Pressure-mediated contact quality improvement between monolayer MoS_2 and graphite

Two-dimensional(2D) materials and their heterostructures have attracted a lot of attention due to their unique electronic and optical properties. MoS_2 as the most typical 2D semiconductors has great application potential in thin film transistors, photodetector, hydrogen evolution reaction, memory device, etc. However, the performance of MoS_2 devices is limited by the contact resistance and the improvement of its contact quality is important. In this work, we report the experimental investigation of pressure-enhanced contact quality between monolayer MoS_2 and graphite by conductive atom force microscope(C-AFM). It was found that at high pressure, the contact quality between graphite and MoS_2 is significantly improved. This pressure-mediated contact quality improvement between MoS_2 and graphite comes from the enhanced charge transfer between MoS_2 and graphite when MoS_2 is stretched. Our results provide a new way to enhance the contact quality between MoS_2 and graphite for further applications.     

Picosecond pulse generation in a mono-layer MoS_2 mode-locked Ytterbium-doped thin disk laser

A mode-locked(ML)picosecond ytterbium-doped thin disk laser using a monolayer Mo S2as the saturable absorber(SA)is demonstrated.The monolayer MoS_2 is fabricated through the method of low-pressure chemical vapor deposition.The laser directly produces stable ML picosecond pulses at a slope efficiency of 9.71%.The maximum output power is approximately 890 mW,while the corresponding repetition,pulse energy,and pulse duration are 48.6 MHz,18.3 nJ,and 13.1 ps,respectively.Results suggest that the monolayer MoS_2 is a promising SA for ultrafast lasers system.     

Gap plasmon-enhanced photoluminescence of monolayer MoS_2 in hybrid nanostructure

Monolayer transition-metal dichalcogenides(TMDs) have attracted a lot of attention for their applications in optics and optoelectronics.Molybdenum disulfide(MoS_2),as one of those important materials,has been widely investigated due to its direct band gap and photoluminescence(PL) in visible range.Owing to the fact that the monolayer MoS_2 suffers low light absorption and emission,surface plasmon polaritons(SPPs) are used to enhance both the excitation and emission efficiencies.Here,we demonstrate that the PL of MoS_2 sandwiched between 200-nm-diameter gold nanoparticle(Au NP) and 150-nm-thick gold film is improved by more than 4 times compared with bare MoS_2 sample.This study shows that gap plasmons can possess more optical and optoelectronic applications incorporating with many other emerging two-dimensional materials.     

Nucleation mechanism and morphology evolution of MoS_2 flakes grown by chemical vapor deposition

We study the nucleation mechanism and morphology evolution of MoS_2 flakes grown by chemical vapor deposition(CVD)on SiO_2/Si substrates with using S and MoO_3 powders.The MoS_2 flake is of monolayer with triangular nucleation,which might arise from the initial MoO_3-xthat is deposited on the substrate,and then bonded with S to form MoS_2 flake.The ratio of Mo and S is higher than 1:2 at the beginning with Mo terminated triangular nucleation formed.After that,the morphology of MoS_2 flake evolves from triangle to similar hexagon,then to truncated triangle which is determined by the faster growth speed of Mo termination than that of S termination under the S rich environment.The nucleation density does not increase linearly with the increase of reactant concentration,which could be explained by the two-dimensional nucleation theory.     

Effect of graphene grain boundaries on MoS_2/graphene heterostructures

The grain boundaries of graphene are disordered topological defects, which would strongly affect the physical and chemical properties of graphene. In this paper, the spectral characteristics and photoresponse of MoS_2/graphene heterostructures are studied. It is found that the blueshift of the G and 2 D peaks of graphene in Raman spectrum is due to doping. The lattice mismatch at the graphene boundaries results in a blueshift of MoS_2 features in the photoluminescence spectra, comparing to the MoS_2 grown on SiO_2. In addition, the photocurrent signal in MoS_2/hexagonal single-crystal graphene heterostructures is successfully captured without bias, but not in MoS_2/polycrystalline graphene heterostructures.The electron scattering at graphene grain boundaries affects the optical response of MoS_2/graphene heterostructures. The photoresponse of the device is attributed to the optical absorption and response of MoS_2 and the high carrier mobility of graphene. These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.     

Modulation of the second-harmonic generation in MoS_2 by graphene covering

Nonlinear optical frequency mixing,which describes new frequencies generation by exciting nonlinear materials with intense light field,has drawn vast interests in the field of photonic devices,material characterization,and optical imaging.Investigating and manipulating the nonlinear optical response of target materials lead us to reveal hidden physics and develop applications in optical devices.Here,we report the realization of facile manipulation of nonlinear optical responses in the example system of MoS₂ monolayer by van der Waals interfacial engineering.We found that,the interfacing of monolayer graphene will weaken the exciton oscillator strength in MoS₂ monolayer and correspondingly suppress the second harmonic generation(SHG)intensity to 30%under band-gap resonance excitation.While with off-resonance excitation,the SHG intensity would enhance up to 130%,which is conjectured to be induced by the interlayer excitation between MoS₂ and graphene.Our investigation provides an effective method for controlling nonlinear optical properties of two-dimensional materials and therefore facilitates their future applications in optoelectronic and photonic devices.     

对氨基苯甲酸在不同基底上的SERS研究

用氢气还原氧化银的方法合成了粒径约为100nm的银纳米粒子,然后在表面经PVP修饰的载玻片上以自组装的方式构筑了银纳米粒子单层和双层二维有序阵列,在铝基底上通过对氨基苯甲酸成功构筑了银纳米粒子的有序组装体,用扫描电镜进行了表征;对不同基底二维有序阵列的表面增强拉曼光谱进行了研究,结果表明,对氨基苯甲酸在银和铝基基底上都是垂直吸附,其表面拉曼增强效应是电磁场增强占主导地位,但同时也有化学增强的贡献。     

Interfacial Charge Transfer Induced Electronic Property Tuning of MoS_2 by Molecular Functionalization

The modulation of electrical properties of MoS_2 has attracted extensive research interest because of its potential applications in electronic and optoelectronic devices.Herein,interfacial charge transfer induced electronic property tuning of MoS_2 are investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy measurements.A downward band-bending of MoS_2-related electronic states along with the decreasing work function,which are induced by the electron transfer from Cs overlayers to MoS_2,is observed after the functionalization of MoS_2 with Cs,leading to n-type doping.Meanwhile,when MoS_2 is modified with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F_4-TCNQ),an upward band-bending of MoS_2-related electronic states along with the increasing work function is observed at the interfaces.This is attributed to the electron depletion within MoS_2 due to the strong electron withdrawing property of F_4-TCNQ,indicating p-type doping of MoS_2.Our findings reveal that surface transfer doping is an effective approach for electronic property tuning of MoS_2 and paves the way to optimize its performance in electronic and optoelectronic devices.     

Raman analysis of bilayer graphene film prepared on commercial Cu(0.5 at% Ni) foil

This study reports the Raman analysis of bilayer graphene films prepared on commercial dilute Cu(0.5 at% Ni) foils using atmospheric pressure chemical vapor deposition. A bilayer graphene film obtained on Cu foil is known to have small areas of bilayer (islands) with a significant fraction of non‐Bernal stacking, while that obtained on Cu/Ni is known to grow over a large area with Bernal stacking. In the Raman optical microscope images, a wafer‐scale monolayer and large‐area bilayer graphene films were distinguished and confirmed with Raman spectra intensities ratios of 2D to G peaks. The large‐area part of bilayer graphene film obtained was assisted by Ni surface segregation because Ni has higher methane decomposition rate and carbon solubility compared with Cu. The Raman data suggest a Bernal stacking order in the prepared bilayer graphene film. A four‐point probe sheet resistance of graphene films confirmed a bilayer graphene film sheet resistance distinguished from that of monolayer graphene. A relatively higher Ni surface concentration in Cu(0.5 at% Ni) foil was confirmed with time‐of‐flight secondary ion mass spectrometry. The inhomogeneous distribution of Ni in a foil and the diverse crystallographic surface of a foil (confirmed with proton‐induced X‐ray emission and electron backscatter diffraction, respectively) could be a reason for incomplete wafer‐scale bilayer graphene film. The Ni surface segregation in dilute Cu(0.5 at% Ni) foil has a potential to impact on atmospheric pressure chemical vapor deposition growth of large‐area bilayer graphene film. Copyright © 2015 John Wiley & Sons, Ltd.     

Strain-modulated excitonic gaps in mono-and bi-layer MoSe_2

Photoluminescence(PL) and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe_2 to comparatively investigate the evolution of excitonic gaps and Raman phonons with strain. We observed that the strain dependence of excitonic gaps shows a nearly linear behavior in both flakes. One percent of strain increase gives a reduction of ~42 meV(~35 me V) in A-exciton gap in monolayer(bilayer) MoSe_2. The PL width remains little changed in monolayer MoSe_2 while it increases rapidly with strain in the bilayer case. We have made detailed discussions on the observed strain-modulated results and compared the difference between monolayer and bilayer cases. The hybridization between 4d orbits of Mo and 4p orbits of Se, which is controlled by the Se–Mo–Se bond angle under strain, can be employed to consistently explain the observations. The study may shed light into exciton physics in few-layer MoSe_2 and provides a basis for their applications.     

基于硫蒸气处理转移的单层二硫化钼的荧光增强

采用化学气相沉积法在SiO2/Si衬底上制备了单层MoS2,再通过300℃硫蒸气处理用聚甲基丙烯酸甲酯(Polymethyl Methacrylate,PMMA)转移下的单层MoS2.使用原子力显微镜、真空荧光检测和拉曼光谱等手段表征了样品的形貌和光致发光性能.结果表明:经过硫蒸气处理转移后的单层MoS2的光致发光强度比由化学气相沉积法制备的未处理的单层MoS2的光致发光强度增强了约5倍.光致发光强度增强是由于在硫蒸气处理过程中,单层MoS2的部分硫空位被硫原子纳米团簇所填补,从而提高了光致发光效率.此外,分别将单层MoS2转移到SiO2/Si衬底、石英、三氧化铝及氟化镁衬底再经过硫处理后,也观察到了类似的荧光增强现象.     

含偶氮两亲分子自组装单层膜的扫描力显微镜和共聚焦拉曼光谱研究

两亲性分子在液 /固界面组装成二维有序的结构是目前超分子科学研究的热点。含有偶氮苯单链季铵盐化合物 4-[(4’ -十二烷氧基 )偶氮苯氧基 ]己基三乙基溴化铵 (简写为C12 AzoC6 N+)在水溶液中能聚集成胶束 ,当它自组织到新鲜的云母片上时 ,则形成二维有序的纳米尺寸的树枝状条带结构。为了进一步系统的研究该自组织树枝状条带结构单层膜的性质 ,本文利用扫描力显微镜 (SFM)研究了该化合物在大于其临界胶束浓度时在新鲜的云母片上自组织成树枝状条带结构的行为 ,并通过共聚焦拉曼光谱对该自组织树枝状条带结构单层膜进行了测试 ,指认了固体和自组织单层膜的拉曼位移。通过比较 ,我们进一步确认了在云母片上的自组织的二维有序的纳米尺寸树枝状条带单层膜的结构。这将为这种二维有序的纳米尺寸树枝状条带结构将来的应用奠定基础。     

Effect of defects on the electronic structure of a PbI_2/MoS_2 van der Waals heterostructure: A first-principles study

PbI_2/MoS_2, as a typical van der Waals(vdW) heterostructure, has attracted intensive attention owing to its remarkable electronic and optoelectronic properties. In this work, the effect of defects on the electronic structures of a PbI_2/MoS_2 heterointerface has been systematically investigated. The manner in which the defects modulate the band structure of PbI_2/MoS_2, including the band gap, band edge, band alignment, and defect energy-level density within the band gap is discussed herein. It is shown that sulfur defects tune the band gaps, iodine defects shift the positions of the band edge and Fermi level, and lead defects realize the conversions between the straddling-gap band alignment and valence-band-aligned gap, thus enhancing the light-absorption ability of the material.     

Layer-number determination of two-dimensional materials by optical characterization

Initiated by graphene, two-dimensional(2D) layered materials have attracted much attention owing to their novel layer-number-dependent physical and chemical properties. To fully utilize those properties, a fast and accurate determination of their layer number is the priority. Compared with conventional structural characterization tools, including atomic force microscopy, scanning electron microscopy, and transmission electron microscopy, the optical characterization methods such as optical contrast, Raman spectroscopy, photoluminescence, multiphoton imaging, and hyperspectral imaging have the distinctive advantages of a high-throughput and nondestructive examination. Here, taking the most studied 2D materials like graphene, MoS_2, and black phosphorus as examples, we summarize the principles and applications of those optical characterization methods. The comparison of those methods may help us to select proper ones in a cost-effective way.     

Magnus Hall Effect in Two-Dimensional Materials

The Magnus Hall effect(MHE) is a new type of linear-response Hall effect, recently proposed to appear in two-dimensional(2D) nonmagnetic systems at zero magnetic field in the ballistic limit. The MHE arises from a self-rotating Bloch electron moving under a gradient-electrostatic potential, analogous to the Magnus effect in the macrocosm. Unfortunately, the MHE is usually accompanied by a trivial transverse signal, which hinders its experimental observation. We systematically investigate the material realization and experimental measurement of the MHE, based on symmetry analysis and first-principles calculations. It is found that both the out-ofplane mirror and in-plane two-fold symmetries can neutralize the trivial transverse signal to generate clean MHE signals. We choose two representative 2D materials, monolayer MoS_2, and bilayer WTe_2, to study the quantitative dependency of MHE signals on the direction of the electric field. The results are qualitatively consistent with the symmetry analysis, and suggest that an observable MHE signal requires giant Berry curvatures. Our results provide detailed guidance for the future experimental exploration of MHE.     

Raman Suppression in a Kilowatt Narrow-Band Fiber Amplifier

A novel technique to suppress stimulated Raman scattering in a high power narrow-band fiber amplifier is reported.By seeding with a combination of a broadband amplified spontaneous emission seed and a narrowband master oscillator seed,the Raman Stokes components can be reduced about 16 dB at a total output power of1 kW.Raman suppression results are depicted in a different wavelengths seeding case and the same wavelength seeding case,respectively,with different seed power ratios.     

Resonant surface enhanced Raman scattering by ultra thin Langmuir-Blodgett polydiacetylene polymer films

Using resonant Raman scattering and surface enhanced Raman scattering techniques, changes in the structural and electronic properties of Langmuir-Blodgett (L-B) polydiacetylene films were observed as film thicknesses were increased from one monolayer (or one bilayer) to several bilayers. The L-B films, starting with a single monolayer (or one bilayer) in a disordered “red” phase, were found to change into a mixed phase (ordered “blue” and disordered “red”) as one or more additional bilayers were deposited. This is the first observation of a “disorder to order” transformation in a L-B film. The observed effect is attributed to the ordering brought about by interactions between the initial and subsequent L-B PDA bilayers.     

Raman Scattering Modification in Monolayer ReS_2 Controlled by Strain Engineering

Regulation of optical properties and electronic structure of two-dimensional layered ReS_2 materials has attracted much attention due to their potential in electronic devices.However,the identification of structure transformation of monolayer ReS_2 induced by strain is greatly lacking.In this work,the Raman spectra of monolayer ReS_2 with external strain are determined theoretically based on the density function theory.Due to the lower structural symmetry,deformation induced by external strain can only regulate the Raman mode intensity but cannot lead to Raman mode shifts.Our calculations suggest that structural deformation induced by external strain can be identified by Raman scattering.     

Influencing factors of external fluorescence seeding enhancing stimulated Raman scattering in liquid‐core optical fiber

This paper reports an external fluorescence seeding technology enhancing the stimulated Raman scattering (SRS) in liquid‐core optical fiber (LCOF). By surrounding a small section of LCOF with a glass capillary and a solution of Rhodamine 6G filled between them to separate fluorescent dye and Raman medium, the initial intensity of SRS is linearly amplified by external fluorescence seeding, and then the SRS of LCOF can be enhanced effectively. Experimental results show that both the concentration of fluorescent dye and the seeding position have an influence on enhancement of SRS. The maximum enhancement of Stokes lines is obtained when the concentration of dye solution is optimized at ~10⁻⁶ mol/l and seeding position is located at the input end. Copyright © 2013 John Wiley & Sons, Ltd.     

Integration of correlative Raman microscopy in a dualbeam FIB SEM

We present an integrated confocal Raman microscope in a focused ion beam scanning electron microscope (FIB SEM). The integrated system enables correlative Raman and electron microscopic analysis combined with focused ion beam sample modification on the same sample location. This provides new opportunities, for example the combination of nanometer resolution with Raman advances the analysis of sub‐diffraction‐sized particles. Further direct Raman analysis of FIB engineered samples enables in situ investigation of sample changes. The Raman microscope is an add‐on module to the electron microscope. The optical objective is brought into the sample chamber, and the laser source, and spectrometer are placed in a module attached onto and outside the chamber. We demonstrate the integrated Raman FIB SEM function with several experiments. First, correlative Raman and electron microscopy is used for the investigation of (sub‐)micrometer‐sized crystals. Different crystals are identified with Raman, and in combination with SEM the spectral information is combined with structurally visible polymorphs and particle sizes. Analysis of sample changes made with the ion beam is performed on (1) structures milled in a silicon substrate and (2) after milling with the FIB on an organic polymer. Experiments demonstrate the new capabilities of an integrated correlative Raman–FIB–SEM. Copyright © 2016 John Wiley & Sons, Ltd.