparent electrode (TE) in a trilayer-stacked geometry and exhibiting high optoelectronic performance was fabricated by using the spin coating method as shown in Fig. 6. The trilayer structure is composed of a graphene film sandwiched between a transparent and colorless polyimide (TCPI) layer and a methanesulfonic acid (MSA)-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer containing dimethylsulfoxide and Zonyl fluorosurfactant (designated as MSA-PDZ film). Particularly, from multivariate and repetitive harsh environmental tests (⁠ $T =−50$ to 90  $°$C, over 90 RH%), the TCPI/Gr heterostructure was shown to exhibit almost excellent tolerance to mechanical and thermal stresses. Moreover, the gas barrier properties that protected the MSA-PDZ film from exposure to moisture were observed to be sufficiently good. Owing to the synergetic effect from the TCPI/Gr/MSA-PDZ anode structure, the TCPI/Gr/MSA-PDZ-based polymer light-emitting diodes displayed significantly improved current and power efficiencies with the values of 20.84 cd/A and 22.92 lm/W, respectively.⁵⁵ In another study, mixed-dimensional transparent conducting electrodes (TCEs) were fabricated using graphene and one-dimensional electrospun metal fiber. In comparison with other TCEs, the Ag-fiber/Gr hybrid electrodes were shown to exhibit a highly stable morphology (67% lower peak-to-valley ratio), high oxidation stability, low sheet resistance (approximately 11  $Ω$/sq), high transmittance (approximately 94%), also excellent flexibility, and outstanding chemical stability. Such multiple functionalities of the transparent and flexible hybrid structure highlight its potential for applications in electronics and highly stable optoelectronics.⁵⁶ 

While the lack of a bandgap in graphene limits its applications in optoelectronic devices, being highly transparent makes graphene attractive for photonic applications. Zhou et al. created a photodetector composed of graphene and carbon nanotubes (CNTs).⁵⁷ Side-polished optical fiber along with CNTs was deposited on single layer graphene in order to have a fiber integrated hybrid graphene/CNT photodetector. The interaction between the fiber and graphene was enhanced by CNTs; thus, high responsivity over the visible and infrared region was obtained from the photodetector. The photoresponsivity at 1550 nm was found to be $1.48× 10 5$ A W $− 1$⁠, which is six times higher compared to that of CNT-free photodetectors.⁵⁷ It tested the feasibility of the TiO $2$/GO composite for enhancing the photocatalytic degradation of MB in synthetic wastewater. By considering optimum conditions under UV-vis irradiation, the effects of photocatalyst dose, pH, and reaction time on MB removal were evaluated by the composites. In addition, TEM, SEM, XRD, FTIR, and BET analyses were used to demonstrate any changes in the physico-chemical properties before and after the treatment. The photodegradation pathways of the target pollutant by the composite and its removal mechanisms were explained. It was found that the same composite with a 1:2 wt ratio of GO/TiO $2$ possesses the largest surface area of 104.51 m $2$/g. Under optimum reactions (0.2 g/l of dose, pH 10, and 5 mg/l of pollutant’s concentration), an almost complete MB removal could be attained within 4 h, which is higher than that of pure TiO $2$ (30%) under the same conditions.⁵⁸ 

The storage of energy has been always the focus of interest in science and technology due to the disappearance of the energy sources. Graphene and GO are utilized due to their large surface area and exceptional conductivity, thereby enhancing the capabilities of batteries. Functionalization of graphene and GO improves their binding with the electrolytes, therefore enhancing the performance of the produced overall device. Mendoza et al. functionalized graphene with triruthenium acetate and characterized the structure with SEM, IR, Raman, and XPS.⁵⁹ For electrochemical characterization, cyclic voltammetry and discharge curves were applied and the results showed that the charge–discharge capacity of functionalized graphene is spectacular with a cycling retention over 98% after 5000 cycles. Compared to graphene, the capacity of triruthenium cluster-functionalized graphene (11 F/g) is 1.2 times higher at a specific discharge current of 0.25 A/g; however, at high current densities and scan rates, the capacity is limited by charge transport.⁵⁹ With the purpose of solving the current problems of the solid electrolyte interphase, (SEI) such as single functional, mechanical crushing, and peeling, LiF-intercalated-graphene SEI was proposed as anodic protection. The LiF-intercalated-graphene SEI exhibits an elastic modulus of 430 MPa and Young’s modulus of 20 GPa, which proves the mechanical strength. Batteries in which Li-anodes protected by LiF-intercalated-graphene SEI showed enormous electrochemical performance as the ultralow capacity decay rate was found 0.022% after 300 cycles at 1  $°$C and high discharge capacity was found to be 1092 mAh/g at 0.5  $°$C.⁶⁰ Since catalyst-based anion exchange membrane fuel cells (AEMFCs) require electrocatalysts with different structures than conventional structures to increase the efficiency of AEMFCs. Sharma et al. proposed the work function tailoring of graphene by encapsulating the Co nanoarchitecture.⁶¹ Both theoretical and experimental results indicated that Co nanoparticles affect the electronic structure of graphene shell and reduce the work function. Oxygen reduction reaction, catalytic activity, and durability were enhanced, and a high maximum power density of 412 mW/cm $2$ was produced when graphene shell-encapsulated Co nanoparticles were used as cathode catalysts.⁶¹ Platinum nanoparticle decorated graphene sheets (Pt@FGSs) were added to liquid hydrocarbon fuels and observation was 24% improvement in fuel conversation rates.⁶² In addition, in pyrolysis low-molecular-weight species produced with a better yield, enhanced hydrogen yield by a factor of 12.5. The results were also backed up with molecular dynamics simulations.⁶² NiCo $2$O $4$ microspheres embedded graphene oxide aerogel was used in both anode and cathode in order to manufacture high-performance and flexible Li–O $2$ batteries whose preparation and characterization are illustrated in Fig. 7. An ultrahigh capacity of 3398.4 mAh/g and volume expansion properties of the Li-anode were observed. Also, O $2$-cathode displayed spectacular energy efficiency. The two results were combined with Li–O $2$ batteries with amazing long-term cycling stability with highly reversible discharge/charge for more than 400 cycles.⁶³ In another study, Liu et al. functionalized graphene oxide with tetraethylenepentamine (TEPA) using the modified Hummer’s method to increase CO $2$ adsorption. They used ultrasound to exfoliate graphene oxide for increasing the available surface area for TEPA attachment. According to the analysis of textural properties, thermal stability, and elemental compositions of graphene oxide before and after the functionalization, binding of TEPA to GO was observed. For CO $2$ adsorption, TEPA activated GO showed more adsorption capacity (1.2 mmol/g) compared to pristine GO (0.3 mmol/g).⁶⁴ 

The oxygen-containing groups in GO serve as active sites where water molecules can reside on and grounds for subsequent catalytic reactions for water splitting. By introducing metal-oxides or -hydroxides, hydrophilicity of graphene derivative can be enhanced. In another strategy for the performance enhancement of graphene and graphene oxide, modifications with functionalizing molecules can be made in order to halt the carrier recombination to increase hydrogen or oxygen evolution rates. Restuccia et al. showed that structures having reactive edges increase the friction, which was demonstrated using graphene flakes between sliding diamond surfaces.⁶⁵ In addition, it was shown that due to carbon re-hybridization, spontaneously formed graphene under tribological conditions is highly reactive. Such an observation may offer a key for interpreting the dependence of friction on the number of layers of graphene. Both water and oxygen molecules were reported to be effective in extinguishing the reactivity of defects by dissociative chemisorption. However, humidity is more effective than oxygen at enabling the lubricity of graphitic media due to peculiar water molecule processes. Collective mechanisms like Grotthus-type proton diffusion amplified by confinement and the significant alteration in the hydrophilic nature of the passivated media are among them.⁶⁵ Liu et al. studied the development and testing of a novel composite membrane based on GO and MXene. Due to its two-dimensional interlayer channels and hydrophilicity, a composite membrane (⁠ $∼$550 nm) with a GO/MXene mass ratio of 1/4 exhibited much greater water flux (71.9 l m $− 2$ h $− 1$ bar $− 1$⁠) as compared to that of a GO membrane (6.5 l m $− 2$ h $− 1$ bar $− 1$⁠). The rejection of common small molecule organic dyes (NR, MB, CV, BB) was found to exceed 99.5%, and similar excellent removal efficiencies were found for two representative types of natural organic matter in raw waters (HA and BSA). Such superior water flux of the GO/MXene was attributed to the moderate increase in interlayer spacing of the membrane and also to the decrease in oxygen-containing functional groups.⁶⁶ In another study, Yao et al. performed a comparative study on the adsorption of typical antibiotics (namely, tetracycline and sulfadiazine) and heavy metals [Cu(II) and Zn(II)] onto GO. Interaction strength of contaminants toward GO was shown to follow the order of Cu(II) > Zn(II)  $≫$ tetracycline > sulfadiazine (coordination of heavy metal-GO in sp $3$ regions; $π$– $π$ stacking of antibiotic-GO in sp $2$ and H-bonding in sp $3$ regions). While heavy metals were shown to demonstrate the enhancement effect for the adsorption capacities (Qe) for antibiotics, antibiotics displayed slight promotion to Qe for heavy metals. Coexisting salt ions, especially Ca $2 +$⁠, inhibited the adsorption. Humic acid provided more sites for heavy metal uptake but competed with antibiotics for adsorption.⁶⁷ 

TMDs are a group of 2D materials having the formula of MX $2$⁠, where M stands for transition metal atoms namely, Mo, W, Nb, V, etc., while X stands for chalcogenide atoms, S, Se, or Te. Transition metal dichalcogenides form X–M–X bonds, creating tetragonal, hexagonal, and rhombohedral geometries with 1T, 2H, and 3R phases. The mentioned conformers and stacking orientations give rise to various optic and electronic properties of TMDs due to their charge distributions and interlayer interactions. For instance, the electronic characteristics of TMDs change as semiconducting or metallic, which are also used in different fields by functionalizing with various molecules or structures (see Fig. 8). Interlayer interactions between TMD layers are dominated by van der Waals interactions, which make the exfoliation and functionalization process feasible. Even though MoS $2$ conforms in the trigonal prismatic semiconducting 2H phase under standard conditions, in situ changes in the structure play a key role in the functionalization process. Chemical exfoliation involves initiating butyllithium (n-BuLi) intercalation between MoS $2$ layers, which leads to the exfoliation of the layers. On the other hand, this process converts structurally 2H-MoS $2$ to 1T-MoS $2$ due to the electron transfer from n-BuLi to MoS $2$⁠,⁶⁸ as confirmed by zeta-potential measurements, benefiting from changed charge values upon phase change.⁶⁹ Methodologies in the functionalization of TMD involve the usage of ce-1T-MoS $2$ (chemically exfoliated).⁶⁸ The authors devised a strategy where they incorporate iodo-organic compounds or diazonium salts⁷⁰ on chemically exfoliated MoS $2$⁠, WS $2$⁠, and MoSe $2$ nanosheets. It was also reported that for the MoSe $2$ and WS $2$ nanosheets, the yields of the functionalization of MoS $2$ are quite proximate for MoS $2$ functionalization, according to the XPS results. Another type of functionalization involves sulfur conjugation by benefiting from the sulfur vacancies on the basal plane of the TMD layer.⁷¹ On organic functionalization of MoS $2$⁠, while the covalent and ligand conjugation functionalization was usually carried out on 1T-MoS $2$⁠, coordination-based functionalization was demonstrated by utilizing either sulfur-edged molecules to coordinate with molybdenum or metal-edged molecules to coordinate with sulfide atoms on the surface.^72–74 A variety of applications of TMDs investigated over the years show a great potential toward applications in solar cells,⁷⁵ catalysis,⁷⁶ sensors,⁷⁷ photodynamic/photothermal therapy,^78,79 biosensors,⁸⁰ and cell imaging.⁸¹ To further expand the applications of TMDs, covalent and non-covalent basal and edge-plane functionalization have been utilized over the years. These types of functionalization can be classified as covalent–noncovalent and organic–inorganic in the subjects of techniques and applications, respectively. While organic functionalization usually leads to sensor and optoelectronic applications, inorganic functionalized TMDs lead to catalytic and therapeutic applications.^82,83

As an alternative and/or successor to graphene, TMDs have been considered strongly as fertile materials for sensor applications. Since TMD materials are known for their unique band structures and strong interaction with generally explored target analytes, they are considered very useful for sensor applications. Their easy functionality contributes to their availability in the field. TMDs can be tailored into having higher binding affinity and charge transfer values between sensor and analyte species. As an example, Kim et al. constructed a model for a fluorescent sensor for the detection of Alzheimer’s disease by utilizing weak interactions in the interface of WS $2$ layers as the fluorescence quencher.⁸⁴ By changing the functional groups when functionalizing WS $2$⁠, it was found that the carboxylic acid dextran or trimethylammonium dextran functionalized WS $2$ nanosheets exhibit 3.6 times faster adsorption rates compared to pristine or phenoxy-dextran functionalized WS $2$⁠. In addition, biosensor designs have also been demonstrated with a wide range of target molecules derived from different problems. Yang et al. reported a fluorescent biosensor for the detection of glycated hemoglobin.⁸⁵ Fluorescence quenching was observed in increasing concentrations of HbA1c, as a general trend. FTIR, XPS, and the PL data of B-PVA-WS $2$ samples indicated the functionalization; also, PL data suggest a direct bandgap, and, therefore, the existence of monolayer nanosheets can be justified. Simultaneous functionalization and exfoliation of WS $2$ were achieved by an esterification reaction between polyvinyl alcohol and carboxy-phenylboronic acid to form B-PVA. The reason for boronic acid to be selected as the functional group is that the boronic acid region of the sensing moiety is able to form reversible bonds with cis-diol groups of carbohydrates, making it an effective group for the sensing of HbA1c. In the biosensor technology, MoS $2$ and several other TMDs, such as WS $2$⁠, WSe $2$⁠, and MoSe $2$⁠, were widely studied because of their biocompatibility, label-free bio-detection, and easy fabrication. One of the main setbacks in field-effect transistor (FET) sensor technology is the fact that many ambient unwanted species adsorb on the sensing area, such as H $2$O, CO, and CO $2$⁠; therefore, a deviation appears in sensor responses. Zhang et al. provided a novel and eco-friendly pathway to weaken the effect of this very problem, by functionalizing the MoS $2$ layer using the conjugation of a DNA tetrahedron and biotin-streptavidin (B-SA).⁸⁶ 

As mentioned earlier, the functionalization of TMD nanostructures allows a novel and wide range of applications in sensor field. In order to enhance the performance and sensing properties of exfoliated MoS $2$ nanosheets at room temperature, Xia et al. designed a NIR light activated NO $2$ sensor by enriching the sulfur vacancies (SV) and functionalizing them with ZnO quantum dots (QDs).⁸⁷ A superior performance enhancement was achieved upon functionalization of SV-MoS $2$ with ZnO QDs. The response of the fabricated sensor was increased approximately fivefold with the incorporation of ZnO quantum dots. To overcome the obstacles such as an unstable baseline and poor sensitivity of MoS $2$-based H $2$ sensors, Gottam et al. devised a setup to functionalize the MoS $2$ with H $2$PtCl $6 ⋅6$H $2$O in the aqueous phase and form a composite film of MoS $2$-Pt.⁸⁸ Platinum is known to be a prominent catalyst for gas sensing environments because of its promotion for oxygen dissociation. The MoS $2$–Pt composite film H $2$ sensor device took precedence over preexisting metal-sulfide sensors with its outstanding stability over 70 days. Using WS $2$ nanosheets as the photoactive material, and boronic acid-functionalized carbon dots as a signal amplifier in biosensor, Sui et al. demonstrated a model that was used for the sensing of 5-hydroxymethylcytosine (5 hmC) and $β$-GT activity assessment and $β$-GT activity assessment $β$-GT inhibitor monitoring.⁸⁹ The procedure proceeded as AuNP decoration on an ITO electrode was followed by probing DNA immobilization with an Au–S bond at the infrastructure. Kaur et al. reported a biolayer interferometric in vitro selection technique (BLI-SELEX) for the detection of E. coli toxin subtypes.⁹⁰ Two different aptamers were effectively immobilized onto a chitosan exfoliated nanosheet substrate to create a voltammetric diagnostic test. Because of the chemical stability of TMDs, functionalization usually involves specific chemical reagents and is limited to the chalcogen vacancies on layers. Lihter et al.⁹¹ utilize electrografting to modify the MoS $2$ surface and clarify the advantages of electrochemical functionalization of MoS $2$⁠, using 3,5-bis(trifluoromethyl) benzenediazonium tetrafluoroborate.

TMDs, with their flexible structure, easy functionality, and high biocompatibility, have attracted much attention in the field of cancer therapy over the past decade. Additionally, with functionalization, their intrinsic properties can be enhanced by incorporating polymers, DNA strands, and polar ligands on the TMD plane. The functionalization with biomolecules enables a good binding with the cells and target drugs, making the material more suitable to bioapplications. Utilizing MoS $2$ in a versatile nanostructure, Peng et al. proposed a process in which they inhibited the generation of reactive oxygen species in the system.⁹² First, the model entailed stabilization of MoS $2$ with lipoic acid modified polyethylene glycol (LA-PEG) and then added a charge-convertible peptide [LA-K11(DMA)], which is expected to be pH-responsive. After the mentioned steps, toluidine blue oxygen (TBO), as a positively charged photosensitizer, was adsorbed onto the MoS $2$ layer. Therefore, it was expected to see the release of the TBO molecule from the MoS $2$ layer, upon photoinduced overheating of MoS $2$⁠, thus observing photothermal therapy (PTT). In Fig. 9, the photoluminescence mapping upon TBO release, pharmacokinetics of MKT and PTT under NIR light irradiation are shown. By using the 1T-phase of MoS $2$⁠, Zhou et al. studied the effects of photoacoustic imaging (PAI) and photothermal therapy (PTT).⁹³ The preparation of different phases of MoS $2$ was demonstrated by means of their phase-dependent performances for PAI guided PTT in near-infrared-II window. It turned out that 1T-MoS $2$ nanodots have nearly fivefold extinction coefficient compared to the 2H phase of MoS $2$ nanodots. The authors claimed that upon after being modified with polyvinylpyrrolidone, 1T-MoS $2$ can be used as an effective PAI guided PTT agent, under 1064 nm NIR laser irradiation. It is known that MoS $2$ has a large number of drug-loading sites. Xu et al. proposed a polyglycerol functionalized MoS $2$ nanosheets with PTT and pH-responsive properties for the simultaneous delivery of doxorubicin (DOX) and chloroquine for the treatment of HeLa-R cells.⁹⁴ Under optimal conditions, at pH 5.5, laser-assisted drug release from MoS $2$ nanosheets was significantly increased. In vitro investigations proved that devised nanostructure has a remarkable ability to kill HeLa-R cells. Jiang et al. engineered a device that is able to conduct three different types of anti-cancer treatments, including PTT, CDT, and immunotherapy.⁹⁵ This multifunctional and intelligent nanoplatform system consists of BSA-R837 (Bovine Serum Albumin-Immunoadjuvant Imiquimod) functionalized MoS $2$-CuO hetero-nanocomposite. The procedure for the synthesis of the mentioned hetero-nanocomposite involves the hydrothermal reaction of CuO and MoS $2$ nanoflowers (see Fig. 10). Subsequently, BSA and R837 molecules were loaded onto the nanostructure. While CuO mimics peroxidase behavior, it converts H $2$O $2$ into OH $−$ radical for CDT. The 808 nm NIR laser photocatalytic process has been proven to be an anti-cancer agent. Upon light irradiation, the tumor-associated antigens were generated and strengthened by R837 when tumors were being eliminated. Figure 9 schematically illustrates the intelligent multifunctional therapeutic nanosystem. The PTT requires hyperthermia (overheating), and Cai et al. proposed that the heating could potentially harm adjacent tissues.⁹⁶ In a one-pot synthesis, they used Bovine Serum Albumin-Gadolinum Oxide nanoparticles (BSA-Gd $2$O $3$⁠) as an exfoliation agent and magnetic resonance imaging (MRI) T1 contrast agent. A natural heat shock protein 90 inhibitor was used to reduce the resistance to heat of the cells and reduce the required heat for the PTT process. In overall conclusion, the PTT temperature was reduced from 50 to 43–45  $°$C. In the field of cancer treatment, while it is challenging to establish a system that achieved simultaneous PTT/PDT (photodynamic therapy) processes, Liu et al. constructed a model nanostructure in order to refer work on the challenge.⁹⁷ MoSe $2$ nanoparticles were functionalized with photosensitizer ICG and pH-responsive PDA to be used as drug carriers for PTT/PDT in NIR light irradiation. MoSe $2$@ICG-PDA-HA was shown to inhibit 4t1 cell-growth and, thus, provide a new path to effectively cure breast cancer. WS $2$ nanostructures are also limelight for the photo-thermal therapy field. In this manner, Xie et al. proposed the functionalization of WS $2$ with a biomimetic liposome.⁹⁸ After lipid coating, WS $2$-lipids were seen to have enhanced stability of WS $2$ nanosheets, with or without drug loads. In vivo and in vitro tests showed that the system is cytotoxic to MCF-7 cells.

Transition metal dichalcogenides have bandgap values dependent on their number of layers and strong light–matter interaction, making them an excellent class of materials as candidates for solar cells. The efficiency of solar cells constructed can be enhanced by employing the addition of quantum dots or plasmonic nanoparticles on TMDs, to enhance light absorption and light separation.⁹⁹ As an efficient counter electrode that was utilized in solar cells, Li et al. synthesized MoS $2$@Co $3$S $4$-0.5 hetero-nanocomposite by the hydrothermal method.¹⁰⁰ The nanocomposite not only prevented MoS $2$ from aggregating but also increased the nanocomposite $′$ specific surface area. The composite played an important role in the synergistic effect in the triiodide reduction catalysis. The main emphasis of the study was that MoS $2$@Co $3$S $4$-0.5 shows superior catalytic activity compared to Pt electrodes by 0.87% in liquid-state solar cells. In the study by Peng et al., an impressively long-lasting biocomposite was constructed, which consists of tannic acid (TA) modified cellulose nanofibers (TA-CNFs) and MoS $2$ nanosheets (TA-MoS $2$⁠) coated polyacrylonitrile (PAN) strengthened by Fe $3 +$ chelating with TA and borate cross-linking. As shown in Fig. 11(a), at the surface of the photothermal (TFMCB) $8$@PAN (PTP) fabric, the solar energy was gathered during solar irradiation and transformed into thermal energy, by doing so, the temperature difference between the hot side and the cold side of the PTP-TEG considerably increased.¹⁰¹ The striking prohibition of CO and HCN releases was discovered from TG-IR tests. For this model of PAN-coupled and modified thermoelectric generator solar cell, the voltage output is three times greater than the blank version. While Si was widely used as an excellent light harvester in the field of solar cells, the overpotential, required to conduct the photoelectrochemical hydrogen evolution reaction (PEC-HER), was rigorously high. In another work, Chen et al. synthesized MoS $2$-WS $2$ composite and decorated it onto a Si photo-absorber to be used as a cocatalyst to amplify PEC-HER performance.¹⁰² While this composite experimentally showed superior catalytic activity compared to pristine MoS $2$ and WS $2$⁠, it was also seen to have a larger amount of hole concentrations of unoccupied electronic states that were utilized by accepting Si-carriers. Likewise, the randomly generated sulfur vacancies upon the experimental procedure act as HER active sites in systems. To investigate the hierarchy of the core/shell structure, nanocomposites of TMDs were widely used in the fields of both solar energy and therapy. Long et al. produced different ratios of Co $x$Fe $3 − x$O $4$/MoS $2$ (⁠ $x=1.5$⁠, 0.75, and 0.5) and MoS $2$/Co $x$Fe $3 − x$O $4$ (⁠ $x=0.5$⁠) using the hydrothermal method and interchanging the core-shell hierarchy.¹⁰³ With MoS $2$ used as a shell, better microwave absorbing and comprehensive absorbing performance were observed. This performance evaluation consists of a broader frequency bandwidth, smaller optimal reflection loss, and high chemical stability.

Just as in the field of solar cells, the tunability of bandgap and mobility of the materials are crucial parameters for the construction of optoelectronic devices. The mentioned parameters can be tuned by functionalizing, exfoliating, or changing the film thickness. In order to investigate the effect of film thickness on charge mobility in Lipoic acid-MoS $2$ (LA-MoS $2$⁠) transistor systems, Nie et al. reported a simple method to evaluate the enhancement of electronic mobility.¹⁰⁴ The theoretical model was proposed for the explanation of charge impurity (CI) effect by vacancy filling, and it was shown that theoretical and experimental values are in adequate agreement. It was agreed that the sulfur healing is more effective on the thin films rather than the thick films, in terms of charge mobility. This issue originated from the scales of distance differences between CI and charge carriers in thin and thick films. The subject of thin-film transistors (TFTs) is a wide area of different applications. One of the major problems in thin-film transistors built with exfoliated TMD films is the hardship of controlling their electrical properties when compared to CVD grown TMDs. Referring to this challenge, Lee et al. investigated the effect of doping behavior of chemical functional groups bonded to the TMD layers on the electrical properties.¹⁰⁵ During the process, different functionalization techniques, such as n-BuLi assisted chemical exfoliation and diazonium group functionalization, were used. Depending on the reactivity and electron-donating or electron-withdrawing nature of the functional groups, the enhancement of the charge mobility of the transistor was discussed within a wide range of parameters. McClellan et al. devised a model for low-temperature sheet resistance as $∼$7 k $Ω$/sq, and an appreciable contact resistance of $∼$480  $Ω$  $μ$m in transistors built by using CVD MoS $2$⁠.¹⁰⁶ An extraordinary current density was also achieved, which is around 700 mA/mm along the MoS $2$ transistor, by keeping the transistor on/off ratio above 10 $6$⁠, simultaneously. It was also proposed with better device heat sinking, which was performed for the prevention of self-heating, and the maximum current could reach above 1 mA/mm. While physical challenges slow down and inhibit the growth of Si technology, transition metal dichalcogenides are seen as the alternative for the transistor field. While the functionalization of TMDs is still a bit ambiguous, it is possible to diversify, enhance, and optimize the usage of TMD materials. In their work, Seo et al. functionalized MoTe $2$ with octadecyl trichlorosilane (ODTS) and poly-L-lysine (PLL) molecules of electron-accepting and electron-donating behavior, respectively.¹⁰⁷ An increased sustainability was seen after the charge-enhancing counter doping was applied to MoTe $2$ layers. Oh et al. studied the n-type conductivity suppressing property of Te-doped few-layers MoS $2$⁠, synthesized by the metal-organic chemical vapor deposition (MOCVD) method.¹⁰⁸ The challenge of the n-type and p-type MoS $2$ modeled transistor device of few-layers MoS $2$-Te FET showed a p-type behavior. The detailed qualifications of the pFET with high-K Al $2$O $3$ insulator have shown superior mobility as compared to earlier works (187 cm $2$/V s) and have possessed a Schottky barrier of 32 meV. In another study, Sar et al. proposed a performance-enhancing process for MoS $2$ phototransistors that involve functionalization with colloidal quantum wells and incorporation of a highly efficient Förster Resonance Energy Transfer (FRET) mechanism.¹⁰⁹ While the CdSe/CdSe CQW thin film, whose HAADF-TEM (High-angle annular dark-field Transmission Electron Microscopy) images are shown in Fig. 12(f), acts as the photosensitizer on the FET because of the dipole–dipole coupling between MoS $2$-CQWs, the system that is illustrated in Fig. 12(a), allows enhanced photoresponsivity at maximum gate voltage and tenfold specific detectivity. The scales of the substrate of the fabricated transistor device is shown in Fig. 12(b). While chalcogen (S, Se, Te) vacancies or defects in TMD layers are accepted to weaken the optoelectronic properties of the material, they are also gateways to both vacancy healing and modification of TMDs. The 2LA peak, which is activated upon S-vacancies, was found to appear at around 450 cm $− 1$ in the Raman spectra of MoS $2$-CQW nanohybrid on glass and Al $2$O $3$ substrates as shown in Fig. 12(c). Moreover, the S-healing process was widely studied and relatively better understood compared to Se-vacancy healing. After the functionalization of MoS $2$ via CQWs, the total structure of the transistor allowed strong light absorption in the operative region of the UV-Vis absorption spectra. The efficiency of FRET mechanism is decided according to the distance and the spectral overlap between donor–acceptor pairs,¹¹⁰ as shown in Fig. 12(g). Zhao et al. used thiophenol groups to modify monolayer defected WSe $2$ and investigated the performance of the treated material transistor and photoluminescence-wise.¹¹¹ They have shown a tenfold increase in current density, electron mobility, and on/off ratio of the transistor, and a threefold increase in the photoluminescence count in healed WSe $2$ layers. The sulfur healing may cause planarization and heteroatomization of the structure. The increase in photoluminescence might originate from sulfur-healing, considering for the molecules, these are the factors that increase photoluminescence in systems.

The usage of TMDs in water splitting applications has gained attention for their tunable electronic structure due to their synthesis procedure. TMDs have the means to convert solar energy to hydrogen and oxygen, since they have excellent carrier mobility and suitable bandgap energies. Furthermore, functionalizing TMDs may optimize their catalytic activity, surface reactivity, and charge separation and, hence, increase their availability in water splitting applications.

The investigation of photocatalytic potential in Janus TMDs depends on their electrostatic potential difference between their inner and outer surfaces (⁠ $ΔΦ$⁠). Though the complications stemming from the tubular structures of the nanotubes, calculating $ΔΦ$ has been a challenge. In a responsive manner, Zhang et al. present an innovative approach to this problem. In their study, $ΔΦ$ of Janus MoSSe tubular structures of single-walled nanotubes (A-SWNTs) has been calculated employing first-principles calculations. Findings in the study state that $ΔΦ$ and diameter are inversely proportional; therefore, the decreased diameter of particles leads to an augmented $ΔΦ$⁠. Through these findings, it becomes clear that MoSSe A-SWNTs showcase strategically positioned band edges for the facilitation of water splitting and excellent solar to hydrogen conversion efficiency. The nanotubes, being Janus and double-walled, allow for tremendous electrostatic potential difference and hence, electric field between the surfaces, initiate oxygen (OER) and hydrogen evolution reaction (HER) on inner and outer surfaces, while the excited carriers feed the reactions of HER and OER.¹¹² Forming effective electrocatalysts that perform well in normal conditions without getting damaged is a challenge and a crucial task. Chiang et al. propose a novel microreactor that is composed of a graphene/WSe $2$ within a medium with a pH of 7, which is optimal for water splitting applications. The introduction of Cr atoms onto the surface of synthesized nanoparticles induce an easier charge transfer between solid–liquid interface. Moreover, direct growth of graphene onto the two-dimensional WSe $2$ reduces contact resistance of the device dramatically, enhancing water splitting efficiency. As a result, Cr-doped WSe $2$/graphene device exhibits an overpotential reduction of 1000 mV at 10 mA $− 2$ than its pristine bare WSe $2$ counterpart.¹¹³ Forming effective HER and OER aqueous electrocatalysts and enhancing the performance of the developed device is crucial for sustainable H $2$ production. In this study, Tran et al. an electrocatalyst cast upon iron phthalocyanine (FePc) and vanadium oxide phthalocyanine (VOPc), which reside on MoS $2$-coated MXene Mo $2$TiC $2$T $x$ heterostructure. Via this newly formed device structure, water adsorption and activation were improved, and kinetics for HER and OER are improved.¹¹⁴ In a recently published paper, Singh et al. focused on the optoelectronic potential of MoS $2$-based vdW and Janus structures. With first-principles calculations, they explore the photocatalytic properties of bilayer MoS $2$ and Janus vdW heterostructures with various TMDs and TMOs. The authors examined the photocatalytic performance of heterostructures. The study, therefore, unveils that, despite the modest photocatalytic performance of each monolayer, the van der Waals heterostructures exhibit type-II band alignment, claiming them available as photocatalysts using a Z-scheme mechanism.¹¹⁵ Constructing innovative devices through heterostructures provides versatile means to finely adjust the resulting devices’ physical, chemical, and electronic attributes. Using Density Functional Theory (DFT), the authors investigate the CdS/PtSSe (CPHS), CdS/SPtSe, and CdS/SePtS heterostructures’ photocatalytic potential. Relatively low lattice mismatches and negative formation energies imply the feasibility of visible light-absorbant CPHSs. While the electronic structures of the heterostructures are unaffected by the stackings, the band alignments switch between type-I and type-II. While type-I alignments exhibit more potential for light-emitting devices, type-II alignments create enhanced charge separation through the device. The band edges of CPHS(S)s are compatible with water splitting reaction and the yield meets 70% solar-to-hydrogen (STH) efficiency.¹¹⁶ Wang et al. propose a design for a 1T-MoS $2$ incorporated precious-metal-free electrocatalyst for effective HER in acidic media. Nevertheless, the limitations of 1T-MoS $2$ in alkine media and OER disability slow the material’s broader applications down. The authors hydrothermally incorporated Ni, Co, and Fe metals into flower-like 1T-MoS $2$ to overcome the challenges surrounding the material. Among the alkali metals, Ni-1T-MoS $2$ showcased high current density and improved catalytic activity for HER and OER in alkaline media.¹¹⁷ 

Black phosphorus (bP) is a two-dimensional anisotropic material, which has received much attention since it was isolated by mechanical exfoliation in 2014.^118–120 Its structure is formed such that the phosphorus atoms are covalently bonded to three neighboring phosphorus atoms through $s p 3$ hybridized orbitals, leading to the formation of orthorhombic puckered structure^121–125 and adjacent layers are held together by weak van der Waals interaction as shown in Fig. 13(a). The atomic arrangement of bP expands two inequivalent directions including the zigzag (parallel to the atomic ridges) and the armchair (perpendicular to the ridges), which exhibit in-plane anisotropy.¹²⁶ On the other hand, Fig. 13(b) represents the evolution of the electronic band structures of bP from monolayer to trilayer form and it is seen that the bandgap remains to be direct, whose band edges reside at the $Γ$ point, with decreasing energy bandgap. As compared to the case of TMDs, which exhibit direct to indirect transition even at bilayer structure,^99,127 bP can be considered to be more suitable for optoelectronic applications owing to its robust direct bandgap behavior. As shown in Fig. 13(c), the bandgap of bP is tunable with a number of layers and reaches to its highest value (2.0 eV) in the monolayer structure. In addition to its thickness-dependent direct bandgap (0.3–2.0 eV from bulk to monolayer), bP exhibits high charge carrier mobility (about 1000 cm $2$/V.s), ambipolar transport behavior, and unusual in-plane anisotropy.^128–131 Such characteristics make bP a potential candidate for applications in a wide range of functional devices, including field-effect transistors (FETs),^132,133 optoelectronic devices,^134,135 energy storage,^136–138 sensors,¹³⁹ and biomedicine.¹⁴⁰ 

Even though extensive research has been performed on pristine bP, the development of the pristine bP for such applications has been hampered by its several inherent properties. First, bP is known to be easily oxidized due to its lone electron pairs. Since oxygen is chemisorbed on bP layer, preferably on top of phosphorus atoms, there exists a high energy barrier (⁠ $>$3 eV) for the desorption of oxygen from the surface.^141–143 This gives rise to ambient instability of the bP surface, leading to short device lifetimes.¹⁴⁴ In addition, there is lack of an efficient method for the synthesis of high-quality large scale single-crystal bP.^145,146 Up to date, top-down techniques, such as mechanical exfoliation, liquid-phase exfoliation, and plasma etching, have demonstrated either relatively low crystal quality or small flake sizes of bP.^147–149 Furthermore, since bP is poorly soluble in common organic solvents, it is hard to manufacture bP using solution-process methods.¹⁵⁰ In order to overcome such intrinsic obstacles of bP, it is necessary to study functionalization methods for further developments of its potential applications.

One of the effective approaches for passivation and altering properties of few-layers bP is chemical functionalization, which enables either chemical reaction or absorption of the functional chemicals onto bP surface.¹⁵⁰ With the chemical functionalization, the lone electron pairs of bP atoms are covalently attached with foreign atoms and form stable chemical bonds including P–C, P–O–C, P–N, and P=N bonds, thus avoiding interactions between the reactive lone electron pairs and oxygen, which are considered to be the main cause of the degradation.

Diazonium compounds have been widely considered for the modification of semiconductors through covalent bonding, in which the functionalization process takes place via a free radical mechanism.¹⁵¹ Covalent functionalization of bP surface with aryl diazonium salts was first reported in 2016 and has been imparted to be advantageous for not only the effective passivation of bP but also for altering its electronic properties.¹⁵² Hersam et al. studied varying aryl diazonium salts including 4-nitrobenzenediazonium (4-NBD) and 4-methoxybenzenediazonium tetrafluoroborate salts (4-MBD) [Fig. 14(a)] for the modification of bP surface. XPS measurements revealed the formation of P–C bonds at 284 eV in the C-1s and a broad P–C peak at 133 eV in the P-2p spectrum after 30 min. of reaction time with both 4-NBD and 4-MBD. Also, the existence of covalent functionalization with 4-NBD and 4- MBD was verified by Raman spectroscopy measurements, in which decreasing peak intensity of A $g 1$ with increasing functionalization was reported to be a sign of the surface modification. In addition, the degradation rate of bP was drastically hindered, as proved by the stable surface morphology indicated by atomic force microscopy (AFM) after exposure to air up to 10 days [see Figs. 14(b) and 14(c)]. As a result, passivation of bP was successfully achieved via covalent attachment of aryl groups on P atoms (forming P–C bonds). On the other hand, according to the DFT calculations, the generation of P–C linkage was thermodynamically favored and four coordinate attachments were shown to be responsible for lattice disruption in bP. It was reported that by decreasing the amount of aryl diazonium molecule through reaction with 4-NBD, the chemical modification rate of bP can be controlled. Hence, the level of chemical modification of bP could be adjusted. The authors implemented bP-based FET devices to investigate the effect of covalent functionalization using aryl diazonium salts on the electronic properties of bP, verifying that aryl diazonium covalent functionalization altered the electronic properties of bP, revealing excellent and tunable p-type doping and consequently improving the FET efficiency and the ON/OFF current ratio up to 10 $6$⁠.¹⁵² In contrast, Collins et al. reported that the chemical functionalization via aryl diazonium salts causes oxidation of bP and results in generation of aryl multilayers, leading to ambient instability. Instead, they investigated the chemical functionalization of iodonium and diazonium salts, resulting in aryl iodonium compounds, providing good ambient stability after exposure to air for up to a week (Fig. 15). Also, it was revealed that iodonium salts were separated into aryl iodide and Ar-F groups, yielding an Ar-F modified bP surface through covalently P–C bonding confirmed by spectroscopic characterizations. Consequently, aryl functionalization with iodonium salts hindered the formation of oxidation by covering available surface oxygen sites, leading to a stable ambient environment that allowed the electrical properties of bP to be altered via covalent linkage of electron-donating and withdrawing groups on the aryl compound.¹⁵³ 

Layered bP has been utilized as an electrode material in energy storage devices due to its high specific surface area and good electrical conductivity. However, the material suffers from capacity decay and poor cycling stability. To overcome these issues, it is logical to propose functionalization with conductive polymers or decoration with graphitic materials. These modifications on the materials may increase the conductivity of hybrid material and solve the problems for poor cycling.¹⁵⁴ Based on the high theoretical capacity (2596 mAh/g), bP has been considered as an intriguing electrode material in lithium-ion and sodium-ion batteries (LIBs and SIBs).^154,155 Compared to traditional graphite anode materials, the charge and discharge capacities of bP-based anode materials were improved for LIBs.¹⁵⁶ On the other hand, during the periodic lithiation and delithiation process, capacity decay was observed in bP-based anode materials as a result of intensively breaking anode materials caused by the high volume expansions. This obstacle can be solved by the chemical functionalization of bP via carbon materials, Hence, the generation of P–C bonds makes bP more durable during lithium insertion/extraction.

In the study by Cui et al., the battery performance of the bP-based anode materials with a variety of carbon sources evaluated by a high energy mechanical milling process (HEMM) were compared and reported.¹⁵⁴ The charge/discharge measurements were performed on the electrochemical lithium storage properties of red phosphorous (RP), a mixture of bP and graphite (bP/G) without HEMM, and chemically bonded bP-G composite with HEMM. The charge capacity, C $p$⁠, of RP, bP/G electrode, and bP-G were measured to be 1856, 2479, and 2786 mAh/g, respectively. In addition, the charge and discharge hysteresis (E $p$⁠) features were shown to decrease from 0.43 to 0.31 V, verified by the TEM, XPS and Raman data, as evidenced by the formation of stable P–C bonding improved electrochemical performance of bP-graphite composite during lithium insertion/extraction. It was also compared between cycling performance of bP/G and bP-G. It was concluded that due to the increasing Coulombic efficiency and improving contact between phosphorus and carbon particles, the bP-G composite exhibited reversible behavior and relatively higher capacity compared to the bP/G after the 100th cycle. Consequently, the bP-G electrode-based device showed high electrochemical performance owing to its relatively high electrical conductivity, fast charge transfer process, and good lithiation and delithiation process, as proved by the generation of stable P–C bonding.

On the other hand, Wang et al. used 4-nitrobenzenediazonium (4-NBD) modified bP covalently bonded with reduced graphene oxide (RGO) hybrid (4-RbP) in order to enhance the SIB anode performance of bP. They considered the modification of bP via 4-NBD, which chemically bonds to nitrobenzene by the formation of P–C bonding (type 1), decreasing surface energy and improving stability. In addition, functionalized bP with 4-NBD was passivated by RGO, attached by the generation of P–C (type 2) and P–O–C bonding, enabling to storage of more sodium ions to improve cyclic performance. The existence of two different P–C bonds was revealed by Raman and FT-IR spectra that realized the difference in intensity in XPS. Furthermore, the electrochemical performance of the SIB anode of bP was evaluated. The 4-RbP anode displayed a reversible behavior with a high capacity of 1472 mAh/g at a current density of 0.1 A/g after the 50th switching cycle and a 650 mAh/g current density of 1 A/g after the 200th switching cycle. This is attributed to reduced surface energy by the formation of P–C bonding, which widens the channel between bP and RGO. The widened channels stored more sodium ions for improving cycle performance in SIB.¹⁵⁵ 

Li diffusion mechanism of an oxidized bP/graphene oxide (bP/GO) layer for bendable LIBs was investigated by Kim et al. bP was chemically bonded with GO, leading to a remarkably high mechanical strength and bending ability of the bP/GO heterostructure. The formation of P–C bonding was confirmed by Raman spectra, which revealed the appearance of the broad peak between 600 and 700 cm $− 1$ and blueshift of G band up to 1600 cm $− 1$⁠. The electrochemical performance of the bP/GO electrode was measured, revealing reversible characteristics with a capacity of 737 mAh/g at a current density of 0.5 A/g and retaining a superior capacity of 477 mAh/g after 500 cycles. The authors also compared the bP/GO and bP/neat graphene electrode. It was noted that oxygenated functional groups in GO affected the battery performance and played a crucial role in sustaining a stable cycle of the electrode. Furthermore, electrical impedance spectroscopy (EIS) was used to confirm increased Li diffusion kinetics of the bP/GO electrode. D $L i$ values obtained for bP/neat graphene and bP/GO were $8.19× 10 12$ and $6.61× 10 11$⁠, respectively. When the diffusion coefficient of nanosilicon was taken into account (10 $12$⁠),¹⁵⁷ D $L i$ of bP/GO exhibited superior Li diffusion kinetics. Finally, the authors applied a flexible Li-ion battery using the bP/GO anode. The device displayed high gravimetric and volumetric energy densities of 389 Wh/kg and 498 Wh/l, respectively, with a high retention rate of 92.3 after 100 cycles.¹⁵⁸ 

A major drawback of pristine bP is the short lifetime of its intrinsic p-type charge carrier in as-synthesized samples.¹⁵⁹ It is essential that bPs with p- and n-type charge carriers have controllable characteristics while sustaining their mobility. Functionalization has been an effective technique for tackling these two shortages, which greatly restrict the further improvement of bP-based transistors.^152,160

Guo et al. studied stability and transistor performance of metal ion modified bP.¹⁶¹ Mechanically, exfoliated bP was transferred onto Si wafer with a 300 nm thick SiO $2$ and immersed in silver nitrate to be modified bP via Ag $+$ [see Fig. 16(a)]. It is well known that the Ag $+$ and the conjugated $π$ bond via the cation- $π$ display significant interaction. The modification of bP via Ag $+$ was revealed by XPS and appeared to peak at 133.0 eV, as proved by the interaction between bP and Ag $+$⁠. In addition, there was no evidence of P $x$O $y$ peak from bP $A g +$ when compared to bare bP, observing a peak at 134.0 eV, related to the oxidization of bP (P $x$O $y$⁠). Furthermore, Raman spectra verified the modification of bP via Ag $+$ and the peak intensity of Ag $1$ was considerably increased, affirming adsorption of Ag $+$ on the bP surface. Finally, the authors applied the Ag $+$ modified bP-based FET as shown in Fig. 16(b). The attachment of Ag $+$ on the bP surface not only improved device stability but also increased hole mobility up to 1666 cm $2$/Vs, realizing a twofold enhancement compared to bare bP device. In addition, Ag $+$ modified bP-based FET exhibited a superior ON/OFF ratio to $2.6× 10 6$⁠, which was 44 times higher than bare bP [see Fig. 16(c)].

In another report, Chen et al. investigated functionalized bP via potassium (K) on the complementary devices. XPS and UPS measurements verified surface functionalization as well as remarkable charge transfer between bP and K. It was noted that the modification of bP with K reduced the bandgap of bP due to the induced strong electric field, giving rise to enhanced electron mobility up to 262 (377) cm $2$/V s in FET. Consequently, complementary devices were evaluated using a spatially controlled K doping to obtain a p-n based-diode, having a near-unity ideality factor of 1.007 with an on/off ratio of 10 $4$⁠.¹⁶⁰ 

Similarly to the TMDs, strong light–matter interaction and tunability of bandgap of bP, highly favor its use in the field of optoelectronics. This tunability of bandgap of the black phosphorus is taken advantage of and improved for their use in optoelectronics by using polymers and metal-oxide nanoparticles. As shown in Fig. 17(a), Zhang et al. used tetraphenyl porphyrin (TTP), covalently bonded to black phosphorus nanosheets (bP NSs) via diazonium chemistry for photophysical properties. The existence of the covalent attachment between porphyrin and bP NSs was observed by spectroscopic characterizations and revealed a new peak feature at 131.16 eV in the XPS spectrum. Additionally, the authors made a control experiment with diazonium-free porphyrins to justify the formation of P–C bonding between porphyrin and bP NSs. Moreover, fluorescence spectroscopy was conducted to investigate photophysical characteristics of bP NSs and bP NSs-TTP hybrid structures. Under an excitation wavelength of 415 nm, the fluorescence spectra of bP NSs-TTP exhibited considerably quenched when compared with TTP, and the quantum yield of tetraphenyl porphyrin (TPP) and bP NSs-TTP were found to be 5.9% and 3.5%, respectively [see Fig. 17(b)]. The reduced quantum yield associated with electron/energy transfer between the excited porphyrins and the bP NSs, as supported by the fluorescence lifetime of TTP and bP NSs–TPP was 8.49 ns (67.6%) and 0.38 ns (32.4%), respectively [see Fig. 17(c)]. As a result, the porphyrin functionalized bP NSs pave the way for appealing applications such as solar energy harvesting and optoelectronic devices.¹⁶² 

Li et al. improved diazonium functionalization by covalently binding redox-active triphenylamine (TPA) on bP NSs, reflecting good electrochemical activity and ambient stability. They also used TPA-functionalized bP NSs to improve nonvolatile memory devices, indicating good long-term stability.¹⁶³ 

Covalently functionalized bP utilizing conjugated polymers through the diazonium salt method was reported by Chen et al. Using a 4-bromobenzene-diazonium template, a conjugated polymer poly[(1,4- diethynylbenzene)-alt-9,9-bis(4-diphenylaminophenyl) fluorine] (PDDF) was achieved by covalently grafting onto bP. Spectroscopic characterizations also verified the existence of covalent functionalization of bP with 4-BBD. Finally, the authors applied the modified bP-based resistive random memory devices. The result displayed good nonvolatile rewritable memory performance with a high ON/OFF current ratio up to 10 $4$⁠, and the voltages of turn-on and turn-off were measured as +1.95 and $−2.34$ V, respectively. In addition, the current flowing through the memory device in either the ON or OFF states remained constant after the 200th switching cycle.¹⁶⁴ 

Functionalized bP has been used in many sensor applications such as gas, ion, and bio with enhanced detection of limits, response speed, and selectivity. Sabherwal et al. reported modified bP via poly-L-lysine (PLL) to detect cardiac biomarker myoglobin (Mb) using aptamer-functionalized black phosphorus nanostructured electrodes. The authors implemented the attachment between PLL functionalized bP and anti-Mb aptamers via Coulombic interaction and monitored layer-by-layer modification of DNA aptamer by XPS. As a result, PLL-bP-based DNA aptasensor displayed remarkable Mb sensing with 36  $μ$pg $− 1$ ml cm $− 2$ and a detection limit of around 0.524 pg/ml as well as a dynamic response between 1 pg m/l and 16  $μ$g m/l.¹⁶⁵ 

Fiber optic biosensor for high-sensitivity monitoring of human neuron-specific enolase (NSE) cancer biomarkers was conducted by Chen et al. Liquid exfoliated bP was coated on tilted fiber grating (BF-TFG) to improve the light-matter interactions. It was reported that the PLL functionalized bP induces to have a biocompatibility surrounding for the immobilization of anti-NSE while keeping an available attaching site for the identification of NSE biomarkers with high affinity. The authors detected the NSE biomarkers with a detection limit of 1.0 pg/ml. The reported value was four times lower than the NSE cutoff of small cell lung cancer and 100-fold higher than GO and AuNPs-based biosensors.¹⁶⁶ 

Jung et al. reported the functionalization of few-layers bP with noble metals such as Au and Pt in order to enhance chemical sensing properties. The dynamic sensing measurements were performed on pristine bP, Pt/bP, and Au/bP to detect VOCs, NO $2$⁠, and H $2$ in ambient conditions. It was noted that the pristine bP showed only sensitivity to paramagnetic molecules such as NO and NO $2$ due to adsorbent paramagnetic gas molecules on the PB surface, leading to spin-polarized current on the active surface area. In addition, compared with pristine bP, P-functionalized bP demonstrated a remarkable selective of H $2$ response with a concentration down to 1% and an electrical channel variation of 500%. This was attributed to tunable doping levels in bP via electron transfer from Pt, leading to a reduction in hole concentration in bP as a result of increased channel resistance. Furthermore, a variable detection of NO $2$ ranging from negative to positive was achieved after the functionalization of Au. Such a response associated with amounts of n-doping of Au altered the charge carriers of the bP from holes to electrons. Consequently, Au- or Pt-incorporated bP systems not only enhanced gas sensing ability but also improved long-term device stability as well.¹⁶⁷ 

In another work, Han et al. coated MIL-53 metal-organic framework (MOF) on functionalized bP via Al ion to detect N0 $2$ gas.¹⁶⁸ A MIL-53 coated bP (bP@MIL-53) was prepared by growing bP/Al $3 +$ under solvothermal conditions with 2-aminoterephthalic acid (NH $2$-BDC)[Fig. 18(a)]. The authors proposed that functionalization of bP via Al resulted in a decrease in bP’s electron density, leading to a decrease in bP’s reaction ability to O $2$ and H $2$O. Hence, coated MOF enabled further improvement in the thermal and ambient stability of bP. Gas sensor measurements of bP@MIL-53 revealed a high response and repeatable behavior to NO $2$ at 70  $°$C, with concentrations of 1, 2, and 5 ppm, as well as being remarkably selective toward NO $2$ when compared to CO, NH $3$⁠, and H $2$S gases, with a detection limit of 1 ppb [Figs. 18(b) and 18(c).]

A broad range of growth techniques and functional approaches provide bP with controlled flake size and high surface interactions, and also unique physical and chemical characteristics, making them especially attractive in biological systems. Furthermore, functionalization stays a very effective way to further improve the beneficial properties of bP, and the binding of targeted drugs and activate the bP surface for any interacting biomolecule with functionalizing polymers.

Electrocatalytic performance toward H $2$O $2$ by modifying bP with poly-L-Lysine (pLL) was improved by Zhao et al. First, the pLL was attached to the surface of bP with hydrophobic interactions, and then a negatively charged immobilized hemoglobin (Hb) was placed in the hybrid pLL-bP structure. The immobilized Hb also exhibited rapid and reversal direct electron transfer (DET), resulting in relatively high conductivity and bioactivity of bP. The reported rate constant (kET) was found to be 11.24 s $− 1$ for DET of Hb on pLL-bP hybrid structure. Finally, it was shown by electrochemical measurements of Hb-pLL-bP GCE that the electrochemical response to H $2$O $2$ with various concentrations from 10 to 700  $μ$M displayed linear features and better electrocatalytic performance toward reduction of O $2$ and H $2$O $2$⁠. Based on these findings, the was suggested that the pLL-bP hybrid may be used for the development of novel biofuel cells, bioelectronics, and biosensors.¹⁶⁹ 

Mei et al. developed bP-based delivery mechanism for cancer theranostics. Positively charged polyethylene glycol–amine (PEG-NH $2$⁠) was attached to mechanically exfoliated bP via electrostatic interaction in order to create PEGylated bP NSs. It was noted that it was possible to deliver theranostic drugs, such as doxorubicin (DOX) in conjunction with cyanine7 (CY7) and chemotherapy for near-infrared imaging in vivo, via PEGlated bP NSs. In addition, in vitro and in vivo test measurements showed the improvement in anticancer activity in DOX-loaded PEGylated bP NSs with high biocompatibility. Consequently, the functionalized approach of bP NSs offered a strong framework for cancer theranostics due to their adaptability to modified PEG-NH $2$ and high biosafety.¹⁷⁰ 

Fluorescent Nile Blue dye (NB) was used to functionalize bP via covalent bonding for photothermal therapy (PTT). The NB dye was transformed into diazonium tetrafluoroborate (NB-D) that interacts with bPs, and generates P–C linkages on the surface of bP via aryl diazonium chemistry. Yu et al. reported that the functionalized bP via NB (NB@bPs) demonstrates impressive optical performance durability over quick deterioration and has significant near-infrared (NIR) fluorescence. The in vitro and in vivo tests were performed in order to investigate the practical application of NB@bPs for photothermal therapy. In vitro measurements revealed that the dye-modified bPs were not only biocompatible but also had relatively high PTT and NIR imaging capability. On the other hand, in vivo measurements indicated that NB@bPs identified the cancer location with red fluorescence and led to effective cancer ablation when exposed to NIR illumination. These experimental findings point to the NB functionalized bP-based biomedicine holds promise for tumor applications.¹⁷¹ 

Qui et al. created a novel light-activated bP NSs-based hydrogel for tumor treatment.¹⁷² Positively charged polyethylene glycolamine was utilized to electrostatically functionalize liquid exfoliated bP NSs to create PEGylated bPNSs. The releasing kinetics of doxorubicin (DOX) may be precisely regulated by adjusting the light intensity and dose rate. In vitro and in vivo studies revealed that the bP NSs-based hydrogel had an exceptionally high killing power for tumor cells, as well as a tumor ablation impact. The strategy has the ability to be used toward clinical cancer research.

bP was modified via polydopamine (PDA) to improve stability and photothermal efficiency.¹⁷³ For modeling therapeutic agents, DOX and P-gp siRNA were utilized. P-gp siRNA diminished the expression of the permeability glycoprotein (P-gp) on the tumor cellular membranes, resulting in P-gp mediated multidrug resistance (MDR). In addition, the drug-loading measurements revealed pH-sensitive and NIR-radiation activated drug release characteristics. Finally, the authors applied both in vitro and in vivo tests. The findings revealed the multifunctional treatment had a strong anti-proliferative impact on cancer cells.

Engineering the properties of materials has been the main interest of the researchers to create derivatives of such materials making them suitable for various applications. While the application of external effects, such as mechanical strain or gating, has been widely considered for tuning properties of materials, chemical functionalization of structures has always been the main purpose not only for tuning of individual properties but also for creating derivatives of the material with a wide range of features. Although functionalization is an old process that has been applied to many different materials, recent advances in materials science revealed the demonstration of ultrathin 2D materials, which are excellent candidates for such chemical engineering. Fired up by the successful synthesis of graphene, various types of 2D materials either one-atom-thick or with few-atom thickness have been added to the library of the 2D family. The confinement of electrons in one dimension makes 2D materials suitable candidates for surface functionalization through their unpaired electrons residing on the surface.

As recent studies reported a novel way for doping 2D materials, modulation doping of a 2D material through constructing a heterostructure with other suitable materials may be useful in order to tune carriers in the material. Within this strategy, one is able to introduce carriers through the charge transfer from defects inside an adjacent encapsulation layer.^174–176 It is also possible to control either n-type doping or p-type doping of the 2D material using modulation doping. Moreover, such a strategy allows not only effectively adjusting the magnitude of carriers but also eliminating the effects of Coulomb impurity scattering. Therefore, the doping modulation strategy could be an important alternative and useful way to tune carriers in a 2D material for their use in various application areas mentioned in this Perspective. Different ultrathin materials have been reported to exhibit various features mostly unique to their chemical formation or structural organization. For instance, while graphene possesses many important properties, such as high electrical and thermal conductivity, optical transparency, and high mechanical stiffness and strength, the monolayer of MoS $2$ is a very good candidate for optoelectronic applications with its suitable direct electronic bandgap. Researchers have always been looking to define a bandgap to graphene, for which chemical functionalization is an effective method and has been successfully examined through atomic adsorption on the graphene surface using different candidates, such as H, F, Li, etc. Not only isotropic 2D materials but also monolayer structures exhibiting in-plane anisotropy, such as black phosphorous, have been widely studied by means of chemical functionalization to use them in nanotechnology applications.

In sensor applications, the use of graphene has been facing problems, such as its low efficiency in sensing the chemicals, which are not useful and dangerous for our daily life. To solve such problems, the modification of the basal plane of graphene using various chemicals was demonstrated, and it was observed that the sensing ability of graphene was improved. Similar attempts have been achieved in monolayers of TMDs for their use as nanosensors. One of the main routes to improve the sensing ability of such 2D materials has to be preserving the material surface from the detection of unwanted adsorbents. For this purpose, using chemical species for the surface modification to stabilize the material can be crucial to block the interaction of the modified 2D material with those irrelevant unwanted chemicals and this makes the sensor more responsible for the real chemicals that are aimed to be sensed. For different purposes of applications, the surface modification of the 2D material through chemical functionalization has to improve various properties of the material. While for wearable technology applications, extending the mechanical response of the material is important, for optoelectronic and photonic applications, the control of the electronic and optical properties becomes crucial. On the other hand, the use of 2D materials for energy conservation purposes, the capacitance of the material has to be tuned via the modification of its surface and suitable chemicals have to be chosen for such purposes. While increasing the efficiency of the material in many applications areas as mentioned, one of the main purposes is also to stabilize the functionalized material against the environmental effects. For instance, one of the main problems with the anisotropic monolayer of bP is its easy oxidation. Therefore, researchers studied the chemical modification of bP surface not only to tune its properties but also to make it more stable against environmental conditions. However, the choice of the functional group that saturates the surface of bP is quite critical. For instance, chemical functionalization of bP via aryl diazonium salts increases its oxidation ability, and, thus, this leads to the instability of bP. In order to overcome such problems, sandwiching chemically modified bP layer between different 2D materials may enhance its stability and it can be used for more purposes. An advantage of using bP layers for surface modification can be its in-plane anisotropic feature such that if successfully achieved, orientation-controlled functionalization could make bP a more potential candidate for the sensor or in transport applications.

In the present Perspective, we reviewed the recent advances in the functionalization of 2D materials such as graphene, TMDs, and anisotropic bP. Possible experimental routes of chemical engineering of the surface of those materials were presented, and the improvement of their efficiencies in device applications was discussed. The main problems and potential solutions through the chemical modifications of materials’ surface were reviewed. Future technologies depend on the manipulation of ultrathin 2D materials and on how effectively such modifications can be achieved. One of the main challenges of experimental issues is the synthesis of high-quality and large-size 2D materials, which affect the functionalization ability of the material. Another challenge in the demonstrated 2D materials is to stabilize them even in their free-standing form. Chemical modification is an important attempt to solve such a problem once suitable chemical groups are chosen. The future of advanced nanotechnology lies in the synthesis of high-quality large-area 2D materials and in their successful surface functionalization for their use in various applications. A 2D material can be a good nanosensor or a potential candidate for the purification of wastewater or can be used in energy storage applications, all of which can be made more efficient by the chemical modification of the surface of the material using suitable candidates. Although the applications of functionalized layered 2D materials are discussed in the present Perspective, it is important to note that the 2D form of non-layered materials is gaining much attention due to their more chemically reactive surfaces and potentially can be used in various nanotechnological applications.

The authors have no conflicts to disclose.

T. A. Duran: Investigation (equal); Writing – original draft (equal). Y. O. Yayak: Investigation (equal); Writing – original draft (equal). H. Aydin: Investigation (equal); Writing – original draft (equal). F. M. Peeters: Writing – review & editing (equal). M. Yagmurcukardes: Supervision (lead); Writing – review & editing (lead).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.