Few-layer Research Articles

Monolayer NbSe2 Favors Ultralow Friction and Super Wear Resistance.

The urgent demand for atomically thin, superlubricating, and super wear-resistant materials in micro/nanoelectromechanical systems has stimulated the research of friction-reducing and antiwear materials. However, the fabrication of subnanometer-thick films with superlubricating and super wear-resistant properties under ambient conditions remains a huge challenge. Herein, high-quality monolayer (ML) NbSe2 (∼0.8 nm) with ultralow friction and super wear resistance in an atmospheric environment was successfully grown by chemical vapor deposition (CVD) for the first time. Moreover, compared with few-layered (FL) NbSe2, ML NbSe2 has a lower friction coefficient and better wear resistance. On the basis of density function theory (DFT) calculations, the adhesion and the degree of charge transfer between ML NbSe2 and the substrate is larger than that of the topmost layer to the underlying layers of NbSe2 with two or more layers, which can be used to explain that the ML NbSe2 favors ultralow friction and super wear resistance.

Sulfurization engineering of single-zone CVD vertical and horizontal MoS2 on p-GaN heterostructures for self-powered UV photodetectors.

Molybdenum disulfide (MoS2) has been attracting considerable attention due to its excellent electrical and optical properties. We successfully grew high-quality, large-area and uniform few-layer (FL)-MoS2 on p-doped gallium nitride (p-GaN) using a simplified sulfurization technique by the single-zone CVD of a Mo seed layer via E-beam evaporation. Tuning the sulfurization parameters, namely temperature and duration, has been discovered to be an effective strategy for improving MoS2 orientation (horizontally aligned and vertically aligned) and quality, which affects photodetector (PD) performance. The increase in the sulfurization temperature to 850 °C results in improved structural quality and crystallite size. However, a prolonged sulfurization duration of 60 minutes caused the degradation of the film quality. The close lattice match between p-GaN and MoS2 contributes to the excellent quality growth of deposited MoS2. Following this, an n-MoS2/p-GaN heterostructure PD was successfully built by a MoS2 position-selectivity method. We report a highly sensitive and self-powered GaN/MoS2 p-n heterojunction PD with a relatively high responsivity of 14.3 A W-1, a high specific detectivity of 1.12 × 1013 Jones, and a fast response speed of 8.3/13.4 μs (20 kHz) under a UV light of 355 nm at zero-bias voltage. Our PD exhibits superior performance to that of the previously reported MoS2/GaN p-n PD. Our findings suggest a more efficient and straightforward approach to building high-performance self-powered UV PDs.

Load‐Dependent Friction Hysteresis for Graphitic Surfaces in n‐Hexadecane

AbstractSliding‐induced friction behavior of a single‐asperity silica probe against few‐layer (FL) graphene and bulk graphite is measured in the presence of n‐hexadecane using an atomic force microscope (AFM). The load‐dependent nanoscale friction measurements display friction hysteresis, i.e., higher friction forces during unloading of the contact than loading, at a given normal load. However, unlike hysteresis in friction of graphene measured in ambient, several unique trends are observed when the contact is immersed in n‐hexadecane. First, the friction hysteresis is measured up to a transition load and beyond which is found negligible; second, a similar behavior is observed on bulk graphite; and third, a friction strengthening of the contact persisted up to several nanometers. Quasi‐static force‐separation curves identify up to four layers of n‐hexadecane solvation layers on graphitic surfaces. Molecular dynamic simulations illustrate that the solvated n‐hexadecane molecules within the contact carry the probe load and determine the generated contact area, affecting friction hysteresis. Further, during AFM probe sliding, instead of a pucker, a molecular pile‐up of n‐hexadecane, in front of the tip is observed. These findings provide new perspectives on understanding of the dissipation mechanisms of graphene that predominantly are surrounded by structured liquid molecules.

Controllable Synthesis of Atomically Thin 1T‐SnSe2 Flakes and Its Linear Second Harmonic Generation with Layer Thickness

AbstractAs an important member of the IVA–VIA group compounds, 2D SnSe2 has emerged as a perfect platform for developing diverse applications, especially in high‐performance optoelectronic devices and data storage, etc. However, the bottom‐up synthesis of large‐area uniform, atomically thin SnSe2 crystals with controlled thicknesses has not yet been realized. Herein, we report the large‐area uniform growth of monolayer (1L), bilayer (2L), and few‐layer (FL) 1T‐SnSe2 single‐crystal flakes on mica substrates via a facile chemical vapor deposition (CVD) route. The feeding amount of Sn precursor and flow rate of hydrogen carrier are found to be the key parameters for the thickness‐controlled growth of uniform SnSe2 flakes. More intriguingly, obvious second harmonic generation (SHG) is revealed in the retained inversion symmetry structure of 1T‐SnSe2, with its intensity showing linear dependence with the thickness from monolayer to multilayers. The new findings reported herein should pave the ways for the thickness‐tunable growth of atomically thin SnSe2 crystals, and their unique optical property explorations and applications in nonlinear optics.

Few-layer antimonene electrical properties

Antimonene -a single layer of antimony atoms- and its few layer forms are among the latest additions to the 2D mono-elemental materials family. Numerous predictions and experimental evidence of its remarkable properties including (opto)electronic, energetic or biomedical, among others, together with its robustness under ambient conditions, have attracted the attention of the scientific community. However, experimental evidence of its electrical properties is still lacking. Here, we characterized the electronic properties of mechanically exfoliated flakes of few-layer (FL) antimonene of different thicknesses (∼ 2–40 nm) through photoemission electron microscopy, kelvin probe force microscopy and transport measurements, which allows us to estimate a sheet resistance of ∼ 1200 Ω sq−1 and a mobility of ∼ 150 cm2V−1s−1 in ambient conditions, independent of the flake thickness. Alternatively, our theoretical calculations indicate that topologically protected surface states (TPSS) should play a key role in the electronic properties of FL antimonene, which supports our experimental findings. We anticipate our work will trigger further experimental studies on TPSS in FL antimonene thanks to its simple structure and significant stability in ambient environments.

Trion Binding Energy Variation on Photoluminescence Excitation Energy and Power during Direct to Indirect Bandgap Crossover in Monolayer and Few-Layer MoS2

The behavior of the trion and exciton photoluminescence (PL) under various laser excitation conditions, that is, energy and power, is studied in monolayer and few-layer (FL) chemical vapor deposition (CVD)-MoS2 flakes grown on SiO2/Si. The room-temperature μ-PL spectroscopy of the flakes shows that the A exciton at 1.86 eV is overlapped by the trion component. First, the thickness-dependent characterization of the flakes has been carried out. The trion spectral weight and dissociation energy are observed to increase with the number of layers, which is correlated with an increase in nonequilibrium electron density. We propose the presence of many-body effects explaining this unusual dependence in CVD-MoS2 on n-type SiO2/Si. In the power-dependent μ-PL measurements, the deconvolution of spectra for the exciton and trion bands shows a faster intensification of the trion component compared to the A exciton with increasing laser power. The trion binding (dissociation) energy varied from 28 to 33 meV in the monolayer and from 32 to 46 meV in FL MoS2, when increasing the power of excitation light. The phenomenon is found to be more prominent when increasing the energy of excitation from 2.33 to 3.06 eV. The enhancement of the trion binding energy leads to a strong intensification of the trion component. Thereby, the intense A exciton/trion PL band redshifts and becomes more asymmetric when increasing the power of excitation at both energies of 2.33 or 3.06 eV. Simultaneously, the B exciton contribution to the spectrum also increases. Such an enhanced formation of the trions and B excitons is explained by a rate of photogenerated electron–hole pairs, leading to a higher population of nonequilibrium electrons. In the measurements under a higher excitation energy of 3.06 eV, the PL redshift is more prominent because the trion component is more intense compared to the A band. This is explained by considering a higher absorption of MoS2 at higher energies.

Angular dependence of nanofriction of mono- and few-layer MoSe2

The frictional properties of two-dimensional (2D) materials are strongly dependent on the crystallographic orientation and number of layers. Although friction anisotropy caused by crystallographic orientations has been reported for various 2D materials, the flexural deformations and different defects complicate the insight into the mechanism of the in-plane friction anisotropy of these materials. Here, the anisotropic friction behavior between an atomic force microscopy tip and monolayer (ML) and few-layer (FL) MoSe2 flakes grown by CVD was performed by nanofriction measurements at different crystallographic directions, applied loads (FA), and tip scanning velocities. Our results reveal that the angular dependence of the friction forces is highly anisotropic for both types of MoSe2 flakes, and the anisotropy decreases with FA. Importantly, the anisotropies demonstrate opposite angular dependence for ML and FL MoSe2 flakes. As confirmed by the friction scanning velocity dependences, this difference seems to originate from different friction mechanisms for ML and FL MoSe2 flakes. The friction of FL flakes was predominantly influenced by atomic stick–slip motion. In contrast, ML MoSe2 is characterized by a coexistence of deformation-induced and atomic stick–slip motion. The experimental results presented here extend the understanding of the tribological properties of dry lubricants operating at the nanoscale.

Davydov Splitting, Resonance Effect and Phonon Dynamics in Chemical Vapor Deposition Grown Layered MoS2

We present comprehensive temperature dependent Raman measurements for chemical vapor deposition grown horizontally aligned layered MoS2 in a temperature range of 4–330 K under a resonance condition. Our analysis of temperature dependent phonon frequency shift and linewidth suggests a finite role of three and four phonon anharmonic effect. We observe Davydov splitting of the out-of-plane (A 1g ) and in-plane () modes for both three layer (3L) and few layer (FL) systems. The number of Davydov splitting components are found more in FL compared to 3L MoS2, which suggests that it increases with an increasing number of layers. Further, Davydov splitting is analyzed as a function of temperature. Temperature evaluation of the Raman spectra shows that the Davydov splitting, especially for A 1g mode, is very strong and well resolved at low temperature. We observe that A 1g mode shows the splitting at low temperature, while mode is split even at room temperature, which suggests a prominent role of A 1g mode in the interlayer interaction at low temperature. Further, an almost 60-fold increase in the intensity of the phonon modes at low temperature clearly shows the temperature dependent tuning of the resonance effect.

Reduced Fracture Strength of 2D Materials Induced by Interlayer Friction.

The potential applications of 2D layered materials (2DLMs) as the functional membranes in flexible electronics and nano-electromechanical systems emphasize the role of the mechanical properties of these materials. Interlayer interactions play critical roles in affecting the mechanical properties of 2DLMs, and nevertheless the understanding of their relationship remains incomplete. In the present work, it is reported that the fracture strength of few-layer (FL) WS2 can be weakened by the interlayer friction among individual layers with the assistance of finite element simulations and density functional theory (DFT) calculations. The reduced fracture strength can be also observed in FL WSe2 but with a lesser extent, which is attributed to the difference in the interlayer sliding energies of WS2 and WSe2 as confirmed by DFT calculations. Moreover, the tip-membrane friction can give rise to the underestimation of the Young's modulus except for the membrane nonlinearity. These results give deep insights into the influence of interfacial interactions on the mechanical properties of 2DLMs, and suggest that importance should be also attached to the interlayer interactions during the design of nanodevices with 2DLMs as the functional materials.

Spin-phonon coupling and magnon scattering in few-layer antiferromagnetic FePS3

Temperature-dependent Raman spectroscopic studies of spin-phonon (SP) coupling and magnon scattering in bulk and few-layer (FL) antiferromagnet (AFM) ${\text{FePS}}_{3}$ with N\'eel temperature (${T}_{N}$) $\ensuremath{\approx}$ 120 K were performed. Bulk and FL (2--5 atomic layers) samples show four distinct modes at room temperature between 150 and $400 {\text{cm}}^{\ensuremath{-}1}$ and a broad peak at $\ensuremath{\approx}105 {\text{cm}}^{\ensuremath{-}1}$. On lowering the temperature, three distinct phenomena are observed. First, we see the SP coupling, identified by the deviation from the usual two- or three-phonon anharmonic behavior of the higher wave number peaks ($\ensuremath{\ge}150 {\text{cm}}^{\ensuremath{-}1}$) at or below ${T}_{N}$. The strength of SP coupling can be calculated for bulk and FL flakes considering mean-field approximations. Secondly, we see the spin ordering marked by the evolution of three peaks at lower wave numbers (around $105 {\text{cm}}^{\ensuremath{-}1}$) below ${T}_{N}$ due to incommensurate magnetic cells at low temperature. Thirdly, magnon excitation in FL pristine ${\text{FePS}}_{3}$ is detected by the emergence of a distinct peak at $120 {\text{cm}}^{\ensuremath{-}1}$ ($\ensuremath{\sim}3.6$ THz) at a temperature much lower than ${T}_{N}$ ($\ensuremath{\approx}60$ K). Tracking the magnon mode in the designed van der Waals heterostructures with ${\text{Bi}}_{2}{\text{Te}}_{3}$ and ${\text{Cr}}_{2}{\text{Ge}}_{2}{\text{Te}}_{6}$ reveal interfacial electron and hole transfer from ${\text{FePS}}_{3}$, respectively. Raman spectroscopy can thus predict the magnetic transition temperature of FL magnetic insulators via SP coupling, zone-boundary phonons, and magnons. Quasi-two-dimensional AFMs and their heterostructures involving different electronic and magnetic orders may be promising candidates for ultrafast magnon transport involving magnetoelastic waves.

Electrical and electrothermal properties of few-layer 2D devices

While two-dimensional (2D) materials have emerged as a new platform for nanoelectronic devices with improved electronic, optical, and thermal properties, and their heightened sensitivity to electrostatic and mechanical interactions with their environment has proved to be a bottleneck. Few-layer (FL) 2D devices retain the desirable thinness of their monolayer cousins while boosting carrier mobility. Here, we employ an electrothermal model to study FL field-effect devices made from transition metal dichalcogenides MoS2 and WSe2 and examine the effect of both electrical and thermal interlayer resistances, as well as the thermal boundary resistance to the substrate, on device performance. We show that overall conductance improves with increasing thickness (number of layers) at small gate voltages, but exhibits a peak for large gate voltages. Joule heating impacts performance due to relatively poor thermal conductance to the substrate and this impact, along with the location of the hot spot in the FL stack, varies with carrier screening length of the material. We conclude that coupled electrothermal simulation can be employed to design FL 2D devices with improved performance.

Thermomechanical Response of Supported Hexagonal Boron Nitride Sheets of Various Thicknesses

Raman spectroscopy is employed to investigate the temperature dependence of the E2g phonon mode of single-layer, few-layer (FL), and bulk hexagonal boron nitride (hBN) sheets, situated over Si/SiO2 (90 nm). Depending on the sample, two temperature regimes are recorded. In the low to mid range of the examined temperatures, the monolayer and FL samples exhibit significantly higher slopes compared to the thicker ones because of the thermal expansion of the underlying substrate. In the high-temperature region, all the examined samples show almost the same temperature slope, indicating slippage of the hBN sheets relative to the substrate and providing strong evidence that the slopes of the monolayer, FL, and bulk hBN are quite similar of about −0.020 cm–1/K. This is further justified from the full width at half-maximum versus T of the studied samples and the observed similarities of the thermal expansion coefficient (TEC) of one to three layers and bulk hBN, revealing a comparable level of anharmonicity for the E2g mode from the monolayer up to bulk hBN. Moreover, using a finite element method, we have determined the TEC of the underling substrate and the strain induced by TEC mismatch between the single-layer hBN and the substrate. Consequently, the intrinsic frequency shift of the E2g mode for the monolayer hBN upon temperature change is extracted. Finally, the vibrational response of monolayer hBN upon temperature is also examined by means of computational simulations, and satisfactory agreement with the experiment is obtained.

(Invited) Self-Heating in Advanced CMOS-Compatible and 2-Dimensional Semiconductor Devices

For many years the computer industry has relied for progress on the stellar rate of scaling of MOSFETs in integrated circuits. The usual expectation, based on Moore's law, is that the number of transistors packed on a chip doubles every eighteen months. Sustaining this pace requires aggressive research into the numerous bottlenecks that threaten to slow it down. In the past decade, an issue has emerged that threatens to impose an absolute limit on how many transistors can be packed onto a die. This is the issue of heat dissipation. The persistent down-scaling of nanostructures, electronic devices, sensors, and NEMS increases the surface-to-volume ratio and introduces atomic-scale disorder at boundaries and interfaces, which scatters heat carriers (phonons) and increases thermal resistance.In this invited talk, I will present my recent work on numerical simulation and modeling of heat dissipation and phonon transport in a broad range of nanostructures, focusing primarily on extrinsic and disorder effects such as grain/sample boundaries, interfaces, edges, and alloy mass disorder. The talk revolves around the phonon Boltzmann transport equation (pBTE), while the salient feature of the work is that it employs a full phonon dispersion computed from first principles and combined with a momentum-dependent model of electron and phonon scattering. I will show that the thermoelectric response of SOI and Si-membrane-based nanostructures can be improved by employing the anisotropy of the lattice thermal conductivity, revealed in ultrathin SOI nanostructures due to the interplay between the anisotropy of the phonon dispersion and the strong boundary scattering. Next, I explore the consequences of nanostructuring on silicon/germanium and Si/Si-Ge alloy superlattices, and show that the drastic reduction and high anisotropy of thermal conductivity comes from the increased interaction of lattice waves with rough interfaces and boundaries. I will close the loop with applications of my phonon transport models to thermal effects in ultrascaled gate-all-around and junctionless MOS devices.Going beyond silicon CMOS, two-dimensional (2D) materials have tremendous potential for next-generation nano- and optoelectronics. However, heat dissipation and its removal from hot spots in the monolayer remains a critical concern to the design of 2D-based devices. When a 2D material is supported by a substrate, the interfacial area formed between it and the substrate is often far larger than the lateral source/drain contact area. Thus, the majority of heat is removed across the 2D-substrate interface and then via the substrate. Interfaces formed between 2D vdW materials and 3D substrates are fundamentally different than same-dimension 3D-3D and 2D-2D interfaces due to the presence of a vdW gap and the different dimensionalities of the phase spaces on either side of the interface. In this invited talk, I will review the progress in understanding lattice thermal transport, both in-plane and cross-plane, in 2D mono and few-layer materials.Several recent papers measured the TBC between various monolayers and mostly the silicon dioxide (SiO2) substrate, reporting a wide range of values due to inconsistent sample quality. Therefore, it is imperative to build predictive methods for quantifying the TBC between MLs and various substrates. Here, we use a combination of phonon dispersions from first-principles density functional perturbation theory simulations and our 2D-3D TBC model. We investigate the TBC between combinations of six atomic layers (h-BN, graphene, MoS2, MoSe2, WS2, and WSe2) and six substrates (SiO2, AlN, GaN, 6H-SiC, diamond, and Al2O3). We show that TBC is higher for softer substrates with smaller speed of sound, but of the 6 substrates we compared, amorphous SiO2 consistently produced higher TBC than crystalline substrates.Another route to improving mobility and boosting drive current in 2D devices is to replace the monolayer with few-layered (FL) TMDs. Despite recent advances on demonstrating improved electrical performance of FL TMD FETs, there has been less attention towards their thermal management, which is crucial for modern nanoelectronic devices. We study heat dissipation in FL-WSe2 stacks using a coupled electro-thermal model and Raman thermometry experiments. The resulting rise in temperature is obtained from a FL-TBC model to shed light on self-heating and heat dissipation in such devices. We find that the temperature rise in the top layers is significantly larger than the bottom layers because the bottom layers have higher TBC and conduct heat more efficiently to the substrate. The higher temperature of top layers, in turn, significantly reduces their mobility, causing the electrical current to reroute toward the bottom layers where the TBC is higher. This rerouting improves heat removal and limits mobility degradation. We conclude that the optimal number of layers is 6-9 and that FL TMD devices are inherently thermally resilient.

Current Rerouting Improves Heat Removal in Few-Layer WSe2 Devices.

Few-layer (FL) transition-metal dichalcogenides have drawn attention for nanoelectronics applications due to their improved mobility, owing to the partial screening of charged impurities at the oxide interface. However, under realistic operating conditions, dissipation leads to self-heating, which is detrimental to electronic and thermal properties. We fabricated a series of FL-WSe2 devices and measured their I-V characteristics, while their temperatures were quantified by Raman thermometry and simulated from first principles. Our tightly integrated electrothermal study shows that Joule heating leads to a significant layer-dependent temperature rise, which affects mobility and alters the flow of current through the stack. This causes the temperatures in the top layers to increase dramatically, degrading their mobility and causing the current to reroute to the bottom of the FL stack where thermal conductance is higher. We discover that this current rerouting phenomenon improves heat removal because the current flows through layers closer to the substrate, limiting the severity of self-heating and its impact on carrier mobility. We also observe significant lateral heat removal via the contacts because of longer thermal healing length in the top layers and explore the optimum number of layers to maximize mobility in FL devices. Our study will impact future device designs and lead to further improvements in thermal management in van der Waals (vdW)-based devices.

Few-layered vdW MoO3 for sensitive, uniform and stable SERS applications

Metal oxides have been considered as active substrates for surface-enhanced Raman scattering (SERS) through morphology-engineering and defect-introduction with the sacrifice of the uniformity and stability of the materials. In this work, large-size, few-layered (FL) wide band gap van der Waals (vdW) MoO3 nanosheets were prepared as SERS substrates. It’s shown that the decrease of layer number converted the intrinsic MoO3 from non-SERS active to sensitive SERS substrate, with limit of detection (LOD) of 2 × 10−8 M for R6G. The SERS of MoO3 showed high homogeneity with RSD as small as 2.95%, The Raman intensity of R6G on the SERS substrate remained higher than 90% after exposing in atmosphere for 160 days, demonstrating excellent air-stability. Experimental results demonstrated that the SERS effect of the plasmon-free MoO3 substrate comes from chemical enhancement mechanism (CM) when reducing its thickness. Furthermore, the CM effect can be further improved through non-destructive atomic intercalation into the vdW gap, resulting in more charges separation between the adsorbed molecules and substrates. Our result showed that non-destructive modulation strategy is effective for preparation of sensitive, uniform and stable SERS substrate.

High mobility ReSe2 field effect transistors: Schottky-barrier-height-dependent photoresponsivity and broadband light detection with Co decoration

2D transition metal dichalcogenides are promising in various electronics and optoelectronics applications and have gained popularity owing to their carrier transport and strong light–matter interactions. To fully realize their potential in field-effect transistors (FETs) and photodetectors, high mobility and high responsivity are imperative. Here, we demonstrate the highest mobility of ~166 cm2 V−1 s−1 at 200 K for single-layer rhenium diselenide (ReSe2) FETs encapsulated between h-BN flakes at Vg = 47 V. The high mobility is attributed to low-resistance contacts of scandium/gold (Sc/Au), with a low Schottky barrier height and reduced charge scattering platform of h-BN. Further, we elucidated the Schottky-barrier-height dependent high photoresponsivity (~3.2 × 106 A W−1) of few-layer ReSe2 (FL-ReSe2) at 532 nm-wavelength laser light on an h-BN substrate with Sc/Au contacts. Moreover, broadband light detection of undoped and Co-doped few-layer (FL) ReSe2 was performed under different laser wavelengths (400–1100 nm). After the deposition of Co nanoparticles, the photocurrent of FL-ReSe2 increased due to n-doping, as confirmed by the transfer curves of the FL-ReSe2-based undoped and co-doped FETs. Further, the work function decreased from 4.856 to 4.791 eV in FL-ReSe2, as measured by Kelvin probe force microscopy. No light signal was observed at 1100 nm for the undoped ReSe2 (1050 nm < λcut-off < 1100 nm); however, after doping with Co nanoparticles, the cut-off wavelength exceeded to (λcut-off > 1100 nm), due to the additional trap states generated in the energy band gap of ReSe2 after Co doping. Further, the transient response of ReSe2 and Co + ReSe2 FETs was estimated so that the rise and decay times are decreased from 1.9 s & 2.7 s to 1.1 s & 1.8 s, respectively. ReSe2 is therefore a promising semiconducting material for electrical and optoelectrical applications.

Interactions of Transition Metal Dichalcogenide Nanosheets With Mucin: Quartz Crystal Microbalance With Dissipation, Surface Plasmon Resonance, and Spectroscopic Probing.

Ultrathin 2-dimensional transition metal dichalcogenides (TMDs) have become a class of high-potential materials in biomedicine due to their intriguing properties. They have been applied to solve biomedical challenges, such as biosensing, bioimaging, drug delivery, and cancer therapy. However, studies of the interactions between these materials and biomolecules are insufficient. Mucous tissue serves as a barrier to foreign hazardous substances and a gel layer for substance exchange. The main organic matter of mucous tissue is mucin, so it was selected as a model biomolecule to study its interactions with six different TMD nanosheets (NSs), including single-layered (SL), few-layered (FL), and small few-layered (SFL) MoS2 and WS2 NSs, using quartz crystal microbalance (QCM) with a dissipation monitor (QCM-D) and surface plasmon resonance (SPR). Additionally, UV absorption, fluorescence, and circular dichroism (CD) spectroscopy were applied to investigate the mechanism of the interactions and to study the conformational change of mucin. We found that the TMD NSs could adsorb on the mucin layer and affect its viscoelasticity. The results indicated that the SL WS2 NSs exhibited the highest initial absorption rate and the maximum absorption amount, while the SL MoS2 NSs exhibited the highest initial desorption rate. During the adsorption, the viscoelasticity variations of the mucin layer caused by the WS2 nanosheets were weaker than those caused by the MoS2 nanosheets. Furthermore, the conformational changes of mucin caused by the SL MoS2, SL WS2, and SFL MoS2 NSs were higher than those resulting from other TMD NSs. These findings provide important information on the interactions between TMD NSs and mucin and provide useful insights into the interfacial behavior of TMD NSs before they enter tissues.

Electronic structures of air-exposed few-layer black phosphorus by optical spectroscopy

The electronic structure of few-layer(FL) black phosphorus(BP) sensitively depends on the sample thickness, strain and doping. In this paper, we show that it is also vulnerable to air-exposure. The oxidation of BP caused by air-exposure gives several optical signatures, including the broadening of resonance peaks and increased Stokes shift between infrared (IR) absorption and photoluminescence (PL) peaks. More importantly, air exposure causes blue shifts of all resonance peaks in IR absorption and PL spectra, with more prominent effects for thinner samples and higher order subband transitions. Our study alludes a convenient and exotic way for band-structure engineering of FL-BP through controllable air-exposure or defect creation.

Linear and Nonlinear Optical Properties of Few‐Layer Exfoliated SnSe Nanosheets

AbstractMonochalcogenides of group IV elements have been considered as phosphorene analogs due to their similar crystal and electronic structure. Here, few‐layer SnSe nanosheets are synthesized by a sonication‐assisted liquid phase exfoliation process and their linear and nonlinear optical properties are examined. The as‐exfoliated few‐layer (FL) SnSe demonstrates layer thickness from 2 to 10 nm and lateral size of 150 nm with high crystallinity. Optical measurement confirms the layer‐number dependent bandgap, which is also corroborated by first‐principle calculation. In addition, the FL SnSe shows a transition from saturable absorption to reverse saturable absorption with the increase of pumping power, and the nonlinear absorption is characterized by an ultrafast response time of picosecond scale. The nonlinear optical response in FL SnSe could be explained by defect‐involved recombination of photogenerated carriers as well as the Auger scattering process, which is further enhanced by structural two‐dimensionality.

Atomically Thin 2D‐Arsenene by Liquid‐Phased Exfoliation: Toward Selective Vapor Sensing

AbstractPhosphorene and antimonene, single‐ or few‐layered (FL) semiconductor materials, have recently attracted enormous attention due to their unique properties, provided by their extreme thinness. Here, a liquid‐phase exfoliation (LPE) procedure to prepare FL arsenene, another member of pnictogens, assisted by sonication and without any additional surfactant is reported. The exfoliation process is performed in various solvents. Among those, N‐methylpyrrolidone is found to provide the highest concentration of stable arsenene sheets. Spectroscopic and microscopic analyses confirm the formation of high‐quality few‐layer arsenene nanosheets with large lateral dimensions. An application of this material for construction of vapor sensor based on electrochemical impedance spectroscopy is demonstrated. The device detects selectively methanol or acetone vapors depending on the selected resonance frequency. The results are highly reproducible, and the vapor sensor has long‐term stability.