Suspended Monolayer Research Articles

High-Yield Large-Scale Suspended Graphene Membranes over Closed Cavities for Sensor Applications.

Suspended membranes of monatomic graphene exhibit great potential for applications in electronic and nanoelectromechanical devices. In this work, a "hot and dry" transfer process is demonstrated to address the fabrication and patterning challenges of large-area graphene membranes on top of closed, sealed cavities. Here, "hot" refers to the use of high temperature during transfer, promoting the adhesion. Additionally, "dry" refers to the absence of liquids when graphene and target substrate are brought into contact. The method leads to higher yields of intact suspended monolayer chemical vapor deposition (CVD) graphene and artificially stacked double-layer CVD graphene membranes than previously reported. The yield evaluation is performed using neural-network-based object detection in scanning electron microscopy (SEM) images, ascertaining high yields of intact membranes with large statistical accuracy. The suspended membranes are examined by Raman tomography and atomic force microscopy (AFM). The method is verified by applying the suspended graphene devices as piezoresistive pressure sensors. Our technology advances the application of suspended graphene membranes and can be extended to other two-dimensional materials.

Unusually high thermal conductivity in suspended monolayer MoSi2N4

Two-dimensional semiconductors with high thermal conductivity and charge carrier mobility are of great importance for next-generation electronic and optoelectronic devices. However, constrained by the long-held Slack’s criteria, the reported two-dimensional semiconductors such as monolayers of MoS2, WS2, MoSe2, WSe2 and black phosphorus suffer from much lower thermal conductivity than silicon (~142 W·m–1·K–1) because of the complex crystal structure, large average atomic mass and relatively weak chemical bonds. Despite the more complex crystal structure, the recently emerging monolayer MoSi2N4 semiconductor has been predicted to have high thermal conductivity and charge carrier mobility simultaneously. In this work, using a noncontact optothermal Raman technique, we experimentally measure a high thermal conductivity of ~173 W·m–1·K–1 at room temperature for suspended monolayer MoSi2N4 grown by chemical vapor deposition. First-principles calculations reveal that such unusually high thermal conductivity benefits from the high Debye temperature and small Grüneisen parameter of MoSi2N4, both of which are strongly dependent on the high Young’s modulus induced by the outmost Si-N bilayers. Our study not only establishes monolayer MoSi2N4 as a benchmark 2D semiconductor for next-generation electronic and optoelectronic devices, but also provides an insight into the design of 2D materials for efficient heat conduction.

Tuning instability in suspended monolayer 2D materials

Monolayer two-dimensional (2D) materials possess excellent in-plane mechanical strength yet extremely low bending stiffness, making them particularly susceptible to instability, which is anticipated to have a substantial impact on their physical functionalities such as 2D-based Micro/Nanoelectromechanical systems (M/NEMS), nanochannels, and proton transport membrane. In this work, we achieve quantitatively tuning instability in suspended 2D materials including monolayer graphene and MoS2 by employing a push-to-shear strategy. We comprehensively examine the dynamic wrinkling-splitting-smoothing process and find that monolayer 2D materials experience stepwise instabilities along with different recovery processes. These stepwise instabilities are governed by the materials’ geometry, pretension, and the elastic nonlinearity. We attribute the different instability and recovery paths to the local stress redistribution in monolayer 2D materials. The tunable instability behavior of suspended monolayer 2D materials not only allows measuring their bending stiffness but also opens up new opportunities for programming the nanoscale instability pattern and even physical properties of atomically thin films.

Magneto-transport in the monolayer MoS2 material system for high-performance field-effect transistor applications

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities, and magnetic fields.

Fabrication and Optical Measurement of a Suspended Particle Monolayer

AbstractThe formation of a particle monolayer after the evaporation of a suspension droplet containing micro/nanoparticles is critical for multiple applications, including cell printing, particle immunoassays, and electronics. However, conventional methods for forming monolayers have limitations in terms of the use of solid substrates to fix them. In this paper, a novel method is proposed for forming a microparticle monolayer suspended in air using a through‐hole chip. Utilizing the strong interactions between particles and solid/liquid interfaces at the microscale, a simple yet robust method is developed to fabricate a suspended monolayer via particle suspension evaporation. The particle layers are classified into four types according to the particle concentration and suspension volume, namely, burst, ring, mono and ring, and multi patterns. Reorganization of the particle monolayer is achieved despite the repetitive solvent reinfusion and evaporation processes. Remarkably, the suspended particle monolayer exhibits superior optical characteristics, with a signal‐to‐noise ratio that is 1.78 times higher in fluorescence measurements than the same particles placed on a glass plate. This suspended particle monolayer can be applied in a variety of fields requiring sensitive and reproducible particle detection.

High Strain Engineering of a Suspended WSSe Monolayer Membrane by Indentation and Measured by Tip‐Enhanced Photoluminescence

AbstractStraintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro‐ and nano‐scale strain engineering. In this study, an atomic force microscope (AFM) coupled with an optical system is used to perform indentation measurements and tip‐enhanced photoluminescence (TEPL), allowing to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. The photoluminescence signal is modeled considering the deformation, stress distribution, and strain dependence of the WSSe band structure. An additional TEPL signal is observed that exhibits significant variation under strain, with 64 meV per percent of elongation. This peak is linked to the highly strained 2D material lying right underneath the tip. The amplification of the signal and its relation to the excitonic funneling effect are discussed in a more comprehensive model. The diffusion caused by Auger recombination against the radiative excitonic decay will also be compared. TEPL is used to examine and comprehend the local physics of 2D semi‐conducting materials subjected to extreme mechanical strain. Chemical vapor deposition‐fabricated 2D ternaries possess high strain resistance, comparable to the benchmark MoS2, and a high Young's modulus of 273 GPa.

Sustained robust exciton emission in suspended monolayer WSe2 within the low carrier density regime for quantum emitter applications

The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density regions, where the Shockley–Read–Hall (SRH) recombination may be dominant and seriously reduce the emission efficiency. In order to reduce the effect of carrier trapping and sustain a strong photoluminescence (PL) emission under low power pumping conditions, we investigated the influence of “suspending” a monolayer of tungsten diselenide (WSe2), a novel two-dimensional quantum material. Not only the PL intensity but also the fundamental photoluminescence quantum yield (PLQY) has exhibited a huge, order-scale enhancement through suspending; even surprisingly, we found the PLQY improvement to be far significant under small pumping powers and observed an exponential increase in tendency toward an even lower carrier density region. With its strong excitonic effect, suspended WSe2 offers a solution to reduce carrier trapping and participate in non-radiative processes. Moreover, in the low-power range, where SRH recombination dominates, suspended WSe2 exhibited a remarkably higher percentage of excitonic radiation compared to contacted WSe2. Herein, we quantitatively demonstrate the significance of the suspended WSe2 monolayer in a low carrier density region, highlighting its potential for developing compact, low-power quantum emitters in the future.

Pressure-dependent thermal conductivity transient measurement of graphene

The thermal properties of graphene determine the thermal damage evaluation of graphene diaphragm and the lifetime of graphene-based devices. The conventional transient measurement method tends to result in the low-precision thermal conductivity (κ) for suspended monolayer graphene and complex interferometry setup. Hence, in order to improve κ measurement of graphene, a simple, miniaturized and portable Fabry-Perot (F–P) resonator with 10-layer graphene is proposed to perform the actuation and vibration detection of the suspended graphene. Then, the κ value can be calculated by means of laser-heated graphene's thermomechanical response, along with the established model for thermal time constant (τ) of a uniformly heated circular film. A quadratic fit is applied to the measured κ at different air pressures and an average fitting curve is obtained, thus determining that the measured κ relatively decreases by 13.32% with the increasing air pressure outside the F–P cavity from ∼22 kPa to ∼101 kPa. Subsequently, the COMSOL heat transfer simulation confirms that convective heat transfer is the primary reason for the external air pressure affecting transient measurements of κ of graphene. Hence, after taking the intercept of the average fitting curve, the predicted κ at 0 Pa of 10-layer suspended graphene is calculated to be 194.48 W/(m·K), which is close to the fitting curve calculated from previously reported κ of graphene with different layers. This study contributes to accurate transient measurement of κ in graphene, and is also instructive for investigating transient thermal process in other two-dimensional materials.

Temperature and power-dependent photoluminescence spectroscopy in suspended WSe2 monolayer

This work, the photoluminescence (PL) of a suspended monolayer of WSe2 was investigated to avoid the substrate effect, and its evolution with temperature and excitation energy. Raman spectroscopy and PL demonstrated the monolayer characteristic of the sample due to the absence of the B1 2g peak along with a symmetric PL peak centered at approximately 1.65 eV. The PL spectrum exhibited two main emission bands attributed to a neutral exciton (X0) and a trion (X T ), where X0 showed a redshift with temperature variation from 10 K to 310 K, and the intensity ratio between the bands varied from 2.68 to 3.96 eV. The energy evolution of the bands as a function of temperature was analyzed using the modified Varshni equation, yielding an electron–phonon coupling constant with a value of 1.54 (2.11) for X0 (X T ). The intensity behavior was studied using the multi-level model, which provided activation energies of 58.0 meV for X0 and 94.6 meV for X T . The redshift of the mentioned bands is explained by the Urbach formalism, attributing this shift to an increase in phonon density in the material. Computational calculations demonstrated an increase in the material’s lattice parameter with rising temperature, resulting in an increase in the W–Se bond length and a decrease in the distance between Se atoms from different sublattices, causing the direct transition to decrease with temperature. In conclusion, this study showed that the observed evolution in the PL spectrum of the suspended WSe2 monolayer could be related to the increase of phonon density in the material which is important for the future applications of 2D materials in optoelectronics.

Effects of electron irradiation on graphene drums

Using a scanning electron microscope, we irradiate graphene drums with electrons at an energy of 20 keV and a dosage of about 1.58 × 1017 electrons/cm2. The drums consist of graphene exfoliated in ambient air over holes having a diameter of 4.6 μm and etched into an SiO2 substrate. After irradiation, we observe that the drum’s suspended monolayer (ML) region has a ratio of the Raman D peak height, ID, to the Raman G peak height, IG, as high as 6.3. In contrast, the supported ML on the SiO2 substrate has an ID/IG ratio of 0.49. Previous studies have shown that graphene drums containing air can leak in a vacuum at a low rate. We attribute the high ID/IG ratio of the suspended ML to the air that may be in the drums. We propose that the air produces much adsorbed water on the ML, resulting in a high average defect density during irradiation. We present Raman maps of the full-width-at-half maximum, position, and height of the G, 2D, D, and D’ peaks before and after irradiation and maps of ID/IG and ID/ID’. We anneal the drums at temperatures from 50 to 215 °C and find that ID/IG significantly reduces to 0.42. The annealing data are analyzed using an Arrhenius plot. We also find that ID/ID’ depends on annealing temperature and has values ≥8, in the range expected for sp3 defects, for ID/IG ≤ 3.9. This irradiation method may help achieve high average defect densities in ML graphene, imparting novel and potentially valuable properties.

Anomalous size effect of thermal conductivity of two-dimensional dielectric materials

In this work, we find that two-dimensional characteristics of hydrodynamic phonon transports in two-dimensional dielectric materials lead to the anomalous length dependency and even divergence of thermal conductivities. For suspended monolayer graphene the analytical solution of the two-dimensional Guyer-Krumhansl equations shows that the thermal conductivity increases with the increase of its length from 1 micron to 9 micron. When a specific temperature distribution at the inlet boundary is given, and the positive partial derivative of the heat flux along the length direction at the inlet and outlet boundaries is prescribed, analytical results show that the thermal conductivity of suspended monolayer graphene at the room temperature tends to infinity with the increase of its length. This result can be used to artificially regulate the thermal conductivity of suspended single-layer graphene.

Transmission through graphene of electrons in the 30 – 900 eV range

Here, we report on accurate transmission measurements of electrons below 1 keV through suspended monolayer graphene. Monolayer graphene was grown via chemical vapor deposition and transferred onto transmission electron microscopy (TEM) grids. A monochromatic electron gun has been employed to perform the measurements in the 30 – 900 eV range in ultra-high vacuum. The graphene transparency is obtained from the absolute measurement of the direct beam current and the transmitted one, by means of a Faraday cup. We observed a transmission going from ∼20 to ∼80% for monolayer graphene within the experimental electron energy range. The high quality and the grid coverage of the suspended graphene has been proved via micro-Raman, X-ray photoemission, electron energy loss spectroscopies and field-emission scanning electron microscopy. After a 550 °C in-vacuum annealing of the samples, the main contribution to the C 1s spectrum is due to the sp2 component and the evidence of suspended monolayer graphene has been observed through the π-plasmon excitation.

Comparison of four basis sets on calculating exciton states in a suspended monolayer MoS2 using a variationally optimized diagonalization method

Recent studies on excitons in two-dimensional (2D) materials have been widely calculated using different basis sets. Especially, the accuracy of the basis sets is in demand. Using a variationally optimized diagonalization method, I compare the results of the A exciton state calculations in a suspended monolayer MoS2 obtained using four basis sets: the basis sets of the orthogonal Laguerre, nonorthogonal Laguerre, harmonic oscillator and hydrogen atom. The best accuracies of eigenenergies are obtained by using the orthogonal and nonorthogonal basis sets, while the Hamiltonian matrix element based on the nonorthogonal Laguerre basis set has the simpler form. The eigenenergies calculated by the basis set of hydrogen atom exist the largest discrepancy, thus it is not suitable for calculating the exciton states in 2D materials. Based on these findings, the orthogonal and nonorthogonal basis sets are the best choice in calculating highly accurate exciton states in 2D materials.

Nanoscale mass measurement based on suspended graphene

A highly sensitive nanoscale mass sensor was developed for weight measurement of single microparticles using a suspended graphene structure. The sensor is composed of an array of holes covered with suspended monolayer graphene. Based on the shift of the 2D Raman peak in graphene, originating from the elongation of carbon–carbon bonds under pressure, the mass of microparticles in suspended graphene can be measured. The results show that the sensor can detect microparticles with mass ranging from 0.1 ng to 3 ng. The peak shift ratio is −69.8 cm−1 per 1% strain for the experimental value and −72.3 cm−1 per 1% strain for the calculated value. The demonstrated concept presents a promising path for nano-mass measurement applications.

Electric-Field-Driven Trion Drift and Funneling in MoSe2 Monolayer.

Excitons, electron-hole pairs in semiconductors, can be utilized as information carriers with a spin or valley degree of freedom. However, manipulation of excitons' motion is challenging because of their charge-neutral characteristic and short recombination lifetimes. Here we demonstrate electric-field-driven drift and funneling of charged excitons (i.e., trions) toward the center of a MoSe2 monolayer. Using a simple bottom-gate device, we control the electric fields in the vicinity of the suspended monolayer, which increases the trion density and pulls down the layer. We observe that locally excited trions are subjected to electric force and, consequently, drift toward the center of the stretched layer. The exerting electric force on the trion is estimated to be 102-104 times stronger than the strain-induced force in the stretched monolayer, leading to the successful observation of trion drift under continuous-wave excitation. Our findings provide a new route for manipulating trions and achieving new types of optoelectronic devices.

Pump-probe response, four-wave mixing and self-Kerr effect in a suspended monolayer Z-shaped graphene nanoribbon nanoresonator

Pump-probe response, four-wave mixing and self-Kerr effect in a suspended monolayer Z-shaped graphene nanoribbon nanoresonator

Tunable plasmonic terahertz filter based on a suspended monolayer graphene on a ring resonator

Tunable plasmonic terahertz filter based on a suspended monolayer graphene on a ring resonator

High-yield fabrication of electromechanical devices based on suspended Ti3C2Tx MXene monolayers.

MXenes, two-dimensional transition metal carbides, nitrides, and carbonitrides, are known for their exceptional electronic and mechanical properties. Yet, the experimental efforts toward the realization of MXene-based nanoelectromechanical systems (NEMS) combining electrical and mechanical functionalities of MXenes at the nanoscale remain very limited. Here, we demonstrate a high-yield fabrication of the electromechanical devices based on individual suspended monolayer MXene flakes. We employed Ti3C2Tx, the most popular MXene material to date, that can be produced as high-quality micrometer-scale monolayer flakes with a high electrical conductivity of over 10 000 S cm-1 and a high effective Young's modulus of about 330 GPa. These Ti3C2Tx flakes can be transferred over prefabricated trenches in a Si/Si3N4 substrate at a high yield, potentially enabling fabrication of hundreds of electromechanical devices based on suspended MXene monolayers. We demonstrate very clean, uniform, and well-stretched membranes with different dimensions, with Ti3C2Tx flakes suspended over trenches with gaps ranging from 200 nm to 2 μm. The resulting Ti3C2Tx monolayer membranes were electrostatically actuated, while their vertical displacement was monitored using a tip of an atomic force microscope (AFM). The devices reliably responded to the electrostatic actuation in ambient conditions over multiple cycles and with different measurement parameters, such as AC frequency, AC voltage amplitude, and AFM tip loading force. The demonstration of the high-yield fabrication of working electromechanical devices based on suspended Ti3C2Tx MXene membranes at the ultimate monolayer limit paves the way for the future exploration of the potential of MXenes for NEMS applications.

Tension at intercellular junctions is necessary for accurate orientation of cell division in the epithelium plane

The direction in which a cell divides is set by the orientation of its mitotic spindle and is important for determining cell fate, controlling tissue shape, and maintaining tissue architecture. Divisions parallel to the epithelial plane sustain tissue expansion. By contrast, divisions perpendicular to the plane promote tissue stratification and lead to the loss of epithelial cells from the tissue-an event that has been suggested to promote metastasis. Much is known about the molecular machinery involved in orienting the spindle, but less is known about the contribution of mechanical factors, such as tissue tension, in ensuring spindle orientation in the plane of the epithelium. This is important as epithelia are continuously subjected to mechanical stresses. To explore this further, we subjected suspended epithelial monolayers devoid of extracellular matrix to varying levels of tissue tension to study the orientation of cell divisions relative to the tissue plane. This analysis revealed that lowering tissue tension by compressing epithelial monolayers or by inhibiting myosin contractility increased the frequency of out-of-plane divisions. Reciprocally, increasing tissue tension by elevating cell contractility or by tissue stretching restored accurate in-plane cell divisions. Moreover, a characterization of the geometry of cells within these epithelia suggested that spindles can sense tissue tension through its impact on tension at subcellular surfaces, independently of their shape. Overall, these data suggest that accurate spindle orientation in the plane of the epithelium relies on a threshold level of tension at intercellular junctions.

Intensive and broad bound exciton emission at cryogenic temperature in suspended monolayer transition metal dichalcogenides

Two-dimensional (2D) transition metal chalcogenides (TMDCs) are expected to play an important role in next-generation optoelectronic devices. However, the properties of 2D TMDCs can be largely affected by defects, and thus, many efforts have been made to characterize and eliminate the defects. Here, we investigate defect-related trap states, particularly in large-area suspended ${\mathrm{WS}}_{2}$ monolayers at cryogenic temperatures, using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. We observed an intensive and broad (full width half maximum $\ensuremath{\sim}170\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) photoluminescence (PL) emission centered at 1.83 eV, 0.3 eV lower than the free exciton transition energy. From temperature-dependent PL spectra, the thermal activation energy of excitons escaping from trap states is determined to be 30 meV. The TRPL spectrum shows an ultralong lifetime at 13 K. The long-lived emission with the broad PL linewidth is attributed to excitons bound to defects of possibly chalcogenide-site substitution and sulfur vacancies. Compared with the supported monolayer, the defect-related trap states are found to be deeper in the suspended layer and can trap excitons more efficiently due to less dielectric screening. In this letter, we provide further understanding of trap states and decay channels in monolayer TMDCs and show that suspended 2D TMDCs can provide a pure platform for studying defect-related properties.