Large-scale Monolayer Research Articles

Large-Scale Formation of a Close-Packed Monolayer of Spheres Using Different Colloidal Lithography Techniques.

The possibility of using colloidal lithography at the industrial level depends on the ability to form defect-free coatings over large areas. The spin-coating method has not yet shown acceptable results, but a more detailed studying of the regularities of this process may improve the quality of masks. The Langmuir-Blodgett method is expected to be the most preferable for forming high-quality large-scale monolayers. Real-time controlling the surface pressure of the monolayer can allow to obtain close-packed arrays with long-range order. In this work, to develop the spin-coating technology, the influence of technological parameters (spin-coating speed and time, concentrations of components in suspension) on the substrate coverage area with a monolayer of polystyrene spheres (1.25 μm) was studied. An original automated Langmuir-Blodgett system was developed to study the influence of the monolayer surface pressure on its quality using polystyrene spheres (1.25, 1.8, 2.1 μm). The developed spin-coating technology resulted in a record coverage area (90%) of Si substrate (76 mm) and a defect-free hexagonally ordered domain area of 500 μm2. As a result of the developed Langmuir-Blodgett technique, a close-packed monolayer coating was obtained over the entire substrate area (coverage area 99.5%, defect-free domain area 3000 μm2) without the use of any surfactants.

Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection.

We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of "plasmonic heater" gold nanoparticles and "assembly building block" microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1-10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases-colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light.

Understanding epitaxial growth of two-dimensional materials and their homostructures.

The exceptional physical properties of two-dimensional (2D) van der Waals (vdW) materials have been extensively researched, driving advances in material synthesis. Epitaxial growth, a prominent synthesis strategy, enables the production of large-area, high-quality 2D films compatible with advanced integrated circuits. Typical 2D single crystals, such as graphene, transition metal dichalcogenides and hexagonal boron nitride, have been epitaxially grown at a wafer scale. A systematic summary is required to offer strategic guidance for the epitaxy of emerging 2D materials. Here we focus on the epitaxy methodologies for 2D vdW materials in two directions: the growth of in-plane single-crystal monolayers and the fabrication of out-of-plane homostructures. We first discuss nucleation control of a single domain and orientation control over multiple domains to achieve large-scale single-crystal monolayers. We analyse the defect levels and measures of crystalline quality of typical 2D vdW materials with various epitaxial growth techniques. We then outline technical routes for the growth of homogeneous multilayers and twisted homostructures. We further summarize the current strategies to guide future efforts in optimizing on-demand fabrication of 2D vdW materials, as well as subsequent device manufacturing for their industrial applications.

Visible Light-Illuminated Gold Nanohole Arrays With Tunable On-Chip Plasmonic Sensing Properties

Since the discovery of the extraordinary optical transmission phenomenon, nanohole arrays have attracted much attention and been widely applied in sensing. However, their typical fabrication process, utilizing photolithographic top-down manufacturing technologies, has intrinsic drawbacks including the high costs, time consumption, small footprint, and low throughput. This study presented a low-cost, high-throughput, and scalable method for fabricating centimeter-scale (1×2 cm2) nanohole arrays using the improved nanosphere lithography. The large-scale close-packed polystyrene monolayers obtained by the hemispherical-depression-assisted self-assembly method were employed as colloidal masks for the nanosphere lithography, and the nanohole diameter was tuned from 233 nm to 346 nm with a fixed period of 420 nm via plasma etching. The optical properties and sensing performance of the nanohole arrays were investigated, and two transmission dips were observed due to the resonant coupling of plasmonic modes. Both dips were found to be sensitive to the surrounding environment, and the maximum bulk refractive index sensitivity was up to 162.1 nm/RIU with a 233 nm hole diameter. This study offered a promising approach for fabricating large-scale highly ordered nanohole arrays with various periods and nanohole diameters that could be used for the development of low-cost and high-throughput on-chip plasmonic sensors.

Eight In. Wafer-Scale Epitaxial Monolayer MoS2.

Large-scale, high-quality, and uniform monolayer molybdenum disulfide(MoS2) films are crucial for their applications in next-generation electronics and optoelectronics. Epitaxy is a mainstream technique for achieving high-quality MoS2 films and is demonstrated at a wafer scale up to 4-in. In this study, the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS2 on sapphire is reported as with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition (VCVD) system. Field effect transistors (FETs) based on the as-grown 8-in. wafer-scale monolayer MoS2 film are fabricated and exhibit high performances, with an average mobility and an on/off ratio of 53.5 cm2 V-1 s-1 and 107, respectively. In addition, batch fabrication of logic devices and 11-stage ring oscillators are also demonstrated, showcasing excellent electrical functions. This work may pave the way of MoS2 in practical industry-scale applications.

Restoring the electronic properties of epitaxial graphene on SiC substrate by Ar intercalation

Thermal decomposition of 6H–SiC(0001) under an argon atmosphere is a promising technique to fabricate large-scale epitaxial graphene monolayers for electronic applications. In this manuscript, we report on the intercalation of argon, hydrogen and oxygen atoms underneath a graphene buffer layer (BL) on SiC(0001), involving bond formation between the intercalated agent and Si atoms. Density Functional Theory calculations were performed to investigate the atomic structure, stability and electronic properties of quasi free-standing monolayer graphene and bilayer graphene, accomplished by intercalation. According to our results, as the Ar intercalated atoms saturate the silicon dangling bonds in the upper SiC surface, the BL is completely detached from the substrate and transformed into free-standing graphene. Ar intercalation would be hindered by the concurrent and thermodynamically more spontaneous adsorption of Ar atop the graphene layers, resulting in a pronounced decrease of the process efficiency. High Ar-coverage regimes and low kinetic barriers for Ar intercalation might shift the intercalation/adsorption balance. These factors as well as strategies such as ion implantation could be explored to identify more favorable conditions to experimentally test Ar intercalation as a simple method to achieve control of the electronic properties of the BL and help preserve the unique characteristics of graphene.

High-Performance Monolithic 3D Integrated Complementary Inverters Based on Monolayer n-MoS2 and p-WSe2.

Herein, the structure of integrated M3D inverters are successfully demonstrated where a chemical vapor deposition (CVD) synthesized monolayer WSe2 p-type nanosheet FET is vertically integrated on top of CVD synthesized monolayer MoS2 n-type film FET arrays (2.5×2.5cm) by semiconductor industry techniques, such as transfer, e-beam evaporation (EBV), and plasma etching processes. A low temperature (below 250°C) is employed to protect the WSe2 and MoS2 channel materials from thermal decomposition during the whole fabrication process. The MoS2 NMOS and WSe2 PMOS device fabricated show an on/off current ratio exceeding 106 and the integrated M3D inverters indicate an average voltage gain of ≈9 at VDD = 2V. In addition, the integrated M3D inverter demonstrates an ultra-low power consumption of 0.112nW at a VDD of 1V. Statistical analysis of the fabricated inverters devices shows their high reliability, rendering them suitable for large-area applications. The successful demonstration of M3D inverters based on large-scale 2D monolayer TMDs indicate their high potential for advancing the application of 2D TMDs in future integrated circuits.

Ag-Assisted Dry Exfoliation of Large-Scale and Continuous 2D Monolayers.

Two-dimensional (2D) semiconductors have generated considerable attention for high-performance electronics and optoelectronics. However, to date, it is still challenging to mechanically exfoliate large-area and continuous monolayers while retaining their intrinsic properties. Here, we report a simple dry exfoliation approach to produce large-scale and continuous 2D monolayers by using a Ag film as the peeling tape. Importantly, the conducting Ag layer could be converted into AgOx nanoparticles at low annealing temperature, directly decoupling the conducting Ag with the underlayer 2D monolayers without involving any solution or etching process. Electrical characterization of the monolayer MoS2 transistor shows a decent carrier mobility of 42 cm2 V-1 s-1 and on-state current of 142 μA/μm. Finally, a plasmonic enhancement photodetector could be simultaneously realized due to the direct formation of Ag nanoparticles arrays on MoS2 monolayers, without complex approaches for nanoparticle synthesis and integration processes, demonstrating photoresponsivity and detectivity of 6.3 × 105 A/W and 2.3 × 1013 Jones, respectively.

Microscopic growth mechanism and edge states of monolayer 1T′-MoTe2

Transition metal ditellurides (TMTDs) have versatile physical properties, including non-trivial topology, Weyl semimetal states and unique spin texture. Controlled growth of high-quality and large-scale monolayer TMTDs with preferred crystal phases is crucial for their applications. Here, we demonstrate the epitaxial growth of 1T′-MoTe2 on Au (111) and graphitized silicon carbide (Gr/SiC) by molecular beam epitaxy (MBE). We investigate the morphology of the grown 1T′-MoTe2 at the atomic level by scanning tunnelling microscopy (STM) and reveal the corresponding microscopic growth mechanism. It is found that the unique ordered Te structures preferentially deposited on Au (111) regulate the growth of monolayer single crystal 1T′-MoTe2, while the Mo clusters were preferentially deposited on the Gr/SiC substrate, which impedes the ordered growth of monolayer MoTe2. We confirm that the size of single crystal 1T′-MoTe2 grown on Au (111) is nearly two orders of magnitude larger than that on Gr/SiC. By scanning tunnelling spectroscopy (STS), we observe that the STS spectrum of the monolayer 1T′-MoTe2 nano-island at the edge is different from that at the interior, which exhibits enhanced conductivity.

Controllable Growth of Large-Scale Continuous ReS2 Atomic Layers

In recent years, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received significant attention due to their exceptional electrical and optical properties. Among these 2D materials, ReS2 distinguishes itself through its unique optical and conductance anisotropy. Despite concerted efforts to produce high-quality ReS2, the unique interlayer decoupling properties pose substantial challenges in growing large-area ReS2 thin films, with the preparation of single layers proving even more complex. In this work, large-scale continuous monolayer and bilayer ReS2 films were successfully grown on mica substrates using low-pressure chemical vapor deposition (LPCVD). Photodetectors were fabricated using the prepared high-quality ReS2 films, and the devices presented stable photoresponse and enhanced response sensitivity. The production of continuous ReS2 atomic layers heralds promising prospects for large-scale integrated circuits and advances the practical application of optoelectronics based on 2D layered materials.

MoS2-Thin Film Transistor Based Flexible 2T1C Driving Circuits for Active-Matrix Displays.

Two-dimensional (2D) semiconductors offer great potential as high-performance materials for thin film transistors (TFTs) in displays. Their thin, stable, and flexible nature, along with excellent electrical properties, makes them suitable for flexible displays. However, previous demonstrations lacked clear superiority in pixel resolution and TFT performance. Here we present the flexible 2T1C pixel driving circuit for active-matrix displays based on high-quality large-scale monolayer MoS2. A gate-first fabrication process was developed for flexible MoS2-TFTs, showing a remarkable carrier mobility (average at 52.8 cm2 V-1 s-1), high on/off ratio (average at 1.5 × 108), and negligible hysteresis. The driving current can be modulated by pulsed input voltages and demonstrates a stable and prompt response to both frequency and amplitude. We also demonstrated a 10 × 10 active-matrix with high resolution of 508 pixels per inch, exhibiting 100% yield and high uniformity. The driving circuit works well under bending up to ∼0.91% strain, highlighting its normal functions in flexible displays.

Large-scale monolayer MoS2 preparation and its enhanced photoluminescence performance by ultraviolet-ozone treatment

Large-scale monolayer MoS2 preparation and its enhanced photoluminescence performance by ultraviolet-ozone treatment

Large-scale thallene film with emergent spin-polarized states mediated by tin intercalation for spintronics applications

The quantum effects confined in the ultimate two-dimensional limit are able to address the challenges and provide advances in high-performance spintronic devices. In the paper we show a successful strategy to enrich the electronic properties of thallene, a new honeycomb analogue of graphene, through the interface engineering, which opens a great potential of thallene as an advanced spintronics material. While the thallene has been experimentally realized recently on NiSi2/Si(111) substrate, there remains a lack of attractive electronic properties due to the strong thallene-substrate coupling. This challenge is addressed here through the decoration of thallene/NiSi2 interface by Sn interlayer, which allows to eliminate the thallene-substrate coupling and produce a high-quality large-scale thallene monolayer with exotic electron bands demonstrating colossal spin-polarization just above the Fermi level. It is demonstrated that appropriate electron doping or external electric field are enable the spin-transport regime. The discovered band structure regulation boosts the functionality of the 2D-Tl Xene and makes it a highly attractive material for spintronics applications.

Flexible and Speedy Preparation of Large-Scale Polystyrene Monolayer through Hemispherical-Depression-Assisted Self-Assembling and Vertical Lifting

Liquid interfacial self-assembled two-dimensional nanomonolayers are promising as colloidal crystal templates or lithographic masks for the mass production of periodic nanoarrays. However, the interfacial self-assembled monolayers usually suffer various defects such as grain boundaries, line defects, vacancy defects, and multilayers due to imperfect process technology. Here, we demonstrated a series of optimized strategies including the hemispherical-depression-assisted self-assembly, dot coating approach, and vertical lifting transfer protocol to eliminate surface defects especially multilayers jointly. After stabilizing the hemispherical depression height to 1.5 mm with a 40° tilted silicon slide, the self-assembly time was significantly shortened to 10 min for a 1 × 2 cm2 area of PS monolayer. The series of strategies were successfully implemented not only on glass and silicon flat substrates but also on the side and end face of optical fiber, even on flexible substrates, while maintaining great morphologies. Besides, in association with nanospherical lens photolithography and nanosphere lithography technologies, various two-dimensional nanoarrays such as nanotriangles and nanoholes could be successfully obtained for subsequent optical properties research, which present broad application prospects in biochemical sensing, medicine, environmental protection, and other fields.

Perforated Carbon Nanotube Film Assisted Growth of Uniform Monolayer MoS2.

Scaling up the chemical vapor deposition (CVD) of monolayer transition metal dichalcogenides (TMDCs) is in high demand for practical applications. However, for CVD-grown TMDCs on a large scale, there are many existing factors that result in their poor uniformity. In particular, gas flow, which usually leads to inhomogeneous distributions of precursor concentrations, has yet to be well controlled. In this work, the growth of uniform monolayer MoS2 on a large scale by the delicate control of gas flows of precursors, which is realized by vertically aligning a well-designed perforated carbon nanotube (p-CNT) film face-to-face with the substrate in a horizontal tube furnace, is achieved. The p-CNT film releases gaseous Mo precursor from the solid part and allows S vapor to pass through the hollow part, resulting in uniform distributions of both gas flow rate and precursor concentrations near the substrate. Simulation results further verify that the well-designed p-CNT film guarantees a steady gas flow and a uniform spatial distribution of precursors. Consequently, the as-grown monolayer MoS2 shows quite good uniformity in geometry, density, structure, and electrical properties. This work provides a universal pathway for the synthesis of large-scale uniform monolayer TMDCs, and will advance their applications in high-performance electronic devices.

Si9C15 monolayer: A silicon carbide allotrope with remarkable physical properties

Zero band gap in graphene and silicene is an important obstacle to their application in nanodevices. Combining carbon and silicon atoms to form a silicon carbide (${\mathrm{Si}}_{x}{\mathrm{C}}_{y}$) sheet is an applicable proposal to solve the band gap issue. Very recently, a large-scale monolayer $\mathrm{Si}{}_{9}\mathrm{C}{}_{15}$ was successfully synthesized and exhibited suitable band gap and environment stability [Gao et al., Adv. Mater. 34, 2204779 (2022)]. Motivated by this recent experiment, we explore the electronic, mechanical, optical, and thermoelectric properties of a $\mathrm{Si}{}_{9}\mathrm{C}{}_{15}$ monolayer using first-principles calculations. Our results reveal that the monolayer is an auxetic material with a negative Poisson ratio of 0.175. The monolayer is a semiconductor with a direct band gap of 2.54 eV calculated by the Heyd-Scuseria-Ernzerhof functional (HSE06) functional. The optical properties of the monolayer are investigated by solving the Bethe-Salpeter equation. It is revealed that the optical band gap of the monolayer is 2.73 eV, and the exciton binding energy is equal to 1.14 eV, indicating a strongly bound bright exciton. Thermoelectric properties of the monolayer are calculated by combining density functional theory and Green's function formalism in the linear response regime. The lattice thermal conductance of the monolayer is 0.85 nW/K at room temperature, and the maximum figure of merit is 1.2 at 800 K, higher than that of other silicon carbide allotropes. High flexibility, a suitable band gap, high mobility, high optical absorption in the UV-visible region, and high thermoelectric efficiency make the $\mathrm{Si}{}_{9}\mathrm{C}{}_{15}$ monolayer a promising candidate for applications in mechanics, optoelectronics, and energy converters.

Preparation and photoelectric property of large scale monolayer MoS<sub>2</sub>

Transition metal dichalcogenide (TMDC) monolayers exhibit enhanced electrical and optoelectrical properties, which are promising for next-generation optoelectronic devices. However, large-scale and uniform growth of TMDC monolayers with large grain size is still a considerable challenge. Presented in this work is a simple and effective approach to fabricating largescale molybdenum (MoS<sub>2</sub>) disulfide monolayers by chemical vapor deposition (CVD) method. It is found that MoS<sub>2</sub> grows from single crystal into thin film with the increase of oxide precursor proportion. The photodetector of large scale monolayer layer MoS<sub>2</sub> film is fabricated by depositing metal electrodes on the interdigital electrode mask through using thermal evaporation coating. Finally, the highly stable and repeatable photoelectric responses under the conditions of different voltages and different laser power are characterized under 405-nm laser excitation, with response time decreasing down to the order of milliseconds (ms). In addition, the photodetector achieves a wide spectral detection range from 405 nm to 830 nm, that is, from visible light to near-infrared light wavelength range, with optical response (<i>R</i>) of 291.7 mA/W and optical detection rate (<i>D</i><sup>*</sup>) of 1.629×10<sup>9</sup> Jones. The monolayer MoS<sub>2</sub> thin film photodetector demonstrated here has the advantages of low cost, feasibility of large-scale preparation, and good stability and repeatability in the wide spectrum range from visible light to near infrared light wavelength, providing the possibilities for future applications of electronic and optoelectronic devices .

Ultrahigh photoresponse in strain- and domain-engineered large-scale MoS2 monolayer films

Ultrahigh photoresponse with the record photoresponsivity is achieved by the strain- and domain-engineering of large-scale monolayer MoS2 films for maximum tensile strain and suitable atomic alignments.

Aluminum nitride crystal-based photodetector with bias polarity-dependent spectral selectivity

Visible-blind ultraviolet-selective photodetection and ultraviolet-visible broad spectral photodetection are two essential functions eagerly pursued in each application area. However, they usually cannot be realized simultaneously in a bare photodetector because their different underlying photoexcitation processes would interfere with each other. In this work, a photodetector integrating the two distinct photodetector characteristics is presented. The device is prepared based on the heterojunction of a large-scale aluminum nitride bulk crystal and monolayer graphene. The visible-blind ultraviolet-selective photodetection and the ultraviolet-visible broad spectral photodetection are separately manifested in the device depending on the bias polarity. Under a negative bias, the device is a visible-blind deep-ultraviolet photodetector, demonstrating a 193/785 nm rejection ratio of over 106 for the photocurrent and a 193/405 nm rejection ratio of over 103 for the signal/noise ratio. Under a positive bias, the device performs as a broad spectral photodetector responding to light from 193 to 785 nm. Systematical characterization reveals that different photodetection manners are the synergistical results of the different photon energies of the incident light, wavelength-dependent penetration depths in AlN, and the different working modes of the device under different bias conditions. This work provides a particular dual-functional photodetector, which is of great significance in terms of both application and device physics.

Controllable growth of wafer-scale monolayer transition metal dichalcogenides ternary alloys with tunable band gap

The tuning of band gap is very important for the application of two-dimensional (2D) materials in optoelectronic devices. Alloying of 2D transition metal dichalcogenides (TMDCs) is an important way to tune the wide band gap. In this study, we report a multi-step vapor deposition method to grow monolayer TMDC ternary alloy films with wafer scale, including Mo1−x W x S2, Mo1−x W x Se2 and MoS2x Se2(1−x), which are accurately controllable in the elemental proportion (x is from 0 to 1). The band gap of the three 2D ternary alloy materials are continuously tuned for the whole range of metal and chalcogen compositions. The metal compositions are controlled by the as-deposited thickness. Raman, photoluminescence, elemental maps and TEM show the high spatial homogeneity in the compositions and optical properties across the whole wafer. The band gap can be continuously tuned from 1.86 to 1.99 eV for Mo1−x W x S2, 1.56 to 1.65 eV for Mo1−x W x Se2, 1.56 to 1.86 eV for MoS2x Se2(1−x). Electrical transport measurements indicate that Mo1−x W x S2 and MoS2x Se2(1−x) monolayers show n-type semiconductor behaviors, and the carrier types of Mo1−x W x Se2 can be tuned as n-type, bipolar and p-type. Moreover, this control process can be easily generalized to other 2D alloy films, even to quaternary or multi-element alloy materials. Our study presents a promising route for the preparation of large-scale homogeneous monolayer TMDC alloys and the application for future functional devices.