Design of stimuli-responsive transition metal dichalcogenides

Abstract

Stimuli-responsive systems are an emerging class of materials in fields as diverse as electronics, optoelectronics, cancer detection, drug delivery, or sensing. Especially focusing on nanomaterials, 2D transition metal dichalcogenides have recently attracted the scientific community's attention due to their remarkable intrinsic stimuli-responsive behaviour upon external stimuli such as pH, light, voltage, or certain pathogens. This significant response can be further enhanced by forming mixed-dimensional heterostructures and by molecular functionalization, capitalizing on chemistry to manipulate and boost their intrinsic stimuli-responsive properties. Furthermore, thanks to the endless possibilities of chemistry, a new class of smart materials based on the combination of stimuli-responsive molecular systems with transition metal dichalcogenides has recently been synthesized. In these materials, the physical properties of the 2D layers are reversibly modified by the switchable molecules, not only enhancing their stimuli-responsive behaviour but also providing memory to the hybrid. Therefore, this review explores the recent breakthroughs in the chemical design of smart transition metal dichalcogenides with built-in responsiveness.

Transition metal dichalcogenides not only possess intrinsic stimuli-responsive behaviours upon exposure to external stimuli, but molecular functionalization of these materials and/or combination with other materials to form mixed-dimensional heterostructures enables the manipulation and enhancement of their stimuli-responsive properties. Here, the authors review recent breakthroughs in the chemical design of smart transition metal dichalcogenides with built-in responsiveness.

Introduction

Both animals and plants widely display stimuli-responsive (SR) behaviours. For example, mimosa plants fold up their leaves when stroked. In recent decades, a large number of artificial stimuli-responsive materials (SRMs) have been developed and studied, many of which were nature-inspired¹. Conceptually, SRMs are designed materials whose properties can be significantly altered by external stimuli such as pH, temperature, light, stress, and magnetic or electric fields. These intelligent SRMs can function as sensors, actuators, drug delivery systems, or active components in electronic devices to name a few.

When it comes to designing SRMs, two-dimensional (2D) materials provide new insights due to their unique properties. These include their sub-nanometric thickness, flexibility, transparency, high catalytic activity, and unusual electronic structures^2,3. These characteristics are due to the combination of an enormous specific surface area and electron confinement in two dimensions. As a result, 2D materials are highly sensitive to various stimuli, including electrical currents, strain, molecular absorption, and light irradiation^2,3. The first example of an SR device based on 2D materials belongs to the fabrication of graphene-based transistors (stimulus: voltage gate; response: electrical current), which have proven to be particularly efficient thanks to graphene's high charge mobility, sensitivity to the applied field, and long-term reliability. However, graphene has significant limitations, many of which stem from its lack of natural bandgap or spin-orbit coupling. To find alternatives to graphene, the scientific community has explored a variety of 2D materials⁴. These include monoatomic graphene analogues like antimonene^5,6, or more recently goldene⁷, layered transition-metal carbides and nitrides (MXenes)^8–11, 2D transition-metal oxides and hydroxides (TMOs and TMHs)^12–16, as well as transition-metal dichalcogenides (TMDCs) and phosphorus trichalcogenides (MPS₃)^17–22.

In this context, transition metal dichalcogenides (TMDCs) are the most promising alternative for 2D SRM applications. TMDCs have a general formula MX₂, where M is a transition metal and X is a chalcogenide (S, Se, or Te). The binary MX₂ family encompasses numerous compounds; however, only those based on transition metals from groups 4 (Ti, Zr, or Hf), 5 (V, Nb, or Ta), 6 (Mo or W), or 7 (Tc or Re), and group 10 (Pd or Pt) exhibit a well-defined layered structure^4–7 where MX₂ slabs hold together by easily breakable van der Waals interactions, stacking parallelly one on top of the other^2,3. Inside each slab, the metal is coordinated by six chalcogen atoms and the coordination geometry can be either octahedral (O_h) or trigonal prismatic (D_3h). Furthermore, a range of polytypes arises from the orderly stacking of layers at specific periodicity. Thus, diverse unit cell symmetries can be formed (most common are T: trigonal, H: hexagonal and R: rhombohedral). By the combination of different compositions, coordination geometries and unit cells, TMDCs display completely different properties, Fig. 1. For example, octahedrally coordinated (O_h) Ta in 1T-TaS₂ polytype behaves as a semiconductor while its trigonal prismatic coordination symmetry (D_3h) in 2H-TaS₂ polytype gives rise to a metallic 2D material with a superconducting transition below 1 K. In these systems, the transition from a layered bulk compound to the 2D limit can be easily achieved by different chemical and physical approaches, which modifies most TMDCs properties and functionalities as these are thickness (number of layers) dependent. Therefore, these characteristics make this family of layered materials highly desirable for all applications where graphene falls short, including theragnostic therapy and biomedicine, actuators, and optoelectronics^23–28.

Fig. 1. The properties and structure of TMDCs.

a TMDCs most common polytypes depending on the transition metal and their physical properties. b Schematic side and top view of 1T and 2H polytypes and detail of corresponding O_h and D_3h coordination geometries around the metallic centre.

One step ahead, chemistry plays a major role in the development of TMDCs-based SR devices, enhancing their intrinsic switchable capabilities through chemical design, via their functionalization with molecules, nanoparticles (NPs), or even other 2D materials^23–29. An even more exciting alternative consists of functionalizing TMDCs with SR systems. In such heterostructures, the SR components become the source for altering the physical properties of the TMDC, giving rise to new synergies and SR behaviours. Some archetypal examples of these SR molecules are azobenzene, diarylethene, and spiropyran molecules, which can be used for mechanical actuation upon exposure to light³⁰, or spin-crossover materials that can produce the same mechanical actuation upon exposure to light, pressure or by heating^31,32. These new molecular-induced SRMs offer several advantages over conventional TMDCs-based SR systems, as they impart memory to the TMDCs, enabling the development of ternary or even quaternary SR devices (combining three or four external stimuli)^33,34. Thus, pushing the field of TMDCs-SR materials further, and paving the way for their implementation in practical electronics/optoelectronics devices, theragnostic therapy, sensors, or catalysis.

In this review, we will summarize some of the most relevant contributions in the field of the chemical design of TMDCs-based SRMs. We will begin by discussing the works centred around the intrinsic SR behaviours of TMDCs, where gate voltages (section 1.1), light (section 1.2), or strain (section 1.3) have been used as stimuli. Next, we will comment on the most recent reports on enhancing their SR properties through molecular functionalization (section 2.1), NPs decoration (section 2.2) and fabrication of stacked layered heterostructures (section 2.3). Going a step ahead, we will explore the development of SR TMDCs achieved by combining these 2D layers with intelligent molecular systems that transfer their SR switching properties to the TMDCs (Section 3). Finally, we will conclude with a forward-looking outlook, outlining what we consider to be the most pressing challenges and promising directions for future research in the field of TMDCs as SRMs.

Intrinsic SR behaviour of TMDCs

TMDCs are materials considered to be SR by themselves due to their modulable electronic bandgap. Therefore, by applying an external stimulus such as voltage, light, or strain (input signals) their bandgap can be tuned, which in turns affects their electronic properties or light absorption/emission features (output signals). So, these properties have positioned TMDCs as strong contenders in the development of smart devices. In the following section, we will provide a brief overview of some works which explore this characteristic.

Voltage as stimulus

As mentioned before, emerging semiconducting TMDCs provide an ideal platform for developing Field Effect Transistors (FETs), where electrical currents are turned on/off by setting a voltage through a gate (stimulus: gate voltage; response: electrical current). This is due to their relatively high carrier mobility and adjustable band structures. For this reason, the key features pursued over the years rely on fabricating FETs with as high I_on/I_off ratios and carrier mobilities as possible.

In this context, many 2D semiconductive TMDCs, including MoS₂, MoSe₂, MoTe₂, WS₂, and WSe₂, have been used for preparing highly efficient nanoFETs, (electrical performances summarized in Table 1) as they can maintain high efficiencies at the nanoscale^35–41. In particular, I_on/I_off ratios > 1∙10⁷ have been obtained for MoS₂ and WS₂ FETs^{35,38–40,42,43}, values that compete with those of commercial metal-oxide-semiconductor FETs^42,43. Unfortunately, their field effect electron mobilities, between 2 and 46 cm²·V⁻¹·s⁻¹, are still far from the 100 to 1000 of cm²·V⁻¹·s⁻¹ classically exhibited for metal oxides^42,43. In this regard, the main reasons behind this handicap of TMDCs are commonly attributed to scattering with high-density defects (like sulfur vacancies), dielectric surface roughness, Coulomb impurities, adsorbed molecules, or high electrode ohmic resistances^44–46. To suppress Coulomb scattering, Liu et al., and Yu et al., demonstrated that the doping of source/drain contacts⁴⁷ or the integration of a top high-k dielectric film (Al₂O₃ or ZrO₂) or substrates (HfO₂, Al₂O₃) can be used to enhance electron mobilities of WSe₂ and MoS₂ single-layer-based FET, reaching values in both cases of around of ~200 and ~150 cm²·V⁻¹·s⁻¹, respectively, even at room temperature (Fig. 2a)^48,49.

Table 1.

Comparative table of the charge mobilities in some representative TMDC devices

TMDC	Layers source*	Temperature (K)	Mobility (cm2·V−1·s−1)	Ref
MoS2	CVD	300	46	42
MoS2	ME	5	1000	44
MoS2/Graphene	ME	1.9	1300	133
MoS2	ME + S treatment	300	85.9	157
MoS2	ME + S + H2 treatment	300	98.2	157
MoS2/Si3N4	ME	300	71.8	45
MoS2/HfO2	ME	300	150	158
MoS2/HfO2	ME	4	168	158
MoTe2	CVD	300	4	159
1T/2H-MoTe2	CVD	300	16.2	159
WS2	CVD	300	33	42
WSe2	ME	300	0.2	160
WSe2	ME	300	16	161
WeS2/Br doped	ME	300	27	162
CoSe/ WSe2	ME + Co doping	300	42.1	163
WSe2/Al2O3	ME	300	202	49

*2D layers obtained by CVD Chemical vapour deposition. ME Mechanical exfoliation. S and H₂ = Sulfur and Hydrogen treatments.

Fig. 2. Intrinsic SR of TMDCs.

a Field-effect mobility as a function of temperature for three MoS₂ devices deposited on SiO₂ (black), Al₂O₃ (green), and HfO₂ (red). Reproduced with permission of ref. ^48,174 (b) The photocurrent of the protected MoS₂/HfO₂ devices at 1 V. Reproduced with permission of⁶¹. c Responsivity as a function of light wavelength of the hot-electron-assisted MoS₂ photodetector integrated on a silicon nitride waveguide. Reproduced with permission of ref. ⁶². d MoSe₂ PL (colour scale in counts) is plotted as a function of back-gate voltage. Illustration of the gate-dependent trion and exciton quasi-particles and transitions. Reproduced with permission of ref. ⁶⁸. e Differential reflectance evolution during three strain cycles of MoS₂ strained. (left) The colour map shows reflectance intensity (colour axis), uniaxial strain (%) on the horizontal axis, and energy on the vertical axis. (middle) Comparison of two spectra at 0% and 1% strain, highlighted by dashed lines in blue and red, respectively. Dash lines indicate A and B exciton peaks for unstrained MoS₂. (right) A and B exciton peak energies versus tensile strain, fitted to a linear trend to determine the gauge factor. Reproduced with permission of⁷⁸.

Other charge mobility blocking factors can be partially faced through molecular functionalization and heterostructure formation (see Table 2 and sections 2 and 3). Besides, low current leakages of ultrathin 2D layers can permit the fabrication of ultrascaled transistors by combining nanolithography techniques with a “growth-as-fabrication” approach, pushing the boundaries of FETs size limits^39,40, even on flexible substrates⁵⁰ and going a step further in the actual “more than Moore” race.

Table 2.

Ratio of bandgap closure after applying uniaxial and biaxial strain on semiconductive TMDCs (experimental works)

TMDC	Uniaxial strain (meV/%)	Biaxial strain (meV/%)	Ref.
MoS2 (1L)	-37	-70	164,165
MoSe2 (1L)	-41	-54	166
MoTe2 (1L)	-33	-	164
WS2 (1L)	-44	-130	164
WSe2 (1L)	57	-	164
ReS2 (few-L)	-60	-	167

Light as stimulus

The fact that 2D TMDCs possess strong light-matter interactions leads to efficient generation and manipulation of excitons⁵¹. This makes TMDCs highly promising for use as active components in optoelectronics, photosensors, and electroluminescence SR devices. In these applications, light can function as both input and output signals^52–57.

(i) Light as input signal: light can be used as an external stimulus to create extra charge carriers, reducing the TMDC electrical resistance (stimulus: light irradiation; response: current). In this context, the first single-layer MoS₂ optoelectronic device was a phototransistor introduced by Zhang et al. This work evaluated the photoresponsivity of MoS₂ exfoliated by micromechanical cleavage, reaching very low dark currents and high light-induced I_on/I_off ratios⁵⁸. However, it must be noticed that in phototransistors, a complex interaction between light, TMDCs nature, and voltage gate may give rise to significant performance variati.ons (stimulus: light irradiation + gate voltage; response: current). Subsequently, Kis et al. optimized Zhang’s device, achieving an impressive maximum external photoresponsivity of 880 A·W⁻¹ at a wavelength of 561 nm⁵⁹. Moreover, like for regular FETs, by encapsulating the MoS₂ transistor in HfO₂ it is possible to further enhance charge mobilities and I_on/I_off ratios⁶⁰. Following this protocol, Cheng et al., prepared a phototransistor based on WS₂ protected with HfO₂, which exhibited a high responsivity of 1093.1 A·W^–1, and a remarkably external quantum efficiency of 2.1 × 10⁵ % Fig. 2b⁶¹. On the other hand, wave-guide TMDCs photodetectors, have also great potential as they can operate in the telecom band thanks to their remarkable photoresponse in the near-infrared, Fig. 2c⁶². Recently Deng et al., reported a photodetector based on monolayer MoS₂ field effect transistor that operates under the conditions of zero gate bias voltage⁶³. This effective light-current interaction has also been used in photovoltaic devices, converting sunlight into electrical currents for solar cells, and sensors^64,65. Unfortunately, it is not straightforward to compete with established commercialized technology as in cost or power conversion energy (PCE). In this context, in 2014, Tsai et al. used large-scale high-quality chemical vapour deposited (CVD) MoS₂ to form a p-type II heterojunction with p-Si substrate, reaching for the first time, more than 5% PCE with a single TMDC monolayer⁶⁶.

Parallelly, light can be used as an effective stimulus in photothermal conversion processes (stimulus: light; response: heating). So, chemically exfoliated MoS₂ has been proven as an efficient NIR photothermal agent⁶⁷.

(ii) Light as input and output signal: Some TMDCs, such as 2H-MoS₂, 2H-WS₂, or WSe₂, exhibit significant photoluminescence (PL) emission at visible wavelengths even at room temperature when scaled down to a single layer, which can be also seen as a simple SR behaviour where the light of different wavelengths works both as stimuli and response. One step further, several approaches have been proposed to reversibly modulable TMDCs optical response for broadening the possibilities as SR material. One of the most effective techniques involves tuning PL through electrical gating. This method allows for precise electrostatic control of exciton charging using a standard back-gated FET. In this regard, Fig. 2d illustrates how the prevalence of negative (positive) trions at around 1.65 eV in MoSe₂ single layers can be adjusted by applying positive (negative) electrical back gates with respect to the neutral exciton^68,69. In this case, in spite of PL is originally induced by a light source, the stimuli-responsive behaviour consists of the electric modulation of the emitted light from the 2D material (stimulus: gate voltage; response: photoluminescence).

Strain as stimulus

The band structure of semiconductors can be strongly perturbed by mechanical strain, giving rise to the possibility of using mechanical deformation to tune their electronic and photonic performance⁷⁰. This principle, known as strain engineering, has been used for a long time in semiconductor device manufacturing. From the point of view of TMDCs, this technique is usually carried out in-plane, either as a compressive or tensile force. Hence, the introduction of strain originates an elongation or shortening of the atomic M-X bonds, altering the lattice symmetry. As a result, their electronic structure changes, affecting their optoelectronic properties^71,72. For instance, the application of tensile strain over MoS₂ increases the Mo-Mo and Mo-S length, reducing the orbital hybridization and hence the bandgap⁷³. According to theoretical calculations, under the application of a 1-2 % tensile strain, the direct bandgap of 2H-MoS₂ turns into indirect, and over the 10 % to metallic⁷⁴. This strain/bandgap dependence has been vastly explored by the scientific community, and which results are summarized in Table 2^75–77. From the point of view of SR devices this strain can be used, as Castellanos et al. demonstrated, to modulate the electrical conductivity of MoS₂ as a function of the applied uniaxial strain, Fig. 2e⁷⁸. The effects of compressive strain on the properties of TMDCs remain rather unexplored compared to those of tensile strain, however, biaxial compressive strain can be applied to single-layer TMDCs deposited on polymeric substrates by cooling down the samples, giving rise to photodetectors with strain-modulated bandwidth^79,80 and a considerable strain-induced modulation of neutral exciton energies^79,80, among others.

It is worth considering pressure as an alternative to strain when applying mechanical stimuli to TMDCs, as it can effectively modulate the crystal and electronic structures⁸¹. Several studies have explored the effects of applying low and high pressures in the out-of-plane direction of the layers. The impact of pressure depends on the nature of the TMDC and on whether the sample is single or multilayer. When pressure is applied to a multilayer sample, it mainly affects the space between layers and alters interlayer interactions/hybridizations^82,83. In contrast, for a single-layer sample, pressure directly affects the atomic structure within the layer, thereby altering its band structure⁸⁴.

Chemical enhancement of the intrinsic SR behaviour of TMDCs

Functionalization of 2D TMDCs is an extremely appealing way to enhance their intrinsic voltage, light, or strain-induced SR properties. In this context, depending on the specific SR behaviour to be improved, TMDCs can be modified through molecular functionalization, with metallic nanostructures, or with other 2D materials, Fig. 3. In this section we will discuss these three different approaches to boost the intrinsic SR of TMDCs.

Fig. 3. SR behaviour of functionalized TMDCs.

a Schematic illustrations of TMDC functionalized sheet. b μ–T characteristics for defective WS₂ on SiO₂ (black), and MDPS healed WS₂ on Al₂O₃ (red). Reproduced with permission of⁹⁵. c Infrared thermal images of mice bearing a HEPG2 tumour injected with MoSe₂(Gd³⁺-3)-PEG, on top without irradiating and on bottom with irradiation. Reproduced with permission of¹¹⁵ (d) Schematic illustrations of a TMDC decorated with Pd NPs, and the catalytical process of H₂ reactivity with Pd. e Pt Decorated MoS₂ photocurrent in the presence of different concentrations of SARS-CoV-2. Reproduced with permission of¹¹⁸. f Gas sensing responses for Au decorated MoS₂. Reproduced with permission of¹²¹. g Schematic illustrations of a Graphene/TMDC heterostructure of a SiO₂ substrate. h Polarization curves for C₃N₄ nanofibers (black), N-doped graphene (grey), MoS₂ (red), and the C₃N₄/Graphene/MoS₂ heterostructure (blue). Reproduced with permission of¹²⁸. i Time-dependent photocurrent of the MoTe₂/graphene heterostructure (black) and pure MoTe₂(red) devices under 980 nm irradiation and 1V. Reproduced with permission of¹³⁴.

Molecular functionalization

Molecular functionalization can be done by the non-covalent or covalent attachment of molecules onto 2D TMDCs. Non-covalent functionalization involves attaching molecules via weak van der Waals or electrostatic interactions²⁵. Due to the weak interaction between components, the chemical structure, and physical properties of TMDCs are not strongly affected by this functionalization. On the other hand, the covalent approach involves forming new bonds between the 2D layer and the selected molecules, resulting in a strong coupling between both components and a chemical modification of the layer structure⁸⁵. The two main approaches for this functionalization involve utilizing the nucleophilic character of edges and in-plane sulfur atoms to form new C-S bonds^86–88, and the chemistry at sulfur vacancies to anchor mainly molecules containing thiols^89,90.

Molecular functionalization of TMDCs has been extensively explored as a means to (1) enhance their voltage-induced SR, thanks to the healing effect of covalent functionalization or the doping caused by molecules in direct contact; and (2) increase the colloidal stability of exfoliated TMDCs for their use for example in photothermal therapy and theragnostic, taking advantage of their remarkable light to heat conversion. All these possibilities are discussed below.

• Voltage as stimulus

As mentioned previously, low charge mobility values hinder the efficient performance of TMDC-based electronic devices. In this regard, the main reason behind this handicap of TMDCs is commonly attributed to coulomb impurities, sulfur vacancies, or adsorbed molecules^44–46. In this scenario, molecular functionalization can be employed to achieve a healing effect, reducing electron traps and scattering centers^91,92. Therefore, Leong et al. demonstrated that through the sulfuration of exfoliated MoS₂, the sulfur vacancies issue can be addressed, producing a 'healing' effect that significantly increases the electron mobilities up to 86 cm² V⁻¹ s⁻¹⁴². Building on this concept, S. Ippolito et al. designed a molecular strategy to enhance the electrical performance of devices based on defective liquid phase exfoliated TMDCs. They used dithiolated conjugated molecules to simultaneously repair sulfur vacancies in the chemically exfoliated MoS₂, while covalently bridging adjacent flakes. This approach promotes percolation pathways for charge transport, achieving a reproducible field-effect mobility of 10⁻² cm² V⁻¹ s⁻¹, I_on/I_off ratios of 10⁴, and switching times of 18 ms. This represents an improvement of approximately one order of magnitude compared to unmodified, chemically exfoliated MoS₂⁹³. This healing strategy was also used to functionalize MoS₂ and WS₂ with (3-mercaptopropyl)trimethoxysilane (MPS), achieving charge mobilities as high as 81 cm²V⁻¹s⁻¹ and 83 cm²V⁻¹s⁻¹ (Fig. 3b)^94,95, respectively.

On the other hand, molecular functionalization may be used to produce a doping effect⁹⁶, altering the charge carrier density⁹⁷, and the photo response of TMDCs by around three orders of magnitude⁹⁸. As Table 3 reflects the n (p)-doping increases the electron (holes) density and mobility.

Table 3.

Comparative table of bare TMDCs as how their Mobility, Charge density, and I_on/off ratio can be improved by molecular functionalization at room temperature

TMDC	Molecule	Layers source	Mobility (cm2V−1s−1)	Charge density (cm–2)	Ref
MoS2	BDT	BuLi	10−2	-	93
MoS2	NMP	ME	22.2	1.6 × 1013	168
MoS2	DMF	ME	26.6	9.7 × 1012	168
MoS2	DMSO	ME	29.2	6.4 × 1012	168
MoS2/h-BN/WSe2	PPh3	ME	60	1.1 × 1013	169
MoS2	MPS	ME	81	1.6 × 1013	94
MoS2	CsPbBr3	CVD	4.8 (at 0.1 V)	-	170
MoTe2	NH3	ME	180	6.9 × 1012	171
WS2/Al2O3	MPS	ME	83	-	95
WSe2	DNTT	ME	9.3	-	160
WSe2	PFS	ME	150	-	172
WSe2	AHAPS / PFS	CVD	5.7 (e)/20 (h)	-	172
WSe2	4-NBD/DETA	CVD	25 (e)/ 82 (h)	-	173

BDT (1,4-benzenedithiol), NMP (N-Methyl-2-pyrrolidone), DMF (N,N-Dimethylformamide), DMSO (Dimethyl sulfoxide), PPh3 (Triphenylphosphine), MPS ((3-mercaptopropyl)trimethoxysilane), DNTT (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), PFS (trichloro(1H,1H,2H,2H-5 perfluorooctyl)silane), AHAPS (N-[3-(trimethoxysilyl)propyl]ethylenediamine), 4-NBD (4-Chloro-7-nitrobenzofurazan), DETA (Diethylenetriamine).

• Light as stimulus

The efficiency of phototransistors can be also enhanced by the functionalization of TMDCs. For instance, N. Hue et al. showed how WS₂ phototransistor performance can be significantly enhanced by the absorption of reductive molecules like NH₃ on the 2D layers. Thus, at NH₃ atmosphere, the photocurrent generated per unit power of the incident light on the effective area increases from 5.7 A/W to 884 A/W⁹⁹. Furthermore, through chemical functionalization of TMDC surfaces, it is possible to modulate their carrier density and, thus, their electrical photoresponse. In this regard, functionalization with electron acceptor systems like the octadecyl trichlorosilane or poly(pyrrole-co-O-toluidine) has been demonstrated to p-dope exfoliated MoS₂, increasing the I_on/I_off ratio after light irradiation by up to 400. In contrast, when functionalizing with electron donor groups, the MoS₂ photoresponse is reduced^100,101.

On the other hand, the remarkable Near Infrared (NIR) photothermal conversion of some exfoliated TMDCs makes them extremely appealing as contrast agents, for photothermal therapy, and drug delivery. Unfortunately, most 2D TMDCs do not form long-term stable suspensions in many common polar solvents such as water, hampering their use as SR material for in vivo applications¹⁰². In this regard, a very attractive manner to face this issue is via their molecular functionalization with polymers like polyethylene glycol (PEG), or other molecules that can act as capping agents, enhancing nanosheet stability and biocompatibility, while also acting as a platform for loading different anticancer drugs such as doxorubicin (DOX), chlorin e6 (Ce6), 7-ethyl-10-camptothecin (SN38), or cytosine–phosphate–guanine (CpG)^103,104. Remarkably, DOX-loaded MoS₂-PEG nanosheets exhibit an outstanding synergistic anticancer effect upon NIR light irradiation. Although PEG functionalization is the most commonly employed method to date, other molecular coatings such as Chitosan¹⁰⁵, organosilanes¹⁰⁶, or Polyaniline¹⁰⁷ have been used instead, allowing some of them not only to load drugs but also genes¹⁰⁸. To push the field further, Liu et al. doped PEG functionalized MoSe₂ with Gd³⁺, enabling the use of this smart material not only for drug delivery but also as a contrast agent for photoacoustic imaging, thus obtaining a SR material with theragnostic applications¹⁰⁹. Similarly, other TMDCs like WS₂¹¹⁰, TiS₂¹¹¹, VS₂¹¹², TaS₂¹¹³, and ReS₂^114,115 with different molecular coatings have also been utilized, displaying comparable levels of efficiency and biocompatibility. It should be noted that due to the high atomic number of Re (Z = 75), the PVP-ReS₂ can be used without further doping as contrast agents for computed tomography and photoacoustic imaging. At the same time, it exhibits one of the strongest NIR absorption with an ultrahigh photothermal conversion efficiency (79.2%), being the most promising TMDC for cancer treatment, Fig. 3c¹¹⁵.

NPs decoration

The NPs decoration of TMDCs is usually performed by chemical- (using a reductant, inter-matrix synthesis or the organometallic approach)^116,117 or photo-reduction deposition of the cationic precursors.

With the introduction of plasmonic metallic NPs, the photoelectronic conversion efficiency is busted improving TMDCs sensing performance to different kind of molecules.

The TMDCs decoration with metallic NPs like Ag, Au, or Pt (Fig. 3d) boosts their photoelectrochemical response, improving the conductivity and the separation of photogenerated electron-hole pairs, and thus, enhancing TMDCs performance as biosensors, even for viruses like COVID, Fig. 3e^118–122. Note that in these MoS₂/metallic NPs heterostructures, the detection can be highly selective depending on the choice of metal. Won Jan et al., demonstrated that by decorating 1T MoS₂ with Au NPs, in the presence of volatile organic compounds with oxygen groups like acetone or ethanol, an increase of up to 130 % in electrical response is displayed (Fig. 3f)¹²¹. These TMCD/Au NPs heterostructures have also been extensively utilized for the detection of biomolecules such as mRNA or pathogens^123–125. A singular example of sensing with these heterostructures was provided by Yang et al., using MoS₂ functionalized with Au and Pt NPs with satisfactory results¹¹⁸. The sensitivity of such devices was pushed to the limit by Wang et al., by synthesizing a MoS₂/ReS₂/Au NPs heterostructure, achieving a sensitivity at the attomolar range for miRNA-21 detection¹¹⁹. On the other hand, TMDCs functionalized with NPs can improve the performance of the 2D materials as chemoresistive gas sensors. Thus, MoS₂ with Pd NPs is a great H₂ gas sensor, thanks to the unique interaction between Pd and H₂ gas, Fig. 3d. According to previous reports, when H₂ molecules approach the surface of Pd, they undergo dissociation into two atomic hydrogen species, resulting in a significant increase in the MoS₂/Pd NPs resistance¹²⁰.

2D heterostructures

2D heterostructures have a massive potential due to the strong coupling that can exist between components (Fig. 3g), leading to enhanced light-current interactions, doping, or even dielectric screening effects¹²⁶. Two large families of 2D heterostructures could be distinguished, these are direct contact 2D/2D heterostructures, prepared by dry methods and known as van der Waals heterostructures, and crosslinked 2D/2D heterostructures with molecular bridges and prepared by methods in solution. In any case, in this review, we will not distinguish between two kinds of heterostructures and will tackle them together.  Therefore, TMDCs heterostructures with other 2D materials have been mainly employed as photo/electrocatalysts, and in opto/electronics¹²⁶.

• Catalysis

When the appropriated 2D material is coupled with exfoliated TMDCs a charge transfer between components can occur upon light irradiation, enhancing the TMDCs catalytical activity. A perfect example of this phenomenon was reported by Rao et al., covalently anchoring C₃N₄ on chemically exfoliated MoS₂. This system displays 68 times better performance as a catalyst for the hydrogen evolution reaction (HER) than a mechanical mixture of both individual components, due to a photoinduced charge transfer from C₃N₄ to MoS₂ (Fig. 3h). This occurs because the C₃N₄ HOMO has higher energy and can transfer electrons to the MoS₂ LUMO upon photoexcitation¹²⁷. This same configuration was later improved by growing MoS₂ over a C₃N₄/Graphene heterostructure, where the presence of graphene facilitates the charge transfer from the C₃N₄ to the MoS₂. Reaching unprecedented high yields (6.97 × 10⁻³ F cm⁻²), which maintains over 3000 Cycles¹²⁸. Other heterostructures based on rGO/MoS₂ or MoS₂/MoSe₂ have been prepared over the years, searching for the most efficient HER catalyst^129–132. In this context, the best performances were displayed by the covalent combination of Phosphorene with MoS₂/MoSe₂¹³². The authors examined visible-light-induced HER of the hybrids, obtaining a HER activity of 26.8 mmol h^–1 g^–1 and 20.7 mmol h^–1 g^–1 for the MoS₂ and MoSe₂, respectively. Remarkably, such Phosphorene/MoS₂ heterostructure, presented an overpotential close to that of Pt (the benchmark in HER), indicating it to be a superior electrocatalyst for the HER, and exhibiting activity in alkaline electrolytes comparable with those of commercial electrocatalysts.

• Electronics

Improving the performance of TMDC-based FETs by achieving better carrier mobilities can be done by encapsulating the material with a dielectric gate. This helps to prevent the absorption of oxygen or water molecules on the 2D material. Interestingly, it has been shown that coulomb impurities scattering can be reduced by sandwiching a MoS₂ monolayer between h-BN, at temperatures below 100 K. This allows for high mobilities up to 1000 cm²·V⁻¹·s^−143,133.

The excellent photoelectrical performances of TMDC can be significantly boosted in 2D heterostructures, combining conductive and light SR 2D materials. Following this idea W. Yu et al., fabricated a photodetector based on a Graphene/MoTe₂ heterostructure, combining the high electron mobility of graphene with the MoTe₂ high yield of optical carriers¹³⁴. This device could achieve a 970.82 A·W^–1 photoelectric response at 1064 nm, 120 times higher than the bare MoTe₂ at 980 nm, Fig. 3i. This same concept was expanded to ReTe₂, obtaining a 10³ times higher current for the graphene/ReTe₂ heterostructure than the bare TMDCs¹³⁵. Another interesting approach was presented by Hou et al., who used a few layers of the semimetal PtSe₂ as metal electrodes, in contact with a monolayer of WS₂, which acts as the photoactive material⁶⁵. The produced devices exhibit a satisfactory I_on/I_off ratio of 1438 under 1.24 mW of 532 nm laser irradiation.

Induced SR behaviour in TMDCs

Bistability refers to the ability of a material or device to exist in two distinct states within the same range of external stimuli, allowing it to "remember" its state^136–138. Bistable stimuli-responsive systems take advantage of this by enabling devices to switch between these states with minimal energy and without needing continuous power to maintain them. This makes them more reliable, energy-efficient, and effective compared to conventional SR systems, particularly in applications actuators, and memory storage devices.

Unfortunately, TMDCs do not naturally have bistability in their stimuli-responsive behaviour. Herein, we explore the development of hybrid stimuli-responsive materials by combining TMDCs with reversible bistable molecules or molecular compounds. The stimuli-responsive behaviour in these hybrids arises from the coupling between TMDCs and the two possible configurations of the bistable system. In this section, we will explore in detail this new and exciting approach to fabricating stimuli-responsive devices based on TMDCs, which may exhibit memory through their functionalization with molecules that may cause stimuli-induced reversible doping (SR TMDCs via molecular doping) or molecular materials that can reversibly strain them (SR TMDCs via mechanical strain).

SR TMDCs via molecular doping

Following this new concept, one of the most interesting approaches for obtaining a real smart material consists of the TMDC interaction with light-induced-SR molecules^139–141. Following this idea, Margapoti et al., deposited exfoliated MoS₂ on top of a mixed self-assembled monolayer spacer of photo-switchable azobenzene molecule and mercapto-hexanol, and observed that upon cis/trans azobenzene photoconversion, the MoS₂ PL intensity triples. This was attributed to the n-doping produced by the trans-azobenzene, promoting the non-radiative trion formation, to later decrease back when switched into the cis conformation¹⁴². This effect was later improved by Samori et al., by physisorption of a similar azobenzene molecule on top of exfoliated MoS₂. Exhibiting a higher number of azobenzenes per MoS₂ unit compared to the work of Margapoti et al., Fig. 4a¹⁴³. This not only results in an increase in the PL modulation upon the trans/cis switching but also an energy shift in the A_1g vibrational mode. When the trans conformation is present the n-type doping promotes the MoS₂ trion formation upon light absorption. In contrast, when the azobenzene is in the cis conformation, the neutral exciton predominates, leading to a significantly higher quantum yield, while redshifts the A_1g vibrational mode¹⁴³. Moving to specific chemical functionalization, Morant et al. functionalized chemically exfoliated MoS₂ with photoswitchable diarylethene (DAE) derivatives capable of undergoing ring opening and closure upon light irradiation¹⁴⁰. Therefore, MoS₂ layers were functionalized with two DAE. One has two amine groups, allowing for an electrostatic interaction with the 2D layers, and the second one has two diazonium groups, for their covalent attachment. Unfortunately, the stabilizing role of MoS₂ prevented a complete ring closure/opening, thus, limiting the DAE bistability.

Fig. 4. SR via TMDCs reversible molecular doping.

a Visual representation of the photochromic switching of the (4-(decyloxy)azobenzene) molecule on the MoS₂ (top), and photoluminescence of the MoS₂ and the AZO/MoS₂hybrid in both conformations (bottom). Reproduced with permission of¹⁴² (b) Scheme of the FET device architecture based on WSe₂/DAE device on top, and the electrical characterization of WSe₂/DAE blend FET device upon light irradiation on bottom. Reproduced with permission of¹⁴⁶ (c) Schematic diagram of the DAE/WSe₂/ P(VDF-TrFE) ternary device on top, and the transistor electrical characterization during 4 switching cycles with different stimuli on bottom. Reproduced with permission of³³.

Furthermore, this reversible doping enables precise manipulation of electrical outputs in solid-state devices through the control of the molecule bistability, adjusting the conductivities of 2D materials through external stimuli^144,145. Pursuing this goal, M. Gobbi et al., prepared a transistor based on exfoliated MoS₂ that was electrostatically functionalized with the photochromic Spiropoirano (SP) molecule¹⁴⁶. SPs are photochromic molecules that exhibit reversible photochemical isomerization between a neutral closed-ring and a zwitterionic open-ring isomer known as merocyanine (MC), characterized by a larger molecular dipole. In solution, the SP→MC isomerization is initiated by irradiation with ultraviolet (UV) light, while the MC→SP back isomerization is achieved either thermally or through irradiation with visible light (light stimuli; response-current). In this system, the presence of positive vertical dipoles in the photogenerated MC molecule leads to a reduction in the Work Function (or n-type doping), resulting in a slight shift in the threshold voltage toward negative values. This shift is consistent with the induction of an electron density, n = 4.6 × 10¹² cm⁻², Fig. 4b. Thanks to the non-disruptive nature of the electrostatic functionalization, the devices based on MoS₂ maintained high electrical performance even after the formation of the SP and MC assemblies, retaining mobilities above 25 cm² V⁻¹s⁻¹.

Subsequently, this concept was further extended using the AZO molecule (4-(decyloxy)azobenzene), which was also non-covalently attached to the MoS₂ via the large alkyl groups¹⁴³. When transitioning between trans and cis conformations upon light irradiation, the PL of MoS₂ undergoes a dramatic change. The natural trion PL of exfoliated MoS₂ is suppressed in favour of the uncharged exciton, indicating that cis-AZO is capable of p-doping the 2D material, while trans-AZO n-dopes it. This doping effect is clearly observed in the back-gated functionalized MoS₂ FET, where the p-doped cis-AZO compound alters the MoS₂ carrier concentration (about 1.11 × 10¹² cm^–2), giving rise to a current decrease of ~34% at gate voltage (V_G) equal to 60 V. This cis/trans conductivity modulation finds its origin in the different dipolar moments that both conformations present, acting as an extra molecular gating.

Even more, the molecular gating can also be induced electrochemically (electrical gate-stimulus; current response), as demonstrated by Zhao et al., through the functionalization of MoS₂ with ferrocene-substituted hexanethiol molecule¹⁴⁷. The device featured two gates: a bottom gate, serving as the conventional electrical gate, and a top gate constructed with an ionic liquid, used to electrically reduce and oxidize the Ferrocene. This setup allowed for the clear observation of distinct electrical responses depending on the oxidation state of Ferrocene. In the oxidized state, due to the induced n-type doping of MoS₂, the electron concentration increased by ~1.10 × 10¹² cm⁻², generating an increase in the recorded current of around 25%. The configuration of this device was subsequently enhanced by incorporating a ferroelectric material as the top gate, along with photochromic molecules, enabling the fabrication of ternary or even quaternary responsive transistors capable of responding to multiple stimuli (light, temperature, or current-stimuli; current-response)^33,34. A great example of this multi-stimuli responsive device was presented by H. Qiu et al., They placed exfoliated WSe₂ on a SiO₂ back-gated substrate functionalized with a light-responsive DAE and coated the top surface with an electrically responsive copolymer layer of poly(vinylidene fluoride–trifluoroethylene) (P(VDF-TrFE)), Fig. 4c³³. In this manner, a multi-stimuli-responsive FET was fabricated, enabling reversible and precise modulation of the output current through either light irradiation or an electric field. Focusing on the top ferroelectric gate, when the positive voltage exceeds the coercive voltage of P(VDF-TrFE), the ferroelectric becomes polarized in the downward direction (P_down), engaging the electrons in WSe₂ to compensate for the polarization charges. Conversely, with negative voltages, the polarization occurs in the opposite direction (P_up), with the WSe₂ engaging the holes. This effect results in higher electronic current for P_down. Therefore, by combining ferroelectric polarization with the light-induced molecular gating of DAE, an overall modulation of the output current of up to 87% could be achieved in this DAE/WSe₂/P(VDF-TrFE) FET. This current modulation remained robust for over 20 cycles without noticeable attenuation, and data retention persisted for over 1000 h. These accomplishments meet some of the endurance requirements necessary for practical high-density non-volatile memories.

SR TMDCs via mechanical strain

Alternatively, it has been predicted that straining 2D-TMDCs modifies the size of the bandgap and the effective masses of the carriers. As a result, in our group, we focused our attention on developing strain-responsive materials by combining exfoliated TMDCs with molecular materials of different scales that are capable of reversible changing their volume upon the application of an external stimulus^140,148,149.

In this regard, Torres-Cavanillas et al., developed an approach to get a real SR material based on the spin-crossover (SCO) phenomenon and 2D-TMDCs. The SCO phenomenon is one of the most remarkable examples of molecular bistability observed in certain octahedrally coordinated transition metal atoms^150,151. These compounds can be switched between two different electronic states, namely the low spin and high spin states, through the application of an external stimulus, typically thermal^152–154. Each state exhibits distinct mechanical properties, making them well-suited for mechanical actuation¹⁵⁵. The strategy reported by Torres-Cavanillas et al., relies on the covalent attachment of SCO NPs of the coordination polymer [Fe(Htrz)₂(trz)](BF₄) onto chemically exfoliated MoS₂, following a two-step protocol (Fig. 5a)¹⁴⁷. The first step involved the covalent functionalization of chemically exfoliated 1T-MoS₂ using (3-iodopropyl)trimethoxysilane. It is noteworthy that the functionalization process led to the recovery of the semiconductive 2H polytype, possibly due to electron withdrawal from the 2D layer upon the formation of new C-S covalent bonds. Next, the silane group on the MoS₂ surface served as an anchor, allowing polymerization with silica-coated [Fe(Htrz)₂(trz)](BF₄) nanoparticles (NPs) with sizes of ~70 nm (or 40 nm). The resulting material exhibited the characteristic PL of the bare 2H-MoS₂, but with the possibility to tune the quantum yield and exciton energy via the thermal spin transition of the NPs (Fig. 5b), as well as by light irradiation. Note that the energy shift of the PL, which corresponds to the MoS₂ energy gap, perfectly emulates the spin transition of the bare NPs. Furthermore, this molecular heterostructure not only exhibits a PL energy dependency with the Fe NPs’ spin state but also undergoes a reversible modulation in its electrical conductivity. As seen in Fig. 5c, when MoS₂ is functionalized with the 70 nm NPs, the electrical conductivity of the composite displays a hysteresis resembling the magnetic hysteresis of the NPs. Firstly, there is an abrupt increase in the composite conductivity by around two orders of magnitude at 380 K, induced by the NPs switching from low-spin (LS) to high-spin (HS), stressing the MoS₂ and closing its bandgap. Then, when cooling down 340 K, this stress is released as the NPs return to their original LS state, thus, restoring their initial conductivity. Notably, when smaller NPs (40 nm) are used for mechanical actuation, the same effect is observed, albeit with a less abrupt response, attributed to the reduced actuation caused by the smaller NPs. Even more, the material can be dispersed in many common solvents, facilitating its processability, while demonstrating remarkable robustness, enduring 20 complete thermal cycles (300 K–400 K) without exhibiting any signs of fatigue.

Fig. 5. SR via TMDCs molecular stain.

a Schematic representation of the MoS₂/[Fe(Htrz)₂(trz)](BF₄). b Photoluminescence of the MoS₂/[Fe(Htrz)₂(trz)](BF₄) 70 nm, in the inset at 355 K in both spin state, and the thermal dependence of the PL position. c Thermal variation of the conductance in the heating and cooling modes for MoS₂ functionalized with [Fe(Htrz)₂(trz)](BF₄) NPs of 70 nm (blue empty spheres), and 40 nm (green empty spheres). Reproduced with permission of ref. ¹⁴⁸. d vdWH formed by WSe₂ (purple dashed lines; thicker WSe₂ is enclosed by blue dashed lines) below a SCO crystal (orange dash lines). Scale bar: 5 µm. e-f Normalized photoluminescence at different temperatures for the WSe₂ heterostructure, and its PL position thermal dependency, respectively. Reproduced with permission of¹⁴⁹.

Next, based on the same idea, Boix-Constant et al. developed a van der Waal (vdW) heterostructure of WSe₂ and a crystal of the SCO compound Fe-(3-Clpyridine)₂-[Pt(CN)₄], Fig. 5d¹⁴⁸. In the WSe₂/Fe-(3-Clpyridine)₂-[Pt(CN)₄] hybrid, a clear PL shift, resembling the thermal SCO hysteresis of the bare crystal was observed (Fig. 5e, f).

This modulation is attributed to the closure of the WSe₂ bandgap, induced by the strain resulting from the volume increase associated with the high spin state. Later on, combining the previous two ideas of vertical heterostructures by dry methods and TMDCs chemical functionalization by methods in solution, Fe(Py)₂[Pt(CN)₄] compound was directly grown layer by layer on MoS₂¹⁵⁶. Unfortunately, in this Janus heterostructure, no electrical modulation was observed as a function of the SCO spin state. This absence of modulation is likely attributed to the low thickness of the film (ranging from 2 to 30 nm), a highly defective structure at the interface and the smaller change in volume during the spin transition compared to Fe(Htrz)₂(trz). Nevertheless, the authors, observed that the robustness of the SCO films could be enhanced with the presence of 1T-MoS₂, partially preventing oxidation.

Conclusions and outlook

The diverse range of materials and properties within the TMDC family, combined with the boundless possibilities of chemistry, presents unparalleled opportunities for customizing stimuli-responsive devices.

Starting from the modulable nature of TMDCs originated in tuneable electronic bandgaps, one step ahead, their chemical functionalization enables precise control over charge-carrier doping in 2D semiconductors and the fine-tuning of their electrical, optical, thermal, and sensing properties. Moreover, adding large molecules provides colloidal stability and biocompatibility, making TMDCs perfect candidates for photothermal therapy or smart drug delivery. The field is advancing rapidly, driven by new developments in TMDC chemical design that enhance their intrinsic SR properties. Recent breakthroughs have led to the creation of systems that combine 2D-TMDCs with bistable molecules, enabling reversible control of carrier mobilities and energy gaps with memory properties.

Despite these significant advances, several critical challenges remain to fully harness the potential of TMDC hybrids in SR applications: (i) Maximize the interaction at the TMDC basal plane, to produce more effective functionalization, enhancing TMDCs response to the external stimuli. Since the basal plane, which constitutes most of the TMDC surface, is less reactive than the edges, new strategies must be developed to boost basal plane functionalization in a controlled manner.; (ii) developing effective techniques to prevent desorption or decomposition of functionalized molecules during device operation, such as encapsulation or operating under milder conditions; (iii) Finding strategies to efficiently scale up the production of hybrid heterostructures and composites combining SR molecules with TMDCs, and developing new ones which exhibit closer and more effective couplings.

In summary, the incredible potential of molecular science, combined with the 2D world, including molecular bistability and property modulation, is poised to unlock ground-breaking technologies. TMDC SRMs are still nascent materials which may be the key to realizing this potential, opening the door to revolutionary advancements.

Author contributions

R.T.-C. and A.F.-A. contributed equally to this work by conceiving the review, compiling the data, and writing the manuscript.

Peer review

Peer review information

Communications Chemistry thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-024-01322-z.