Two-dimensional Thin Films Research Articles

A two-dimensional thin film structure with spectral selective emission capability suitable for high-temperature environments

Spectral selective emission materials, characterized by their high emission efficiency within specific wavelength ranges, play a crucial role in various fields such as infrared stealth and radiative cooling. In this study, leveraging the tunneling effect of ultrathin metal layers and the impedance matching principle, we designed a one-dimensional multilayer thin film structure composed of Mo/Ge/Al. This design successfully achieved spectral selective emission within the 3-14 μm infrared spectrum range, showcasing superior high-temperature infrared stealth capabilities. Research findings indicate that the infrared atmospheric window (ε3-5 μm=0.40, ε8-14 μm=0.30) maintains a low emissivity within the temperature range from room temperature to 400°C, while high emissivity in non-infrared atmospheric windows (ε5-8 μm=0.81) enables radiative cooling. The use of high emissivity in the non-infrared atmospheric window reduced the surface temperature of this structure by 15.3°C compared to untreated Si substrate under similar heating conditions. The temperature reduction was 34.1°C when compared to a Si substrate coated with a 200 nm Al film. This straightforward and easily scalable structure not only presents novel solutions for applications in infrared stealth and radiative cooling but also holds vast potential for diverse fields including military, aerospace, and industrial manufacturing, injecting fresh impetus and possibilities into technological advancement.

Controllable and fast growth of high-quality atomically thin and atomically flat Bi2O2Se films

As a promising 2D material, bismuth oxyselenide (Bi2O2Se) has demonstrated significant potential to overcome existing technical barriers in various electronic device applications due to its unique physical properties like high symmetry, adjustable electronic structure, and ultra-high electron mobility. However, the rapid growth of Bi2O2Se films down to a few atomic layers with precise control remains a significant challenge. In this work, the growth of two-dimensional (2D) Bi2O2Se thin films by the pulsed laser deposition (PLD) method is systematically investigated. By controlling temperature, oxygen pressure, laser energy density, and laser emission frequency, we finally prepare atomically thin and flat Bi2O2Se (001) thin films on the (001) surface of SrTiO3. Importantly, we provide a fundamental and unique perspective toward understanding the growth process of atomically thin and flat Bi2O2Se films, and the growth process primarily proceeds in four steps. Moreover, the combined results of the crystallinity quality, surface morphology, and the chemical states demonstrate the PLD-growth of high-quality Bi2O2Se films in a controllable and fast mode.

Sulfuration-induced enhancement of photodetection performance of photoconductive detector based on Bi2O2Se thin film

Significant interlayer transmission obstacles and numerous interface barriers in two-dimensional material thin films impede charge transfer, degrading device performance. However, appropriate impurity ions can enhance charge mobility by creating transfer channels and regulating interface defect states. Herein, a series of Bi2O2SxSe1-x films were prepared by a one-step hydrothermal sulfurization method. The XRD and XPS measurement and analysis can confirm that the Se atoms have been replaced by the S atoms. The introduction of S atoms can effectively enhance the number of defects in Bi2O2Se, resulting in an increase in carrier concentration, thereby improving the conductivity of Bi2O2Se film. The photoconductive detectors fabricated from these Bi2O2SxSe1-x films showed improved detection performance compared to those made from the pristine Bi2O2Se film, and the responsivity is increased by nearly 1000 times. Notably, the detector based on Bi2O2S0.37Se0.63 film has high responsivity (47.7 mA/W) and fast response time (15 ms/25 ms). The simple thin film preparation method and good device performance make it have great potential for large-scale preparation.

Two-Dimensional Materials-Based Thin Films and Coatings

Here, we highlight the research presented in this Special Issue, focusing on the innovative use of graphene and other two-dimensional (2D) materials to develop advanced coating technologies. The contributions herein address critical challenges such as the scalable fabrication and stable dispersion of such materials and their compatibility with conventional coating systems, offering solutions that enhance their mechanical strength, chemical stability, and multifunctionality. The featured studies demonstrate the diverse applications of these materials, from protective anticorrosive barriers to high-performance optoelectronic devices and environmental remediation. Moving forward, future research is encouraged to explore novel 2D materials, hybrid coating strategies, and advanced computational modeling to overcome existing limitations and unlock new possibilities.

Unveiling the impact of organic cation passivation on structural and optoelectronic properties of two-dimensional perovskites thin films

Several organic cations have been used to passivate perovskite films; however, selecting the optimal cation remains challenging. In this work, we carried out density functional theory calculations to understand the effects induced by 17 different organic cations on the passivation (P-cations) of thin two-dimensional P2 (MAn−1)PbnI3n+1 perovskites films, where n=1 and 2. We found that the interactions between different types of P-cations and the inorganic slab affect the length and angles of the bonds within the inorganic framework (PbI6-octahedra). In general, the binding mechanism includes the interactions of organic cations with the inorganic framework, which leads to the accumulation of electron density within the halides, indicative of Brønsted–Lowry acid–base interactions. Oxygenated groups facilitate additional H-bond formation through -OH and -COOH groups, promoting the localization of the electron density between layers and improving the energetic stability of the system. Based on the results and analysis, we found that three P-cations might have higher potential for real-life applications, namely 4-fluorophenylethylammonium (FPEA), phenylethylammonium (PEA) and butylammonium (BtA).

Thermal analysis of GaN HEMTs using nongray multi-speed phonon lattice Boltzmann method under Joule heating effect

Gallium Nitride (GaN) high electron mobility transistors (HEMTs) exhibit superior electrical properties for power and radio frequency applications, but performance is compromised by localized Joule heating, increasing channel temperatures. Precise thermal analysis during design is essential for optimizing device architecture and management strategies. Traditional methods like the Fourier heat diffusion equation (HDE) and the phonon lattice Boltzmann method (PLBM) with the D2Q8 scheme inadequately model phonon ballistic transport at high Knudsen numbers. This study introduces a nongray multi-speed PLBM integrated with the drift-diffusion model to analyze electro-thermal processes in GaN HEMTs. Validated through simulations of thermal conductivities in two-dimensional GaN thin films, the approach examines internal temperature rise in GaN HEMTs under different gate voltages, comparing results with HDE and gray BTE models, and emphasizing the need for non-Fourier effects in thermal analysis. It also evaluates the impact of GaN layer thickness on temperature distribution, providing a robust solution for thermal analysis in GaN HEMTs and other field-effect transistors.

The nucleation mechanism of MoS2 on Au(111) surface

Controllable preparation of two-dimensional MoS2 single crystal thin films is the key to its development and application. Chemical vapor deposition (CVD) is considered one of the most promising methods for large-scale production of 2D MoS2 films. However, at present, the single crystal MoS2 films prepared by CVD have defects and insufficient sizes. The reason may be that the CVD growth mechanism of MoS2 is still unclear. In this paper, density functional theory (DFT) and molecular dynamics (MD) simulations are used to systematically explore the nucleation mechanism of MoS2 grown on Au (111) surface with molybdenum powder and H2S as precursors. In our theoretical study, some key links of MoS2 nucleation have been clearly displayed, such as deposition of molybdenum, dehydrogenation of H2S, lifting of Mo atoms and so on. This study provides a deep understanding of the nucleation of MoS2 during CVD growth and theoretical basis for the synthesis of various TMDs.

Photodeposition of NiS thin film enhanced the visible light hydrogen evolution performance of CdS nanoflowers

The use of loaded co-catalysts has been found to be effective in addressing the issue of rapid recombination of photogenerated carriers, which can limit photocatalytic efficiency. Herein, we report on the adsorption of Ni2+ on the surface of CdS by photodeposition to obtain NiS/CdS composites. It improves visible light response while providing abundant active sites for photocatalytic reaction. NiS/CdS exhibits a superior photocatalytic H2 evolution rate of up to 7557 μmol·g−1·h−1, which is approximately 4.9 times higher than that of CdS. The corresponding AQE is 19.6% at 420 nm. XPS and photoelectrochemical testing provide a possible mechanism, indicating that the heterojunction between NiS and CdS promotes the transfer and separation of interfacial charge, thus accelerating the water decomposition reaction. This work demonstrates the effectiveness of loading two-dimensional thin film co-catalysts to design high-performance and low-cost composites for promoting the photocatalytic reaction.

Trapping and recapturing single DNA molecules with pore-cavity-pore device

Single-molecule detection technology is a technique capable of detecting molecules at the single-molecule level, characterized by high sensitivity, high resolution, and high specificity. Nanopore technology, as one of the single-molecule detection tools, is widely used to study the structure and function of biomolecules. In this study, we constructed a small-sized nanopore with a pore-cavity-pore structure, which can achieve a higher reverse capture rate. Through simulation, we investigated the electrical potential distribution of the nanopore with a pore-cavity-pore structure and analyzed the influence of pore size on the potential distribution. Accordingly, different pore sizes can be designed based on the radius of gyration of the target biomolecules, restricting their escape paths inside the chamber. In the future, nanopores with a pore-cavity-pore structure based on two-dimensional thin film materials are expected to be applied in single-molecule detection research, which provides new insights for various detection needs.

Superconductivity in atomically thin films: Two-dimensional critical state model

The comprehensive understanding of superconductivity is a multiscale task that involves several levels, starting from the electronic scale determining the microscopic mechanism, going to the phenomenological scale describing vortices and the continuum-elastic scale describing vortex matter, to the macroscopic scale relevant in technological applications. The prime example for such a macro-phenomenological description is the Bean model that is hugely successful in describing the magnetic and transport properties of bulk superconducting devices. Motivated by the development of novel devices based on superconductivity in atomically thin films, such as twisted-layer graphene, here, we present a simple macro-phenomenological description of the critical state in such two-dimensional (2D) thin films. While transverse screening and demagnetization can be neglected in these systems, thereby simplifying the task in comparison with usual film- and platelet-shaped samples, surface and bulk pinning are important elements to be included. We use our 2D critical state model to describe the transport and magnetic properties of 2D thin-film devices, including the phenomenon of nonreciprocal transport in devices with asymmetric boundaries and the superconducting diode effect. Published by the American Physical Society 2024

Fabrication and application of two-dimensional tungsten disulfide thin film

To form a tungsten disulfide film, a tungsten trioxide film is deposited first and then hydrogen sulfide is injected into the furnace tube to sulfide the tungsten trioxide film in a high-temperature environment. Due to the need to accurately control the thickness of tungsten trioxide, the power of the RF sputtering machine was reduced as much as possible in a stable condition in the experiment and the bias voltage during each process was monitored. In this experiment, a sapphire substrate and a silicon substrate with 200[Formula: see text]nm silicon dioxide are used. Then use optical instruments such as Raman optics, ellipsometers and high-resolution electron transmission microscopes, atomic force microscopes and other instruments for further measurement. The analysis results show that we have successfully made tungsten disulfide films of different thicknesses. Moreover, two-dimensional tungsten disulfide thin film has a response to light, gas and pH and related devices have been successfully fabricated in experiments. Among them, comparing the single-layer film and the double-layer film, the film quality of the double-layer film is better. The quality of the film grown on the sapphire substrate is also better than the quality of the film grown on the silicon dioxide substrate.

Phonon transport simulation with an extended VOF scheme for nano-structured thin film

Control of phonon transport in solid devices is important for thermoelectric energy conversion and phononic crystal technology, and much attention has been paid to sub-micrometer or nanometer scale structures for that purpose. In order to investigate how various nano-structures affect the phonon transport, we have developed a numerical simulation code based on the Boltzmann transport equation of phonon distribution function in the reciprocal space. To appropriately treat the phonon transport at interface of an arbitrary shape, we newly introduced a volume of fluid like scheme, which was originally developed for multi-phase flow simulation. As a test of the developed simulation code, we investigated two-dimensional thin silicon films with two types of hole shape and two types of hole arrangement. The results essentially agree with recent experiments.

Two-dimensional thin film lithium niobate photonic crystal waveguide for integrated photonic chips

The photonic crystal waveguide (PCW) possesses remarkable capabilities in manipulating light beams and light–matter interactions within the subwavelength range. This property renders it a highly promising structure for the miniaturization of optical devices. We delve into the mode characteristics in two-dimensional PCWs on thin film lithium niobate, establish the correlation between the single-mode region in the PCW and its photonic crystal duty cycle, and observe mode hybridization in the waveguide. A lithium niobate PCW with sidewall angles can realize single-mode transmission or mode conversion by adjusting the width of its defective waveguides, and it is theoretically and experimentally verified that a change in the width of the waveguide shifts the operating wavelength. The results of the mode analysis are useful in the design of waveguide structures for photonic crystal-based electro-optical modulators and optical sensors.

Single particle detection of micro/nano plastics based on recyclable SERS sensor with two-dimensional AuNPs thin films

Micro/nano plastics (MNPs) are widely recognized as a global environmental threat. Due to the lack of viable and reliable analytical methods, the identification and visualization of microplastics, especially nanoplastics, remains difficult to achieve, especially for single particle detection. Here, this study offered a simple, high-sensitive, and low-cost technique for MNPs identification using surface-enhanced Raman scattering (SERS) based on two-dimensional AuNPs thin films. This work realized the single particle detection of polystyrene (PS) and polymethyl methacrylate (PMMA), owing to the hot spot region created by the electromagnetic amplification of two-dimensional AuNPs thin films. The SERS-active substrate had a high SERS property in the single particle detection of MNPs as small as 200 nm. Moreover, this method can be used for the SERS mapping as a rapid detection technique for MNPs. In additon, this SERS-active substrate had good stability and reusable performance after 5 cycles. These findings indicated that this SERS strategy based on two-dimensional AuNPs thin films was an efficient and rapid method to identify nanoplastics in environment. And we believed that this kind of recyclable SERS sensor may create a universal strategy for the detection of MNPs at trace level.

Large anomalous transverse transport properties in atomically thin 2D Fe3GaTe2

Anomalous transverse conductivities, such as anomalous Hall conductivity (AHC), anomalous Nernst conductivity (ANC), and anomalous thermal Hall conductivity (ATHC), play a crucial role in the emerging field of spintronics. Motivated by the recent fabrication of two-dimensional (2D) ferromagnetic thin film Fe3GaTe2, we investigate the thickness-dependent anomalous transverse conductivities of the 2D Fe3GaTe2 system (from one to four layers). The atomically ultrathin 2D Fe3GaTe2 system shows above-room-temperature ferromagnetism with a large perpendicular magnetic anisotropy energy. Furthermore, we obtain a large AHC of −485 S/cm in the four-layer thickness, and this is further enhanced to −550 S/cm with small electron doping. This AHC is seven times larger than the measured AHC in thicker 2D Fe3GaTe2 (178 nm). The ANC also reaches 0.55 A/K.m in the four-layer structure. Along with these, the four-layer system exhibits a large ATHC (−0.105 ~ −0.135 W/K.m). This ATHC is comparable to the large ATHC found in Weyl semimetal Co3Sn2S2. Based on our results, the atomically ultrathin 2D Fe3GaTe2 system shows outstanding anomalous transverse conductivities and can be utilized as a potential platform for future spintronics and spin caloritronic device applications.

Investigations of TiC and SiC films synthesized by DC magnetron sputtering and thermal annealing

In this research article, two-dimensional thin films of titanium, hierarchical titanium-carbon, titanium carbide, hierarchical silicon-carbon, and silicon carbide films were synthesized on the plates of AISI 304 stainless steel and glass slides, individually, using DC magnetron sputtering techniques. During the sputtering process, the creation of stable plasma was affected by the vacuum level, DC potential, plasma current, and Argon gas. The results showed that the coatings of titanium, titanium-carbon, and titanium carbide modified the optical properties of stainless steel. Additionally, titanium carbide coatings displayed a shiny gold appearance with a hardness of 1500 HV. Besides this, X-ray diffraction studies revealed a hexagonal Ti in the metallic films and C60, and diamond-like structures dispersed on its surface after the carbon coating. The spherical particulates of Ti and C60/diamond-like carbon particulates anchored on its surface were observed using scanning electron microscope analysis. The structural analysis indicated C vacant sites, which are available for ionic/electronic charge diffusion, thereby rendering these structures suitable for making Li storage batteries as electrode material.

Two-Dimensional Molecular Ferroelectric Thin Films for Polarization-Driven Tunable Photovoltaic Devices with High Photocurrent Density

Two-Dimensional Molecular Ferroelectric Thin Films for Polarization-Driven Tunable Photovoltaic Devices with High Photocurrent Density

A two-dimensional PtSe2 thin film coupled with a graphene/Si Schottky junction for a high-performance photodetector.

Silicon materials are irreplaceable in the modern information society because of their rich resource, low price, and mature manufacturing technology for optoelectronics. However, improving the responsivity and response speed of silicon-based photodetectors is still a challenge. Here, a double-heterojunction photodetector (PD) by coupling two-dimensional PtSe2 thin film with a graphene/silicon Schottky junction is proposed. The introduction of PtSe2 enhances the built-in electric field of the device, thus suppressing the dark-state current and promoting the separation of photogenerated electron-hole pairs. Under 808 nm laser illumination, the PtSe2/graphene/Si PD exhibits an optimal responsivity, specific detectivity, and response speed of 0.81 A W-1, 1.24 × 109 Jones, and 43.6/51.2 μs, respectively. These performance indexes are obviously better than the corresponding graphene/Si device. Furthermore, the PtSe2/graphene/Si PD has good environmental durability and photoresponse ability from the ultraviolet to near-infrared. This work will provide new possibilities for designing novel silicon-based photodetection devices with high performance and fast response.

Controllable epitaxy of CrTe2 thin films for application as saturable absorbers

Two-dimensional CrTe2 thin films exhibit excellent nonlinear optical characteristics and have enormous prospects in ultrafast optical devices.

Sputter-grown two-dimensional α-MoO3 thin films: Microstructure dependence on growth conditions

Van der Waals epitaxy is an emerging technology for transferring freestanding films grown at high temperatures onto arbitrary substrates using two-dimensional epitaxial layers. This study investigates the microstructure and growth of α-MoO3 thin films prepared via sputtering. Under a narrow deposition temperature range, epitaxial α-MoO3 thin films were successfully grown on SrTiO3 substrates. The films were characterized by a flat surface morphology, corresponding to (0k0) planes, without undergoing reaction with the substrates. To examine the growth behavior and surface morphological variation of the α-MoO3 thin films under various process parameters, we modified the Ar:O2 ratio, working pressure, sputtering power, and crystal structure of the substrates. Transmittance electron microscopy confirmed the epitaxial growth of the α-MoO3 thin films on (001)-oriented SrTiO3 substrates with a two-dimensional layer structure. Finally, we demonstrate the morphological evolution of the α-MoO3 thin films etched in heated water as a function of soaking time. The etching results suggest that the α-MoO3 thin films are promising sacrificial layers for the transfer of large-scale epitaxial thin films onto flexible substrates. External control of the strain states of transferred freestanding thin films by bending or stretching provides new perspectives for the design of flexible or wearable electronic devices.