Table of contents

Droplet evaporation is a ubiquitous phenomenon with numerous applications. It plays a pivotal role in life and industry since it concerns heat transfer in high efficiency to reach a desired temperature. However, to rationally mediate evaporation has always been a significant challenge. Here by studying the interactions of water molecules with graphene-covered substrate, we propose that graphene could effectively affect water evaporation rate by changing the length of contact line. More importantly, evaporation per length of contact line before and after graphene coverage shows negligible change, suggesting graphene is transparent for evaporation (per unit contact length). Molecular dynamics simulations confirm experimental findings and indicate that principal evaporation events take place via single-molecule desorption at the contact line.

The crucial role of anion vacancies on the phase stability of synthetic metal chalcogenides was elucidated by demonstrating the environment-dependent, reversible phase transition (2H ↔ 1T/1T') of the MoS₂ films synthesized by chemical vapor deposition. The origins of the sulfur (S)-vacancy-controlled phase transition of the synthetic MoS₂ films were supported by various transmission electron microscopy (TEM) studies as well as density functional theory calculations. Under high-vacuum conditions, transition to the metastable 1T/1T' phases was induced by weak electron irradiation during a TEM observation and explained by the detachment of molecules chemisorbed on the S-vacancy sites, and the subsequent electron-delocalization (charge-transfer) process. In addition, a spontaneous backward transition to a 2H phase could be triggered by exposing the sample to air, which induced electron localization by re-adsorption of ambient molecules on the S-vacancy sites.

Screening plays a fundamental role in determining the quasi-particle band gap and many-body effects of two-dimensional (2D) semiconductors. However, the electronic and optical properties of individual layers in van der Waals (vdW) heterostructures are often assumed to remain unaltered from those of isolated layers. Here, we study band gap renormalization and exciton binding energy changes in WS₂/WSe₂ hetero-bilayers using absorption and photoluminescence excitation (PLE) spectroscopy. From the scaling behavior of higher order exciton energies, we estimate the exciton binding energy and quasi-particle band gap of constituent layers in the hetero-bilayer. We show that the band gap and the intralayer exciton binding energy of the constituent layers are reduced by as much as ~100 meV and ~70 meV, respectively, due to dielectric screening of the adjacent layer. These observations serve as an important step towards implementing rational design criteria for optoelectronic devices based on vdW heterostructures.

As complementary metal-oxide-semiconductor technology nodes are scaled down, lowering the contact resistance has become a critical problem for continued scaling. In this study, we suggested the reduction method of the Schottky barrier height, one of the main causes of contact resistance, by insertion of atomically thin two-dimensional (2D) materials between the metal and Si interface. Also, we found that the inserted 2D materials could modulate the work function of the metal and mitigate the Fermi level pinning, leading to reduced barrier height and, hence, reduced contact resistance of the metal–semiconductor junction. With the insertion of MoS₂ and WS₂ materials a two-layer thick, we achieved 160 meV reductions in the Schottky barrier height and increased the current density by 14 times for titanium contact to the n-type silicon. Finally, we suggested a modified band diagram of Ti/n-Si contacts with the 2D interfacial layer. Our results showed that employing 2D materials can be an alternative route for overcoming the contact resistance challenges in modern transistors.

Many transition-metal dichalcogenides, such as MoSe₂, are direct-gap semiconductors at monolayer thickness, which hold potentials in nano-electronics, optoelectronics, and some new concept spin- and valley-electronic applications. For device application, however, controllable doping of the materials is essential. Here we report hole doping of epitaxial MoSe₂ by nitrogen (N) plasma treatment with the aim of understanding the defect structure and its electronic characteristics. Examinations by annular dark field scanning transmission electron microscopy clearly reveal substitutional doping of N by replacing Se atoms in MoSe₂ monolayer upon N-plasma treatment, though creation of Se vacancies are also possible. Interestingly, we note an unexpectedly high concentration of 'dual defects', where both Se atoms in the top and bottom Se layers of MoSe₂ at the same lattice site are substituted by N and/or become vacant, suggesting a catalytic effect of defect formation. X-ray photoelectron spectroscopy and electron energy loss spectroscopy confirm the presence of N–Mo bonds. Photoemission spectroscopy reveals an impurity band as well as the Fermi level shift, confirming the p-type doping effect in MoSe₂ monolayer by N-plasma treatment. Consistent with the PES results, scanning tunneling spectroscopy measurement also reveal defect states peaked at 0.6–0.7 eV above the valance band maximum. The effectiveness of N-doping is discussed.

Manipulation of intrinsic electron degrees of freedom, such as charge and spin, gives rise to electronics and spintronics, respectively. Electrons in monolayer materials with a honeycomb lattice structure, such as the Transition-Metal Dichalcogenides (TMD's), can be distinguished according to the region (valley) of the Brillouin zone to which they belong. Valleytronics, the manipulation of this electron's property, is expected to set up a new era in the realm of electronic devices. In this work, we accurately determine the energy spectrum and lifetimes of exciton (electron–hole) bound-states for different TMD materials, namely WSe₂, WS₂ and MoS₂. For all of them, we obtain a splitting of the order of 170 meV between the exciton energies from different valleys, corresponding to an effective Zeeman magnetic field of 1400 T. Our approach, which employs quantum-field theory (QFT) techniques based on the Bethe–Salpeter equation and the Schwinger–Dyson formalism, takes into account the full electromagnetic interaction among the electrons. The valley selection mechanism operates through the dynamical breakdown of the time-reversal (TR) symmetry, which originally interconnects the two valleys. This symmetry is spontaneously broken whenever the full electromagnetic interaction vertex is used to probe the response of the system to an external field.

Black phosphorus (bP) is a promising two-dimensional (2D) material for opto-electronic applications. Strongly bound excitons with binding energies up to 0.3 eV and remarkably large trion binding energies up to 100 meV have been observed for supported monolayer bP. Surprisingly, this trion binding energy is significantly larger than those found in other 2D materials (e.g. about 30 meV in transition metal dichalcogenides). This has previously been ascribed to the quasi-1D nature of bP. In this work we show, using first principles calculations, that the trion binding energy of bP is indeed large (80 meV) when referenced to the lowest bright exciton but only 30 meV when its energy is measured relative to the lowest dark exciton. Our analysis thus shows that the trion binding energy in bP is not larger than in other 2D materials, and the previous conclusions have to be understood incorporating the large splitting between the dark and bright excitons in bP. We also explore the effect of substrate and in-plane strain of the exciton and trion binding energies and show that these effects do not change the main conclusions. Our results correct the misconception that trion binding energies in monolayer bP are particularly large due to its quasi-1D structure and contribute to the establishment of more a detailed understanding of optical properties of atomically thin semiconductors.

Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities.

Two-dimensional (2D) semiconductors—atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers—are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found (Bandurin et al 2017 Nat. Nanotechnol. 12 223–7) that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe/GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

Graphene is a two-dimensional material, a single layer of carbon atoms with a honeycomb lattice, which has increasing demand, especially wafer-scale graphene for applications in electronics industry. However, graphene requires to be supported on different substrates depending on its utilization and the transfer stage must be achieved without damaging the graphene structure. Wet chemical etching is the major route for graphene transfer widely applied right now; the loss of catalyst during the separation process being the main limitation. Mechanical peeling is a simple process but the quality of the separated graphene remains poor. Currently, electrochemical delamination or bubbling method and water assisted delamination are new and most promising methods for both efficient graphene transfer and possible catalyst reuse. This article provides a comprehensive review of the various graphene transfer methods without compromise in catalyst deterioration and it concludes with the future challenges in the domain.

We introduce the Computational 2D Materials Database (C2DB), which organises a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional materials distributed over more than 30 different crystal structures. Material properties are systematically calculated by state-of-the-art density functional theory and many-body perturbation theory ( and the Bethe–Salpeter equation for  ∼250 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and can be browsed online (http://c2db.fysik.dtu.dk) or downloaded in its entirety. In this paper, we describe the workflow behind the database, present an overview of the properties and materials currently available, and explore trends and correlations in the data. Moreover, we identify a large number of new potentially synthesisable 2D materials with interesting properties targeting applications within spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and forms an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.

Two-dimensional (2D) materials, possessing numerous remarkable properties including high electrical conductivity, optical transparency and mechanical strength, have supplied a fertile soil for theoretical research and practical application in electronics and optoelectronics. Due to a persistent need of strain sensors, microelectromechnical system (MEMS), nanorobots and active flexible electronics, piezotronic and piezo-phototronic effect of 2D materials have been attracting growing attentions in latest years. Therefore, a comprehensive and intensive understanding of piezotronic and piezo-phototronic effect of 2D materials is required. Here we review the recent progress in theoretical analysis and experimental observation of piezotronic and piezo-phototronic effect of 2D materials enabled by non-centrosymmetric crystal structure. After introducing the fundamental physics of piezotronic and piezo-phototronic effect concisely, the origination and analysis of the piezoelectricity in 2D materials are discussed in detail. Furthermore, we focus on the application in piezotronic and piezo-phototronic effect of transition-metal dichalcogenides (TMDCs) including transistor, nanogenerator, humidity sensor and photodetector. Moreover, some other 2D piezoelectric materials (PEMs) are also been summarized owing to their potential applications in piezotronics and piezo-phtotronics. Finally, some perspectives are put forward on the following opportunities and challenges for future research in this emerging field.

Graphene bioelectronics is a groundbreaking field which emerged roughly 8 years ago offering important opportunities for developing new kinds of sensors capable of establishing an outstanding interface with soft tissue. Graphene-based transistors, as well as electrode arrays, have emerged as a special group of biosensors with their own peculiarities, advantages and drawbacks. In this review, we show the progress of the field from single device measurements to in vivo neuroprosthetic devices. First, the general architectures of device fabrication and their implementation for extracellular recordings are discussed, along with the basic sensing mechanisms essential for their use as sensors. Then state-of-the-art approaches are introduced with a discussion of advantages and drawbacks in the design/measurement architectures. As a whole, the review highlights the results from the ever-growing discipline of graphene bioelectronics and draws reasonable conclusions for future research directions. The possibility of using other device architectures or other 2D materials such as MoS₂ and MXenes for the same goal are assessed at the end of this review in order to highlight future challenges and directions towards an efficient 2D materials-to-brain interface.

We studied the nonlinear electric response in WTe₂ and MoTe₂ monolayers. When the inversion symmetry is breaking but the the time-reversal symmetry is preserved, a second-order Hall effect called the nonlinear anomalous Hall effect (NLAHE) emerges owing to the nonzero Berry curvature on the nonequilibrium Fermi surface. We reveal a strong NLAHE with a Hall-voltage that is quadratic with respect to the longitudinal current. The optimal current direction is normal to the mirror plane in these two-dimensional (2D) materials. The NLAHE can be sensitively tuned by an out-of-plane electric field, which induces a transition from a topological insulator to a normal insulator. Crossing the critical transition point, the magnitude of the NLAHE increases, and its sign is reversed. Our work paves the way to discover exotic nonlinear phenomena in inversion-symmetry-breaking 2D materials.

Vertically oriented graphenes have been grown for more than a decade, but until now the chemical and physical mechanisms underlying their growth have not been fully defined and understood. For this reason, we build a multi-scale, multi-factor model which is thoroughly verified using a large body of experimental data to provide a significant insight into the chemical and physical processes that determine nucleation, growth and structure formation of vertically aligned graphenes in plasma environments. Roles of chemical and physical processes that cannot be directly characterized using presently available experimental techniques, e.g. surface diffusion of adatoms and radicals, are also studied using this model. The leading role of surface diffusion fluxes, rather than direct influx from the gas phase, is confirmed, with ion bombardment being a key factor in 'switching' the growth modes by generating surface defects and hence, increasing the surface adsorption energy. Thus, the hydrocarbon radicals generated on a substrate as a result of bombardment are shown to diffuse to the nanoflakes and catalyze the reactions, and serve as the primary source of material to build the nanoflakes.

We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, during transitions between the T_d and 1T' phases in type-II Weyl semimetal MoTe₂. Thinning down the MoTe₂ crystal from bulk material to about 25 nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from ~40 K to more than 300 K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning effects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent α  =  1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe₂ crystals of various thicknesses shows results consistent with our transport measurements.

Using ultra-high vacuum scanning tunneling microscopy (STM) and density functional theory (DFT), we investigated the surface structure of 2D hexagonal boron nitride (hBN) domains on Pd(1 1 1). STM images of polydomain hBN monolayers, grown via dissociative chemisorption of borazine on Pd(1 1 1)/Al₂O₃(0 0 0 1) thin films, are acquired as a function of tunneling current and bias. The images reveal moiré patterns with four periodicities λ  =  0.6  ±  0.05 nm, 1.8  ±  0.14 nm, 2.7  ±  0.20 nm, and 2.8  ±  0.14 nm, corresponding to different orientations on Pd(1 1 1). We find that the apparent surface corrugation Δz in STM changes little with tunneling current, exhibits an oscillatory dependence on the bias voltage, and increases from Δz  ≈  14 pm for domains with λ  =  0.6 nm to Δz  ≈  200 pm for λ  =  2.8 nm. We attribute the observed tunneling-parameter dependence in Δz to the electronic structure of the hBN/Pd(1 1 1) system. Unlike any other monolayer hBN-on-metal system, we suggest that hBN/Pd can have either mainly geometric or mainly electronic corrugation, depending on the domain orientation. Furthermore, for the largest periodicities, we observe a bifurcation behavior in which some domains are nearly flat, and others develop significant hill-and-valley geometric corrugations. We expect a similar behavior for other substrates for which the interaction energy with hBN is intermediate, i.e. neither mostly chemical nor van der Waals binding: for these substrates, a similar approach can help identify interlayer interactions and electronic structure modifications.

We show that neutral atom scattering is suitable to determine the coupling strength between a two-dimensional (2D) material and the underlying substrate. This information can be obtained from the thermal attenuation of the specular intensity, as well as from angular distributions of He and Ne atoms in the low incident energy (∼50 meV) regime. For graphene (Gr) grown on several metal substrates, there is a direct correlation between the slope of thermal attenuation measurements and the Gr–substrate coupling strength obtained from surface phonons measurements. In addition, Ne scattering presents a broad, classical angular distribution when the Gr–substrate coupling is weak, like in Gr/Ir(1 1 1), whereas sharp diffraction features are observed for strongly interacting systems, like Gr/Ru(0 0 0 1). This makes neutral atom scattering a quick and sensitive indicator of the coupling strength between any 2D material with their substrates. The influence of the moiré superstructures on this simple picture is also discussed.

MXene-TiO₂ mesoporous membranes on α-Al₂O₃ hollow fiber supports are prepared and regulated by adjusting the two-dimensional (2D) MXene content. The prepared MXene-TiO₂ membranes are characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and x-ray diffraction. The results show that TiO₂ nanoparticles (NPs) are uniformed deposited on 2D platforms and that the 2D structure of the original MXenes is still preserved in the film after calcination, thereby successfully inhibiting sol infiltration into the porous support. The obtained MXene-TiO₂ layer with a controllable thickness exhibits 'ideal' pathways (longitudinal-lateral transport nanochannel) for dextran molecules between the TiO₂ NPs and MXene nanosheets. Furthermore, the MXene-TiO₂ membranes show higher rejection performance of dextran with increasing 2D MXene content. The optimal mesoporous MXene-TiO₂ hollow fiber membranes exhibit a cut-off molecular weight (MWCO) of 14 854 Da and a high pure water flux of 102 l m⁻² h⁻¹ per bar, thereby showing good potential as high-performance interlayers for ceramic nanofiltration membranes.

The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine (Ti_n+1C_n/Ti_n+1N_n) and functionalized MXenes (Ti_n+1C_nT₂/Ti_n+1N_nT₂, T stands for the terminal groups: –F, –O and –OH, n  =  1, 2, 3) are investigated by density functional theory calculations. Carbide-based MXenes possess larger lattice constants and monolayer thicknesses than nitride-based MXenes. The in-plane Young's moduli of Ti_n+1N_n are larger than those of Ti_n+1C_n, whereas in both systems they decrease with the increase of the monolayer thickness. Cohesive energy calculations indicate that MXenes with a larger monolayer thickness have a better structural stability. Adsorption energy calculations imply that Ti_n+1N_n have stronger preference to adhere to the terminal groups, which suggests more active surfaces for nitride-based MXenes. More importantly, nearly free electron states are observed to exist outside the surfaces of –OH functionalized carbide/nitride based MXenes, especially in Ti_n+1N_n(OH)₂, which provide almost perfect transmission channels without nuclear scattering for electron transport. The overall electrical conductivity of nitride-based MXenes is determined to be higher than that of carbide-based MXenes. The exceptional properties of titanium nitride-based MXenes, including strong surface adsorption, high elastic constant and Young's modulus, and good metallic conductivity, make them promising materials for catalysis and energy storage applications.

A major outstanding challenge in the field of intercalation chemistry has been the insertion of heavy metals into a 2D layered material. Heavy metal intercalation is a promising route towards access of chemically tailored materials or enhancement of novel physics. We present a new series of wet chemical strategies to intercalate atomic heavy metal and semimetal species (Bi, Cr, Ge, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru, Sb, and W) into layered chalcogenides. Bismuth selenide, Bi₂Se₃, and niobium diselenide, NbSe₂, are used to demonstrate this chemistry. Atomic intercalation is performed in solution using decomposition of zero-valent coordination compounds at low temperatures (∼50 °C–170 °C) or reduction with tin chloride. This host of chemical routes is non-destructive, general for chalcogens, and can be used to intercalate some lighter elements as well. These intercalation reactions more than double the current number of atomic intercalants and give access to unique physical properties including heterostructures, charge density waves, and polytypic superlattice structures.

We couple photoluminescent semiconducting 7-atom wide armchair edge graphene nanoribbons to plasmonic nanoantenna arrays and demonstrate an enhancement of the photoluminescence and Raman scattering intensity of the nanoribbons by more than an order of magnitude averaged over large areas, and by three orders of magnitude in the hot spots of plasmonic antennas. The increase in signal allows us to study Raman spectra with high signal-to-noise ratio. Using plasmonic enhancement we are able to detect the off-resonant Raman scattering from the modified radial breathing-like mode (RBLM) due to physisorbed molecules, the 3rd order RBLM, and C-H vibrations. We find excellent agreement between experimental data and simulations describing the spectral dependence of the enhancement and modifications of the polarization anisotropy. The strong field gradients in the optical near-field further allow us to probe the subwavelength coherence properties of the phonon modes in the nanoribbons. We theoretically model this considering a finite phonon correlation length along the GNR direction. Our results allow estimating the correlation length in graphene nanoribbons.

We study the temperature-dependent photoluminescence of monolayer and bilayer molybdenum ditelluride in the temperature range between 5 K and room temperature. We disentangle the effects of interactions of excitons with acoustic and optical phonons and show that molybdenum ditelluride excitons have an unusually small coupling with acoustical phonons. This observation, together with the large luminescence yield which can be obtained from the bilayer, puts forward molybdenum ditelluride as a robust and bright light source in the near infrared range. The scaling of luminescence wavelength and linewidth of the molybdenum ditelluride bilayer differs from the observations in the monolayer by effects that can be traced to symmetry and wellwidth. This suggests a similar band alignment of mono- and bilayer, in contrast to other transition metal dichalcogenides.

The topological nature of two-dimensional (2D) chalcogenide platelets, can present novel opportunities in thin, flexible thermoelectrics. In this work, metal dopants are added to the reactive edges of 2D Bi₂Te₃ platelets. We show that along this active edge, an atomically well-ordered heterojunction is formed and facile charge exchange is created, onto the platelet and proximal to its known topological states. Temperature dependent conductivity suggests that local band bending across the interface may act as an injection energy filter for dopant-originated carriers. Moreover, as carrier density increases with increasing edge-dopant, carrier scattering does not appear to increase dramatically. As a result, an apparent decoupling between electrical conductivity and Seebeck coefficient occurs, leading to a surprisingly high power factors (PF): For example, the PF increases in Bi₂Te₃ platelets by eight times when doped with Cu. First principles calculations show that the electronics of the semiconductor-metal interfaces are quite different for edge and facial configurations, thus the site of metal dopant is believed to play an important role in the expected thermoelectric performance. Finally, this work suggests that the topological sensitivity of dopant placement should be considered in the rational design of high performance thermoelectric composites.

VS₂ is a challenging material to prepare stoichiometrically in the bulk, and the single layer has not been successfully isolated before now. Here we report the first realization of single-layer VS₂, which we have prepared epitaxially with high quality on Au(1 1 1) in the octahedral (1T) structure. We find that we can deplete the VS₂ lattice of S by annealing in vacuum so as to create an entirely new 2D compound that has no bulk analogue. The transition is reversible upon annealing in an H₂S gas atmosphere. We report the structural properties of both the stoichiometric and S-depleted compounds on the basis of low-energy electron diffraction, x-ray photoelectron spectroscopy and diffraction, and scanning tunneling microscopy experiments.

Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We demonstrate the growth of high quality 2D WS₂ over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~10¹² cm⁻²), ~10 μm large WS₂ monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS₂.

Centimeter-sized 2D layered α-MoO₃ single crystal has been grown by a facile physical vapor deposition method. The lateral size of α-MoO₃ single crystals can be tunable from micrometer to centimeter by controlling the inner diameter of the quartz tube reaction chamber. The as-grown α-MoO₃ exhibits strong in-plane Raman anisotropy, where A_g Raman modes give the direction of crystallographic orientation, which can be used to rapidly and precisely identify the crystallinity of large-area α-MoO₃ through selected-area angel-resolved polarized Raman spectroscopy. Moreover, the photodetectors based on annealed α-MoO₃ single crystals exhibit excellent optoelectronic performance with a photoresponsivity of 65.6 A W⁻¹ and a fast photoresponse time of 2 s.

Although many possible two-dimensional (2D) topological insulators (TIs) have been predicted in recent years, there is still lack of experimentally realizable 2D TI. Through first-principles and tight-binding simulations, we found an effective way to stabilize the robust quantum spin Hall state with a large nontrivial gap of 227 meV in 2D honeycomb HgTe monolayer by the Al₂O₃(0 0 0 1) substrate. The band topology originates from the band inversion between the s-like and p-like orbitals that are contributed completely by the Hg and Te atoms, so the quantized edge states are restricted within the honeycomb HgTe monolayer. Meanwhile, the strong interaction between HgTe and Al₂O₃(0 0 0 1) ensures high stability of the atomic structure. Therefore, the TI states may be realized in HgTe/Al₂O₃(0 0 0 1) at high temperature.

At half filling, the very flat energy bands in rhombohedral-stacked tetralayer graphene are unstable to electronic interactions, giving rise to electronic states with spontaneous broken symmetries. Using transport measurements on suspended dual-gated devices, we observe an insulating ground state with a large interaction-induced transport gap up to 80 meV, which can be enhanced by a perpendicular magnetic field, and suppressed by an interlayer potential, carrier density, or a critical temperature of ~40 K. This insulating gapped state is proposed to be a layer antiferromagnet with broken time reversal symmetry.

MoTe₂ is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe₂ with a critical temperature up to 5 K significantly exceeding the bulk T_c  =  0.1 K. We measured the derivatives of I–V curves for hetero-contacts of MoTe₂ with Ag or Cu, homo-contacts of MoTe₂ as well as 'soft' point contacts (PCs). Large number of these hetero-contacts exhibit a dV/dI dependence, which is characteristic for Andreev reflection. It allows us to determine the superconducting gap Δ. The average gap values are 2Δ  =  1.30  ±  0.15 meV with a 2Δ/k_BT_c ratio of 3.7  ±  0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS-like behavior, which points to a nodeless superconducting order parameter with some strong-coupling renormalization. Remarkably, the observation of a 'gapless-like' single minimum in the dV/dI of 'soft' PCs may indicate a topological superconducting state of the MoTe₂ surface as these contacts probe mainly the interface and avoid additional pressure effect. Therefore, MoTe₂ might be a suitable material to study new forms of topological superconductivity.

Due to the strong 'lip–lip' interactions between neighboring nanosheets layers, the scalable exfoliated fabrication of few-layer-thick boron nitride nanosheets (BNNSs) from the bulk hexagonal boron nitride (h-BN) is more challenging than these graphene nanosheets. In this work, few-layer (generally less than 10 layers) BNNSs were efficiently synthesized through a simple ball milling technique with the 2-furoic acid (FA) as a modifier. The BNNSs as-prepared with a average thickness of about 2.0 nm and an extremely high production yield (~98%) as well as unprecedentedly high dispersed concentration (~35 mg ml⁻¹) in water. The stable aqueous dispersions of BNNSs can be directly used to fabricate ultralight aerogels with a low density of 2.0 mg cm⁻³ and thermally conductive film with a highly thermal conductivity of 25.2 W m⁻¹ K⁻¹. This approach provided us with an environmentally friendly agent with high efficiency for few-layer BNNSs synthesis and demonstrated promising applications for ultralight aerogels and heat dissipation in electronic components.

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer MoS₂ (1L-MoS₂) inside a Au/p-GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial 1L-MoS₂ in the form of triangle domains are grown by powder vaporization on a p-doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that 1L-MoS₂ remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the 1L-MoS₂ is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical Au/1L-MoS₂/p-GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, 1L-MoS₂ appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a pn-junction. The I–V analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the Au/1L-MoS₂/GaN junction.

Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number N affects the electronic and optical properties. Using both ab initio calculations and a tight-binding model for an effective Hamiltonian describing the excitons, we characterize in detail the symmetry of the excitonic wavefunctions and the selection rules for their coupling to incoming light. We show that for N  >  2, one can distinguish between surface excitons that are mostly localized on the outer layers and inner excitons, leading to an asymmetry in the energy separation between split excitonic states. In particular, the bound surface excitons lie lower in energy than their inner counterparts. Additionally, this enables us to show how the layer thickness affects the shape of the absorption spectrum.

Growth of hexagonal boron nitride (hBN) layers on 2'' sapphire substrate using metal organic vapour phase epitaxy is reported here, where we compare the growth under continuous flow and flow modulation (FM) schemes. hBN films grown under the continuous flow regime exhibit low growth rate and rough surface profiles due to severe parasitic reactions between precursor molecules, which are suppressed by adopting a FM scheme. We also observe spontaneous delamination of hBN films from the substrate when immersed in a water bath and attribute this to be due to relaxation of compressive stress in the films, which was further corroborated using Raman spectroscopy. Carbon is identified as a major impurity which gets incorporated as boron carbide under FM growth and results in large sub-bandgap fluorescence in the range of 1.77–2.25 eV. Overall, hBN films deposited using the FM scheme at low growth rate (~2–3 nm h⁻¹) exhibited the best characteristics in the present study, which will be suitable for applications such as van der Waals epitaxy and 2D hetero-structure devices.

The interaction of partially reduced graphene oxide (prGO) and Huh7.5.1 liver cancer cells was investigated by means of DUV fluorescence bioimaging. The prGO sample was obtained by the reduction (to a certain extent) of the initially prepared graphene oxide (GO) nanosheets with hydrazine. The fluorescence of the GO nanosheets increases with time of the reduction due to a change in ratio of the sp² and sp³ carbon sites and the prGO sample was extracted from the dispersion after 6 min, when the intensity of the fluorescence reached its maximum. The reduction process was left to proceed further to saturation until highly reduced graphene oxide (denoted here as rGO) was obtained. GO, prGO and rGO samples were investigated by structural (scanning electron microscopy (SEM), scanning transmission electron microscopy coupled with energy dispersive spectrometry (STEM-EDS)) and spectroscopic (UV–vis, photoluminescence (PL), Raman) methods. After that, Huh7.5.1 cells were incubated with GO, prGO and rGO nanosheets and used in bioimaging studies, which were performed on DISCO beamline of synchrotron SOLEIL. It was found that the prGO significantly enhanced the fluorescence of the cells and increased the intensity of the signal by ~2.5 times. Time-lapse fluorescence microscopy experiments showed that fluorescence dynamics strongly depends on the type of nanosheets used. The obtained prGO nanostructure can be easily conjugated with aromatic ring containing drugs, which opens a possibility for its applications in fluorescence microscopy monitored drug delivery.

Graphene, the atomically-thin honeycomb carbon lattice, is a highly conducting 2D material whose exposed electronic structure offers an ideal platform for chemical and biological sensing. Its biocompatible, flexible and chemically inert nature associated with the lack of dangling bonds, offers novel perspectives for direct interfacing with biological molecules. Combined with its exceptional electronic and optical properties, this promotes graphene as a unique platform for bioelectronics. Among the successful bio-integrations of graphene, the detection of action potentials in numerous electrogenic cells including neurons has paved the road for the high spatio-temporal and wide-field mapping of neuronal activity. Ultimate resolution of sensing ion channel activity can be achieved with neural interfaces, and it was shown that macroscale electrodes can record extracellular current of individual ion channels in model systems, by charging the quantum capacitance of large graphene monolayer (mm²). Here, we show the field effect detection of ion channel activity within neuron networks, cultured during several weeks above graphene transistor arrays. Dependences upon drugs, reference potential gating and device geometry confirm the field effect detection of individual ion channel and suggest a significant contribution of grain boundaries, which provide highly sensitive nanoscale-sized sensing sites. Our theoretical analysis and simulations demonstrate that the ion gating of a single grain boundary in liquid affects the electronic transmission of the whole transistor channel, resulting in significant conductance variations. Monitoring the ion channels activity is of great interest as most of neurodegenerative diseases relied on channelopathies, which rely on ion channel abnormal activity. Thus, such highly sensitive and biocompatible neuro-electronics which open the way to FET detection at the sub-cell precision should be useful for a wide range of fundamental and applied research areas, including brain-on-chip, pharmacology, and in vivo monitoring or diagnosis.

Nematic liquid crystal (LC) molecules adsorbed on two dimensional materials are aligned along the crystal directions of the hexagonal lattice. It was demonstrated that short electric pulses can reorient the aligned LC molecules in the preferred armchair direction of hexagonal boron nitride (h-BN). Several states with a variety of colors were obtained by changing the direction and strength of the electric pulses. The ab initio calculations based on density functional theory was carried out to determine the favorable adsorption configurations of the LC molecules on the h-BN surface. A non-volatile display, in which pixel resolution can be determined by grains of hexagonal surface, is proposed, which can offer a pathway towards dynamic high-quality pixels with low power consumption, and could define a new paradigm for all non-volatile display applications.

Atomically thin two-dimensional (2D) mechanical resonators should realize highly sensitive force sensors and high performance nano-electro-mechanical systems due to their excellent electrical and mechanical properties. However, practical applications require stability of the resonance frequencies against temperature. Here, we demonstrate the manipulation of the thermal expansion coefficients (TECs) by creating a van der Waals heterojunction using graphene and MoS₂, which have opposite signs of TECs. Our method greatly suppresses the apparent TEC of the 2D heterojunction to 1/3 of the monolayer graphene without the detraction of the tunability of the resonance frequency by electrostatic attraction.

Based on high throughput (HTP) density functional theory calculations and topological characterization, we have identified four topologically nontrivial two dimensional (2D) materials out of 641 most stable systems in the 2D materials database. The HTP screening is carried out based on the automated construction of maximally localized Wannier functions followed by explicit calculations of the surface states, where the Wannier centers are evaluated as a further verification of the nontriviality. The electronic structure of such nontrivial materials is discussed in detail together with the features in the crystal structure and relevant orbitals.

Understanding and controlling the spin degree of freedom in two-dimensional transition metal dichalcogenides offers the potential for designing functional quantum materials. This work investigates the dynamics of photo- and resident carrier spins in an encapsulated MoSe₂ monolayer using non-degenerate time-resolved Kerr-rotation microscopy. The lightly doped monolayer exhibits clear exciton and trion resonances with spin-polarizations that are characterized by a fast (~20 ps) decay attributed to photocarrier relaxation and recombination, followed by a slow (~690 ps) decay associated with resident carrier depolarization. Dual-frequency Kerr-rotation spectra directly reveal exciton-trion coupling on ultrashort timescales and within the spin coherence time of the system. Moreover, the distribution of the exciton-trion coupling features exposes inhomogeneous broadening likely arising from different domains within the excitation spot.

Two-dimensional transition metal dichalcogenides possess large surface-to-volume ratios that make them ideal candidates for sensing applications such as detecting the surface adsorption of specific gas molecules. The resulting changes of the electrical and optical properties allow for detection and analysis of interaction mechanisms at the sensing interface. Specifically, we investigate the influence of O₂ adsorption on monolayer MoS₂ and the role of the Fermi level energy in this process. We record the response in photoluminescence and transport properties of monolayer MoS₂ upon O₂ adsorption and the impact of external electric gating. We find an increase of the photoluminescence intensity and a reduction of the conductivity upon O₂ adsorption, and show that the adsorption can be enhanced by an increase of the Fermi level energy. These results demonstrate that ionosorption of O₂ on MoS₂ by charge transfer only occurs if free carriers are available in the conduction band of MoS₂. This free-carrier-supported adsorption-mechanism is corroborated by density functional calculations. Furthermore, the resulting reduction in screening of the Coulomb interaction between photo-excited electron–hole pairs amplifies the effect of the electron transfer on the excitonic recombination, causing a strong change of the photoluminescence intensity and rendering photoluminescence recording advantageous for sensing applications.

Pursuing two-dimensional (2D) intrinsic ferromagnetism with high Curie temperature and great mechanical flexibility has attracted great interest in flexible spintronics. In the present work, we carried out a density functional theory (DFT) investigation on the 2D M₂Se₃ (M  =  Co, Ni, and Pd) monolayers to understand their structural stabilities, electronic, magnetic and mechanical properties. Our results show that the Co₂Se₃ monolayer exhibits a fascinating half-metallic ferromagnetism with high Curie temperature (~600 K). In addition, due to their unique buckling hinge-like structure, M₂Se₃ monolayers possess the large out-of-plane negative Poisson's ratio (NPR) and superior mechanical flexibility evidenced by their unusual critical strain (~50%–60%) two times greater than the well-known 2D materials. These findings imply that 2D M₂Se₃ family is the promising materials for the applications in the flexible and high-density spintronic nanodevices.

2D materials such as graphene and transition metal dichalcogenides exhibit novel electrical and optoelectrical properties, which are promising for the application in sensing and imaging. However, the photoresponse ability of pure monolayer graphene in room temperature and ratio of I_ph and I_dark are still unsatisfactory, due to the weak light absorption itself, short lifetime of photo-excited carriers and large dark current, especially in the infrared range. Here, we successfully constructed a broadband graphene-based phototransistor consisting of lateral MoS₂-graphene-MoS₂ heterostructure by chemical vapor deposition technique. It shows broadband photodetection and a low dark current less than 1 pA at applied bias of 0.5 V, resulting in excellent specific detectivity more than 10¹² Jones and nearly 10⁹ Jones in visible and infrared band, respectively. Moreover, the ratio of I_ph and I_dark could be modulated by the back-gate voltage when the phototransistor is under the infrared illumination. And a high ratio of 10⁵ was exhibited. This type of graphene-based phototransistor by chemical vapor deposition demonstrates an approach for the room temperature broadband photodetection of monolayer graphene by energy engineering. All the results address key challenges for broadband detection by graphene-based phototransistor, and are promising for the large scale fabrication and integration in the future electronic and optoelectronic applications.

Resistive-switching memories are alternative to Si-based ones, which face scaling and high power consumption issues. Tetrahedral amorphous carbon (ta-C) shows reversible, non-volatile resistive switching. Here we report polarity independent ta-C resistive memory devices with graphene-based electrodes. Our devices show ON/OFF resistance ratios  ∼, ten times higher than with metal electrodes, with no increase in switching power, and low power density  ∼14 μW μm⁻². We attribute this to a suppressed tunneling current due to the low density of states of graphene near the Dirac point, consistent with the current–voltage characteristics derived from a quantum point contact model. Our devices also have multiple resistive states. This allows storing more than one bit per cell. This can be exploited in a range of signal processing/computing-type operations, such as implementing logic, providing synaptic and neuron-like mimics, and performing analogue signal processing in non-von-Neumann architectures.

We demonstrate insights into the three-dimensional (3D) structure of defects in graphene, in particular grain boundaries, obtained via a new approach using two transmission electron microscopy images recorded at different angles. The structure is revealed through an optimization process where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found. We first demonstrate that this method works using an embedded defect in graphene that allows direct comparison to the computationally predicted 3D shape. We then apply the method to a set of grain boundary structures with misorientation angles spanning nearly the whole available range (2.6°–29.8°). The measured height variations at the boundaries reveal a strong correlation with the misorientation angle with lower angles resulting in stronger corrugation and larger kink angles. Our results allow for the first time a direct comparison to theoretical predictions for the corrugation at grain boundaries, revealing the measured kink angles are significantly smaller than the largest predicted ones.

WS₂-based photodetectors were fabricated by sputtering and electron beam irradiation (EBI), and the effect of EBI on the crystallization of WS₂ films was investigated. EBI at 1 kV energy for 1 min transformed the as-deposited amorphous structure of WS₂ film into a two-dimensional (2D) layered crystalline structure with high uniformity over a 50.8 mm diameter wafer. Additionally, EBI enhanced the photoelectrical properties of WS₂-based photodetectors. The as-deposited WS₂ film yielded a responsivity of 0.10 mA · W⁻¹ under 450 nm laser irradiation, but showed no response under 532 and 635 nm laser wavelengths. However, after 1 kV and 3 kV EBI of the WS₂ films, the responsivities under laser irradiation at 450, 532, and 635 nm were 0.36, 1.37, and 0.19 mA · W⁻¹, and 1.68, 2.45, and 1.09 mA · W⁻¹, respectively. The substrate temperatures after 1 min of 1 kV and 3 kV EBI were 102 °C and 591 °C, respectively. The WS₂-based photodetectors exhibited high responsivity in the visible light region despite their unique process conditions of low temperature and fast EBI treatment. Such desirable performance of the EBI-treated WS₂ films shows significant potential for future large-area and low-temperature photoelectronic applications. Thus, we demonstrated that EBI is an attractive method for synthesizing 2D materials as it is fast, simple, controllable, and compatible with sputtering processes.

Few-layer black phosphorus (BP) is a direct band gap material with large exciton binding energies, and shows great promise in optoelectronic applications. Here, we study the excitons in BP-based heterostructures with encapsulation and spacer 2D layers, using first principles GW and Bethe–Salpeter equation (BSE) methods. The 2D layers chosen are germanium sulfide (GeS) and hexagonal boron nitride (hBN), representing respectively strong and weak hybridization with BP. Except for hBN-encapsulated BP, all systems host bright interlayer (or indirect) excitons. In contrast to 2D indirect gap heterostructures, the interlayer excitons here are much brighter. Strong hybridization between GeS and BP increases the effective mass and room temperature exciton lifetimes. In contrast, the hBN spacer layer decouples the BP monolayers in BP/hBN/BP, resulting in the lowest energy exciton being dark. Surprisingly, however, BP/hBN/BP hosts interlayer BP excitons that are even brighter than those in bilayer BP. This lowest energy bright exciton lies very close in energy to the dark state, resulting in an increased effective lifetime. Our work uncovers the interplay between interlayer interactions and the physics of interlayer excitons, and paves the way for the use of bottom-up materials design to optimize the dipole oscillator strengths and lifetimes of interlayer excitons for excitonic device applications.

Lattice deformation and electronic properties are closely linked in two-dimensional materials such as graphene. However, a fine control of the spatial strain distribution is crucial to correctly engineer the electrical properties of atomic-thick materials. Although several solutions have been proposed so far, the flexibility required to fully master and investigate arbitrary strain profiles remains challenging. Here, we locally deform graphene using the poly-methyl-methacrylate (PMMA) shrinkage induced by electron-beam irradiation. Arbitrary design of pulling geometries and different actuation magnitudes can be both defined in the PMMA by electron-beam patterning. Specific graphene strain fields can be obtained using reverse engineering of the PMMA micro-actuators geometry. As proof of principle of operation, we target and we successfully demonstrate a strongly localized and virtually-pure uniaxial strain profile. This configuration is promising for the implementation of the pseudo-magnetic field and allows identifying the graphene crystal orientation. Strain field characterization and out-of-plane graphene deformations are demonstrated and studied by Raman, scanning-electron and atomic-force microscopy. These can all be easily combined with the present device architecture. Remarkably, the induced strain in graphene can be released by heating the sample and reconfigured or restored again by re-irradiating the polymer. In situ observation of nano-mechanical evolution of devices show that micro-actuation can be strong enough to tear the graphene layer. As side result, the in situ failure visualization allows using our technique to qualitative estimate the mechanical quality of chemically synthetized graphene. The relative simplicity and flexibility of our method opens new opportunities for the investigation of straintronics, pseudo-magnetic field and nano-mechanics in two-dimensional materials.

Unfunctionalised few-layer graphene (FLG) refers to graphene materials and formulations which have low or no defects in the graphene sheet structure, thus enhancing their physico-chemical properties. However, the use of FLG and its derivatives in the biomedical field is not without risk to human health. It is largely accepted that the generation of reactive oxygen species and the consecutive inflammatory response is at the basis of its toxicity; nonetheless the signaling pathways triggered by FLG still need to be explored in detail. The present study was aimed at providing some elucidations on the specific molecular signaling induced by low doses of a well characterized FLG material in macrophages. Exposure to low doses of FLG resulted in no significant decrease of macrophage viability. Nevertheless, it elicited a marked oxidative stress. The latter triggered significant inflammatory responses, increasing Tumor Necrosis Factor-alpha (TNF-α) and Interleukin-6 (IL-6) secretion as well as nitric oxide (NO) production, leading to autophagy via endoplasmic reticulum (ER) stress. Indeed, exposure to FLG in the presence of 4-phenylbutyrate (4-PBA) and N-acetyl-L-cysteine (NAC), inhibitors of ER stress and oxidative stress, respectively, decreased ER stress, autophagy, oxidative stress and inflammation. Interestingly, graphene exposure did not induce Interleukin-1Beta (IL-1β) and Interleukin-18 (IL-18) secretion, which are indicators of inflammasome activation. Pre-treatment with 3-Methyladenine (3-MA), an autophagy inhibitor, actually suppressed the autophagy activation triggered by graphene exposure, but leaded to inflammasome activation. Our work highlight for the first time for this type graphene an interplay between oxidative stress and ER stress-mediated autophagy. It also suggests that such a pathway could protect the cells from exaggerated inflammation. This study underlines the importance of having a clear overview on the mechanism behind the interaction between FLG and cells, especially when graphene is used for the development of novel biomedical materials.

One of the most thrilling developments in the photovoltaic field over recent years has been the use of organic–inorganic lead halide perovskite, such as CH₃NH₃PbI₃ (MAPbI₃), as a promising new material for low-cost and highly efficient solar cells. Despite the impressive power conversion efficiency (PCE) exceeding 22% demonstrated on lab-scale devices, large-area material deposition procedures and automatized device fabrication protocols are still challenging to achieve high-throughput serial manufacturing of modules and panels. In this work, we demonstrate that spray coating is an effective technique for the production of mesoscopic small- and large-area perovskite solar cells (PSCs). In particular, we report a sprayed graphene-doped mesoporous TiO₂ (mTiO₂) scaffold for mesoscopic PSCs. By successfully combining the spray coating technique with the insertion of graphene additive into the sprayed mTiO₂ scaffold, a uniform film deposition and a significant enhancement of the electron transport/injection at the mTiO₂/perovskite electrode is achieved. The use of graphene flakes on the sprayed scaffold boosts the PCE of small-area cells up to 17.5% that corresponds to an increase of more than 15% compared to standard cells. For large-area (1.1 cm²) cells, a PCE up to 14.96% is achieved. Moreover, graphene-doped mTiO₂ layer enhances the stability of the PSCs compared to standard devices. The feasibility of PSC fabrication by spray coating deposition of the mesoporous film on large-area 21  ×  24 cm² provides a viable and low-cost route to scale up the manufacturing of low-cost, stable and high-efficiency PSCs.