Mechanical Exfoliation Technique Research Articles

A highly sensitive MoS2 nanosheet gas sensor prepared by mechanically exfoliating for room temperature H2S detection

Hydrogen sulfide (H2S) is a hazardous gas that is highly hazardous to human health, even at extremely low concentrations. Due to surface defects easily caused by hydrothermal or stepwise self-assembly methods, the response of the molybdenum disulfide (MoS2) gas sensor usually does not exceed 60%. In this article, mechanical exfoliation and full dry transfer techniques were used to reduce the surface defects of MoS2 nanosheets, improving the response of gas sensors to H2S at room temperature. The response to H2S was about 80% at a concentration of 15 ppm and about 12% at a concentration as low as 500 ppb. In addition, for 10 ppm H2S, applying a negative gate voltage, the response can be increased by approximately 10% to enhance the gas response. This study demonstrates the enormous potential of the gas sensor based on mechanical exfoliated MoS2 nanosheets for detecting low concentrations of H2S, providing new insight into the materials preparation of highly sensitive gas sensors.

Method of mechanical exfoliation of bismuth with micro-trench structures

The discovery of graphene led to a burst in search for 2D materials originating from layered atomic crystals coupled by van der Waals force. While bulk bismuth crystals share this layered crystal structure, unlike other group V members of the periodic table, its interlayer bonds are stronger such that traditional mechanical cleavage and exfoliation techniques have shown to be inefficient. In this work, we present a novel mechanical cleavage method for exfoliating bismuth by utilizing the stress concentration effect induced by micro-trench SiO2 structures. As a result, the exfoliated bismuth flakes can achieve thicknesses down to the sub-10 nm range, which are analyzed by atomic force microscopy and Raman spectroscopy.

Layer‐Dependent Optical Modulation and Field‐Effect‐Transistor in Two‐Dimensional 4H‐SnS2 Layers

AbstractTwo–dimensional (2D) 4H‐polytype tin disulfide (SnS2) flakes are synthesized using the chemical vapor transport technique. The weak Van der Waals force between the 2D SnS2 layers offers an easy exfoliation of flakes down to a bilayer of thickness ≈2.02 (±0.1) nm using a mechanical exfoliation technique. The optical and field effect transistor (FET) characteristics of the exfoliated 2D 4H‐SnS2 layers are studied. The exfoliated layers are used to fabricate the ≈13‐layered SnS2 FET. The 4H‐SnS2 exhibits a high on/off ratio of ≈106 and mobility ≈1–4 cm2 V−1 s−1. The low mobility of the 4H‐SnS2 FET devices shows an insulating state concordant with the 2D Motts variable range hopping mechanism at varying temperatures. Moreover, it is found that the optical bandgap of the 2D SnS2 single‐crystal layers is largely widened for the bilayers and tri‐layers. The optical bandgap energies vary in the range of 2.56–1.99 eV. The significant alteration in bandgap energies of ≈0.57 eV offers downscaling of the 2D nanoscale semiconducting devices. Such layer‐sensitive changes in optical transmittance, absorbance, and bandgap energies are reflected in Commission Internationale de L'Eclairage (CIE) chromaticity, showing the distinct color of transmittance through various 2D 4H‐SnS2 layers.

Graphene: The Revolutionary 2D Material

Graphene is considered as the initial laboratory-made 2D material which initiated a new revolution in the field of materials science. Since its discovery, lots of researchers have discovered various properties and tested it for many potential applications. The present report discusses a brief overview of graphene, mainly concentrating on the structure, synthesis, and applications of graphene. The mechanical exfoliation and CVD techniques for the synthesis of graphene. Some of the applications of graphene in the field of flexible electronics, sensing, and photovoltaic are reviewed in this report

A new 2D van der Waals material with air stability and in-plane anisotropy

A new 2D van der Waals material MnTeMoO 6 , which is obtained by the facile mechanical exfoliation technique. This material is very durable in the air, as confirmed by the unchanged Raman spectra. Polarization-dependent angle-resolved Raman tests show that MnTeMoO 6 flake displays significant in-plane anisotropy.

Excellent Seebeck coefficient observed in exfoliated N-type Tungsten Disulphide (WS2)

Transition Metal Dichalcogenides (TMDCs) are considered necessary two-dimensional materials in the field of thermoelectricity due to tunable band structure, large effective mass, valley degeneracy and so on. So, in this work, among the TMDCs, the thermoelectric response of mechanically exfoliated Tungsten Disulphide (WS2) has been explored to achieve environment-friendly refrigeration and energy harvesting. Herein, we fabricated the WS2 thermoelectric device through the mechanical exfoliation technique. A few layers of exfoliated WS2 flake were characterized with Raman Spectroscopy, and a thickness of 4.2 nm (∼6 layers) was confirmed through Atomic Force Microscopy. Thermoelectric measurements were carried out using the 2ω method, and the Seebeck coefficient of about ∼ -389 μV/K was recorded at room temperature. The electric and thermoelectric characteristics reveal the n-type conduction in the device and show good mobility of 16.5 cm2V-1s-1. This study experimentally validates the potential of mechanically exfoliated n-type WS2 in thermoelectricity generation.

Tunable performance of indium tin oxide-zinc oxide as Q-switcher

This work successfully demonstrated the tunable performance of a Q-switched pulse erbium-doped fiber laser (QPEDFL) using Indium Tin Oxide-Zinc Oxide (ITO-ZnO)-based SA. The ITO-ZnO is a material growth between transparent conducting oxides and transition metal oxides on a glass slide. The mechanical exfoliation technique is applied to serve the ITO-ZnO as SA and inserted in a single-ring cavity design. The QPEDFL in a single ring cavity gives the repetition rate, pulse width, average output power, pulse energy, and SNR of 38.49 kHz, 5.50 µs, 0.47 mW, 12.72 nJ, and 64.20 dB, respectively. Furthermore, the ITO-ZnO as SA provides a tuning range of 30.40 nm. By tuning the TBF, the wavelength changes from 1531.60 nm to 1562.0 nm, causing the pulse repetition rate and pulse width of the QPEDFL to vary from 41.50 kHz to 17.20 kHz, and 4.22 µs to 10.05 µs, respectively. According to the results obtained, it can confidently say the suggested pulse laser has potential in the application of optical communication systems as a light source.

Biosynthesized zinc oxide and titanium dioxide nanoparticles by aloe vera extract for tunable Q-switched application

Zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles (NPs) were biosynthesized by using aloe vera extract for tunable Q-switched application. ZnO TiO2 NPs was then prepared as a saturable absorber (SA) via mechanical exfoliation technique. By measuring nonlinear optical absorption, the modulation depth and saturation intensity were 1.57 % and 0.05 MW/cm2, respectively. The obtained output pulse had a 1567.60 nm central wavelength and a signal-to-noise ratio (SNR) of 44.94 dB at a pump power of 248.50 mW. The increase of pump power from 178.00 mW to 248.50 mW has led to continuously increase in the repetition rate from 7.70 kHz to 12.99 kHz and decrease from 45.35 µs to 28.93 µs of pulse width, respectively. At maximum pump power, 5.06 nJ of pulse energy was also recorded. When the tunable bandpass filter (TBF) was incorporated into the cavity, the wavelength-tunable Q-switched pulses was constructed, and the tuning wavelength from 1530 nm to 1570 nm that of 40 nm covering wavelength was attained. This finding suggests that a wider range of tunable and simple designs is applicable to optical communications and sensing applications.

Exact Relationship between Black Phosphorus Thickness and Behaviors of Field-Effect Transistors

As a two-dimensional (2D) semiconductor material with excellent optoelectronic properties, black phosphorus (BP) has attracted widespread attention. It was found that the energy band structure of BP crystal changes with its thickness if BP is thin. To explore the accurate effects of the BP thicknesses on devices, BP-FETs with different BP thickness (50 nm, 40 nm, 30 nm, 20 nm, and 6 nm) as the channel material were fabricated by mechanical exfoliation technique. The output characteristics and transfer characteristics of the BP-FETs were analyzed in detail. The source–drain current (Ids) of devices is directly related to the BP thickness. The larger the BP thickness, the larger the Ids obtained under the same gate voltage modulation, but the electric field modulation effect decreases. Especially, the correlation between Ids and BP thickness can be described by a semi-empirical formula, which predicts that only when the BP thickness is less than 21.7 nm, the band structure of BP will be significantly affected by the thickness. The mobility of the carrier increases with the increasing of the BP thickness; for BP thickness of 6 nm, 20 nm, 30 nm, 40 nm, and 50 nm, the mobility is about 52.5 cm2/Vs, 187.5 cm2/Vs, 214.4 cm2/Vs, 252.5 cm2/Vs, and 336.4 cm2/Vs. Finally, the 50 nm BP in FET was etched to 30 nm using plasma etching technology to further verify the above experimental results. It also confirmed that plasma etching methods tend to introduce structural damage and impurity elements, which in turn has an impact on the output characteristics of the device.

Automated mechanical exfoliation technique: a spin pumping study in YIG/TMD heterostructures.

Spintronics devices rely on the generation and manipulation of spin currents. Two-dimensional transition-metal dichalcogenides (TMDs) are among the most promising materials for a spin current generation due to a lack of inversion symmetry at the interface with the magnetic material. Here, we report on the fabrication of Yttrium Iron Garnet(YIG)/TMD heterostructures by means of a crude and fast method. While the magnetic insulator single-crystalline YIG thin films were grown by magnetron sputtering, the TMDs, namely MoS2 and MoSe2, were directly deposited onto YIG films using an automated mechanical abrasion method. Despite the brute force aspect of the method, it produces high-quality interfaces, which are suitable for spintronic device applications. The spin current density and the effective spin mixing conductance were measured by ferromagnetic resonance, whose values found are among the highest reported in the literature. Our method can be scaled to produce ferromagnetic materials/TMD heterostructures on a large scale, further advancing their potential for practical applications.

One-Step Exfoliation Method for Plasmonic Activation of Large-Area 2D Crystals.

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO2 /Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS2 , MoSe2 ) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

Evidence of Charging and Storage at the MoS2/Si Interface

Non-volatile memory chips are important in devices for data storage, where demand kept increasing and the demand of large capacity of storage grew with the development in technology. Flash memory is one of the non-volatile memory devices that has a simple structure that is based on CMOS basic structure. Flash memory was introduced by Toshiba in 1984, which is an electrically erasable/programable non-volatile memory device [1]. Due to the development new technology the demand for size scale down of the memory structure was high, at which the size was kept on being reduced. At sizes less than 65nm flash memory started to show different problem including charge loss and the stress-induced leakage current, due to the tunnelling oxide thickness reduction. [2,3]. The structure of the flash memory has been developed to overcome the issues due to size reduction by introducing nanoparticles to the floating gate, which showed huge development. This development made researchers explore different nanoparticles including 2D materials. 2D materials has distinctive electrical, optical, chemical, and mechanical properties that makes them interesting to be implemented in new technology [4]. One of the interesting 2D materials is Molybdenum disulfide (MoS2). MoS2 have a multitude of features such as high strength, flexibility and has better quantum yield than bulk MoS2 [5]. 2D films of MoS2 can be widely used in various devices for their electronic, optical, and catalytic properties [6]. In this work MoS2 nanoflakes were bonded to silicon to study the charge transport at the interface.The initial bulk MoS2 was in crystal form, which was used to prepare the MoS2 nanoflakes. The preparation procedure for the MoS2 flakes was by using mechanical exfoliation technique. Nitto SPV224 PVC tape had been used to exfoliate bulk MoS2, where part of the bulk MoS2 crystal is placed on the sticky part of the tape. The tape is then folded on the MoS2 piece with the tape surrounding the crystal on the top and bottom. The tape is then separated from each other, which results in exfoliation of the MoS2 into flakes. The more the steps are repeated of this process, the thinner the flakes are. The flakes on the tape were transferred to a p-Si wafer on a hot plate at 50C and bonded to create the MoS2/Si interface.The achieved MoS2 nanoflakes on p-Si wafer sample had been imaged using AFM topography and conductivity measurements. The AFM images were collected to identify the distribution of the flakes. AFM images showed clear flakes on the p-Si wafer at which they were distributed randomly over the surface. The thickness of single monolayer flakes measured was 0.65nm and two monolayer flakes were 1.3 nm. Furthermore, conductive AFM was employed to obtain the IV curve over the flake and obtain their electrical properties, where forward and reverses scanning was done showing a hysteresis loop. The hysteresis observed showed a voltage window gap of around ∆Vth=0.5 V. This indicates electron tunneling and charge storage at the MoS2/Si interface or in the MoS2. Finally, the results, highlight the potential use of 2D MoS2 flakes in future no power memory devices.

(Invited) Synthesis and Application of One-Dimensional Van Der Waals Heterostrucures Based on Single-Walled Carbon Nanotubes

A typical one-dimensional (1D) van der Waals (vdW) heterostructure consists of single-walled carbon nanotubes (SWCNT), boron nitride nanotube (BNNT), and molybdenum disulfide nanotube (MoS2NT), can be grown coaxially by successive chemical vapor deposition (CVD) steps [1]. The coaxially nested structure based on SWCNTs can expand the board application possibilities of 1D vdW heterostructures [2]. Semiconductor SWCNT wrapped with BNNT can be regarded as the ideal building blocks of field-effect transistors (FET) [3]. We have demonstrated the radial semiconductor–insulator–semiconductor (S-I-S) tunneling heterojunction diode by using a micrometer long 1D vdW heterostructure SWCNT@BNNT@MoS2NT [4]. By comparing optical properties of films of BNNT@MoS2NT and SWCNT@BNNT@MoS2NT, we found strong photoluminescence (PL) from monolayer MoS2NT and quenching of PL by coupling to SWCNT through thin BNNT [5]. The remarkable population of free charges and inter-tube excitons are demonstrated by ultrafast optical spectroscopy [6]. The inter-tube excition is regarded as the inter-layer excition for 2D heterostructures. Precise control of the chemical vapor deposition (CVD) process of each layer is essential since the mechanical exfoliation & stacking technique for 2D counterpart is not possible for 1D vdW heterostructures. The surprisingly sharp edge of BNNT grown on SWCNT is the signature of preference of nitrogen terminated zig-zag edge of h-BN common to 2D counterpart [7]. The next challenge is the CVD growth of various transition metal dichalcogenides on BNNTs. In addition to MoS2 nanotubes [1], we will discuss the growth control of WS2 nanotubes and NbS2 nanotubes. A general strategy we can tune the CVD condition from 2D flake to 1D tube is proposed [8]. Part of this work was supported by JSPS KAKENHI Grant Number JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan.

Exploring the structural and optoelectronic properties of natural insulating phlogopite in van der Waals heterostructures

Naturally occurring van der Waals crystals have brought unprecedented interest to nanomaterial researchers in recent years. So far, more than 1800 layered materials (LMs) have been identified but only a few insulating and naturally occurring LMs were deeply investigated. Phyllosilicate minerals, which are a class of natural and abundant LMs, have been recently considered as a low-cost source of insulating nanomaterials. Within this family an almost barely explored material emerges: phlogopite (KMg3(AlSi3)O10(OH)2). Here we carry out a high throughput characterization of this LM by employing several experimental techniques, corroborating the major findings with first-principles calculations. We show that monolayers (1L) and few-layers of this material are air and temperature stable, as well as easily obtained by the standard mechanical exfoliation technique, have an atomically flat surface, and lower bandgap than its bulk counterpart, an unusual trend in LMs. We also systematically study the basic properties of ultrathin phlogopite and demonstrate that natural phlogopite presents iron impurities in its crystal lattice, which decreases its bandgap from about 7 eV to 3.6 eV. Finally, we combine phlogopite crystals with 1L-WS2 in ultrathin van der Waals heterostructures and present a photoluminescence study, revealing a significant enhancement on the 1L-WS2 optical quality (i.e. higher recombination efficiency through neutral excitons) similarly to that obtained on 1L-WS2/hexagonal boron nitride heterostructures. Our proof-of-concept study shows that phlogopite should be regarded as a good and promising candidate for LM-based applications as a low-cost layered nanomaterial.

Thermal release tape assisted mechanical exfoliation of pristine TMD and the performance of the exfoliated TMD saturable absorbers for Q-switched laser generation

This research applied an enhanced material exfoliation and transfer mechanism to process various pristine transition metal dichalcogenide (TMD) materials into a few-layer structure. The mechanical exfoliation of the TMD materials was done using a thermal release tape (TRT) with a different adhesion property compared to the scotch tape (ST) commonly used in the conventional scotch tape assisted mechanical exfoliation (ST-ME) technique. The thermal release tape assisted mechanical exfoliation (TRT-ME) performs better than the ST-ME. Four pristine TMD saturable absorbers, including molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2), have been fabricated using the TRT-ME technique. Subsequently, the performances of these materials were investigated using the same experimental systems and settings. These TMD materials were tested as saturable absorbers (SAs) to induce pulsed lasing in an Erbium-doped fiber laser (EDFL) cavity. The TRT-exfoliated TMD saturable absorbers were incorporated into the laser cavity by sandwiching in between two fiber ferrules using a fiber connector. Incorporating these saturable absorbers into the laser cavity has successfully induced promising Q-switching operations at the C-band region. Besides the pump power-dependent output performance, the stability and wavelength tunability of the Q-switched lasers were also studied at the maximum pump power. The findings from the studies show that the Q-switched lasers induced by the TRT-exfoliated pristine TMD saturable absorbers were stable and wavelength-tunable. In the context of Q-switched laser, the TRT-exfoliated WS2 has the best performance among other pristine saturable absorbers due to its ability to generate the highest pulse energy.

Dry Transfer Process of Single-Layer Graphene on Multi-Layer Hexagonal Boron Nitride for High Quality Heterostructure

Carrier mobility of the graphene thin film is one of the most crucial factors to achieve superior device performance. Factors that influence the carrier mobility include film crystallinity, layer number, presence of contaminations, and effect of the substrate. In this work, to obtain high-quality graphene film, single-layer graphene film was formed on a hexagonal boron nitride (hBN) thin-film via mechanical exfoliation technique. Mechanical exfoliation of graphite was done on a silicon substrate covered with poly (methyl methacrylate)/ poly (acrylic acid). Three adjacent thin films, each having approximately 20 - 30 μm length and 3 - 4 μm width making an average area of 87.5 μm2 for each film. From the measured Raman spectrum, the 2D-to-G peak intensity ratio was 1.7, confirming that exfoliated graphene is a single layer. The absence of the D peak indicates minimum crystal defects in the film. The graphene layers were subsequently dry-transferred onto mechanically exfoliated multi-layer hBN thin film. The area of the exfoliated hBN is around 6,000 μm2. The quality of the transfer process was evaluated through inspection under an optical microscope. All three single-layer graphene thin films were successfully placed exactly on the hBN without any presence of bubble and wrinkle. The process was described very detailed. The formed single-layer graphene on hBN layer is expected to have superior electrical characteristics and can be utilized for the fabrication of high-performance nano device.

New progress and prospects of mechanical exfoliation technology of two-dimensional materials

Since the monolayer graphene was first obtained in the year of 2004, mechanical exfoliation technique has been widely used to prepare various two-dimensional materials such as transition metal dichalcogenides and black phosphorus. Among a variety of preparation techniques of two-dimensional materials, mechanical exfoliation technique shows advantages in its simplicity and universality. More importantly, the exfoliated two-dimensional samples are the ideal ones for many novel phenomena. This paper introduces the background of mechanical exfoliation technique and summarizes the problems of conventional mechanical exfoliation technique in the development of two-dimensional materials. In order to solve the problems of low efficiency and small sample size of conventional mechanical exfoliation technique, some modified mechanical exfoliation techniques have been developed, such as oxygen-plasma-assisted exfoliation method and gold-film-assisted exfoliation method. As a commonly used “top-down” preparation method, the new exfoliation technology is still full of vitality in basic research and application of two-dimensional materials. In the future, larger size and higher quality will be the development direction of exfoliation technology.

Spectroscopic Ellipsometry Investigation of Au‐Assisted Exfoliated Large‐Area Single‐Crystalline Monolayer MoS2

The recent Au‐assisted mechanical exfoliation technique has enabled scalable fabrication of millimeter‐sized, high‐quality monolayer 2D crystals. Herein, spectroscopic ellipsometry characterization of as‐exfoliated single‐crystalline monolayer MoS2 is reported. By developing layer‐by‐layer ellipsometry measurements and analysis, the complex dielectric function of monolayer MoS2 is extracted. Compared with bulk MoS2, the A and B exciton peaks in the dielectric function spectrum nearly disappear in the 1L MoS2–Au hybrid system. The charge transfer at the 1L MoS2–Au interface is responsible for this observation and the conclusion is also supported by Raman, photoluminescence, and differential reflectance measurements. These results reveal the impact of the Au substrate on the dielectric response of 2D monolayers with strong excitonic effect and will deepen our understanding of the 1L MoS2–Au interaction.

Superconducting proximity effect in a van der Waals 2H-TaS2/NbSe2 heterostructure

Superconducting proximity effect in the heterostructures opens up access to engineer superconductivity at the interface, and has inspired substantial research interest in the past decade. Here, we fabricate the van der Waals 2H-TaS2/NbSe2 heterostructure through the mechanical exfoliation and dry transfer technique. Enhanced superconductivity in 2H-TaS2 flakes with increased superconducting transition temperature by about 0.8 K is observed. Such enhancement is believed to be related to the leakage of the Cooper pairs from NbSe2 flakes to 2H-TaS2 flakes. In addition, the critical current of the superconducting transition of NbSe2 flakes is also increased in the heterostructures, which is related to the strengthened pairing strength of Cooper pairs in NbSe2 flakes arising from the interfacial coupling between NbSe2 flakes and 2H-TaS2 flakes. Our findings will open perspectives for insight into the superconducting proximity effect in the van der Waals heterostructures.

Charge density wave transitions in mechanically-exfoliated NbSe3 devices

We studied charge density wave (CDW) transitions in NbSe3 devices with thicknesses t from 20 to 170 nm, fabricated by the mechanical exfoliation technique. NbSe3 exhibits two CDW transitions: one is the linear nesting (CDW1) below K and the other is the diagonal nesting (CDW2) below K. For both the transitions, clear resistivity changes were observed down to t = 20 nm, indicating that the CDW states are robust even for the thin NbSe3 devices. The result also suggests that surface acoustic waves could be useful to modulate the CDW in the NbSe3 devices.