Raman Spectroscopy of Few-Quintuple Layer Topological Insulator Bi2Se3 Nanoplatelets

This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi2Te4·n(Bi2Te3) van der Waals topological insulators, where n is the number of Te–Bi–Te–Bi–Te quintuple layers between two neighboring Te–Bi–Te–Mn–Te–Bi–Te septuple layers. Confocal Raman spectroscopy is applied to study Raman spectra of crystal structures with n = 0,1,2,3,4,5,6, and ∞. The experimental frequencies of vibration modes of the same symmetry in the structures with different n are compared. The lattice dynamics of free-standing one, three, and four quintuple layers, as well as of bulk Bi2Те3(n = infty ) and MnBi2Te4(n = 0), is considered theoretically. Vibrational modes of the last two systems have the same symmetry, but different displacement fields. These fields in the case of a Raman-active mode do not contain displacements of manganese atoms for any finite n. It is shown that two vibrational modes in the low-frequency region of the spectrum (35–70 cm–1) of structures with n = 1,;2,;3,;4,;5, and 6 practically correspond to the lattice dynamics of n free quintuple Bi2Те3 layers. For this reason, the remaining two vibration modes, which are observed in the high-frequency region of the spectrum (100–140 cm–1) and are experimentally indistinguishable in the sense of belonging to quintuple or septuple layer or to both layers simultaneously, should also be assigned to vibrations in quintuple layers under immobile atoms of septuple layers.

Electronic speckle pattern interferometry (ESPI) is a full field, non-contact technique for measuring the surface displacement of a structure subjected to static loading or, especially, to dynamic vibration. In this article we employ an optical system called the amplitude-fluctuation ESPI with out-of-plane and in-plane measurements to investigate the vibration characteristics of piezoceramic plates. Two different configurations of piezoceramic plates, namely the rectangular and the circular plates, are discussed in detail. As compared with the film recording and optical reconstruction procedures used for holographic interferometry, the interferometric fringes of AF-ESPI are produced instantly by a video recording system. Because the clear fringe patterns will be shown only at resonant frequencies, both the resonant frequencies and the corresponding mode shapes are obtained experimentally at the same time by the proposed AF-ESPI method. Excellent quality of the interferometric fringe patterns for both the in-plane and out-of-plane vibration mode shapes is demonstrated. The resonant frequencies of the piezoceramic plates are also measured by the conventional impedance analysis. From experimental results, we find that the out-of-plane vibration modes (type A) with lower resonant frequencies cannot be measured by the impedance analysis and only the in-plane vibration modes (type B) will be shown. However, both the out-of-plane (bending) and in-plane (extensional) vibration modes of piezoceramic plates are obtained by the AF-ESPI method. Finally, the numerical finite element calculations are also performed, and the results are compared with the experimental measurements. It is shown that the numerical calculations and the experimental results agree fairly well for both the resonant frequencies and the mode shapes.

The experimental measurement of resonant frequencies for piezoelectric material is generally performed by impedance analysis. In this paper we employ an optical interferometry method, called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI), to investigate the vibration characteristics of piezoceramic plates. This method demonstrates its advantages of combining noise reduction, like the subtraction method, and high fringe sensitivity, like the time-averaged method. As compared with the film recording and optical reconstruction procedures used for holographic interferometry, the interferometric fringes of AF-ESPI are produced instantly by a video recording system. Based on the fact that clear fringe patterns measured by the AF-ESPI method will be shown only at resonant frequencies, both the resonant frequencies and corresponding mode shapes are obtained experimentally at the same time. Excellent quality for the interferometric fringe patterns of the mode shapes is demonstrated. We find from experimental results that the out-of-plane vibration modes (type A) with lower resonant frequencies cannot be measured by impedance analysis and only the in-plane vibration modes (type B) will be shown. However, both the out-of-plane (bending) and in-plane (extension) vibration modes of piezoceramic plates are obtained by the AF-ESPI method. Finally, numberical finite element calculations are also performed, and the results are compared with the experimental measurements. Excellent agreement for the resonant frequencies and mode shapes are obtained from both results.

Taking the bending stiffness, cable static sag and cable inclinded angle into consideration, the equations of space vibration of the cable-damper system are formulted in this paper. Applying the variable separation strategy and center difference method, the partial differential equations are discrete in space and a set of complex eigenvalue equations are sovled by state space method. Then both the maximum modal damping ratio and the optimal damper parameters are obtained. Some typical stay cables are investigated for both the in-plane and out-plane vibration modes with different cable parameters and damper parameters. The results show the damping ratio for the first in-plane vibration modes with different cable parameters and damper parameters. The resutls show the dampingn ratio for the first in-plane mode is significantly affected by the cable static sag only, but those for the other modes are affected slightly, and cable static sag do not affect the optimal damper parameter for all modes. However the bending stiffness will changes both the maximum modal damping ratios and the optimal damper parameters. Some valuable suggestions are proposed for the optimal damper design.

In this study, a mechanical exfoliation method was developed for the synthesis of high purity Bi2Te3, Sb2Te3, and Bi0.5Sb1.5Te3 flakes. The synthesized Bi2Te3, Sb2Te3, and Bi0.5Sb1.5Te3 flakes were characterized by atomic force microscope, Raman spectroscopy, and photoluminescence (PL). The effect of the thickness of the Bi2Te3, Sb2Te3, and Bi0.5Sb1.5Te3 flakes on PL and Raman spectra was investigated. As the thickness increased in the Bi2Te3 flakes, the out of plane vibration mode (A21g) shows a blue shift. For the thicker Sb2Te3 flakes, the A11g and E2g modes indicated a blue shift and a red shift, respectively. When the thickness of Bi0.5Sb1.5Te3 flakes decreased, the in-plane vibration mode (E2g) shifted to lower frequencies. A new Raman peak has been observed in Bi0.5Sb1.5Te3 flakes, which is not active in the thin films. PL measurements of Bi0.5Sb1.5Te3 flakes with various thickness revealed PL peaks in the range of 2.15 - 2.54 eV at room temperature (300 K). A Bi0.5Sb1.5Te3 flakes-based photodetector exhibited photoresponsivity as high as 661.5 A/W at a 0.02 mW power density with an 1800 nm laser at room temperature (300 K). Compared to the optoelectronic properties of Bi2Te3 flake, a twice higher responsivity at a wavelength of 1800 nm was observed with the Bi0.5Sb1.5Te3 flake-based photodetector.

In this work, a synthesis method for colloidal nanoplatelets (NPLs) of the topological insulator is presented. This method is based on the hot injection of a bismuth precursor into a selenium solution to achieve precise control over both the thickness and lateral dimensions of the NPLs. Through time-resolved aliquot studies, three growth stages are identified: (i) an initial nucleation stage where "baby platelets" are formed, already having the final platelet thickness, (ii) a rapid, reaction-limited lateral growth phase, and (iii) a second lateral growth phase slowed down by precursor depletion. By adapting the reactivity of the bismuth precursor in the nucleation stage, the NPL thickness can be tuned from 7 to 1 quintuple layers, largely independent of lateral size. The NPL lateral dimensions can be altered by adjusting the growth time in stage (ii) or (iii), by adapting the injection temperature, or by injecting additional precursor once the slow growth stage has been reached. This synthesis method, combined with mechanistic insights, can be used to synthesize tailored platelets for future studies on topologicalinsulators.

Ultrathin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak van der Waals interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi(2)Se(3)), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi(2)Se(3) nanoribbons (>50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultrathin nanoribbons, showing drastic difference in sheet resistance between 1-2 QLs and 4-5 QLs. Transport measurement carried out on an exfoliated (<or=5 QLs) Bi(2)Se(3) device shows nonmetallic temperature dependence of resistance, in sharp contrast to the metallic behavior seen in thick (>50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.

Accurate analytical type solutions for the free in-plane vibration of clamped and simply supported rectangular plates

Exploring structural and electronic properties of topological insulator/graphene nano-heterostructures

The fundamental characteristics of semiconductor nanoplatelets (NPLs) have been intensively studied with a view to their use in various optoelectronic devices. In particular, the assembly of highly anisotropic NPLs into stacks allows their optical properties to be easily controlled. Here, we systematically investigate the radiative decay processes of NPL assemblies for two distinct configurations, face-down and edge-up, and find a new way of manipulating their excitonic properties by introducing metamaterial structures. While the exciton lifetime for face-down NPLs is not strongly affected by the metamaterials, a significant reduction in the exciton lifetime for edge-up NPLs is observed. We account for this interesting result in terms of orientation-dependent image dipole interactions by showing that the net transition dipole moments for the two NPL configurations can be affected inversely by metamaterials. These results will facilitate the development of optoelectronic applications requiring the active manipulation of exciton lifetimes.

Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl2, resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9–13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5–11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

When exposed to light, the colloidal perovskite nanoplatelets (NPLs) in the film can fuse into larger grains, and this phenomenon was thought to be closely related to ion migration. However, the available CsPbBr3 NPLs are not conducive to directly distinguishing this hypothesis. Herein, we prepare mixed-halide perovskite CsPbBr2.7I0.3 NPLs by a ligand-assisted reprecipitation method and investigate the photoluminescence evolution of NPLs under laser irradiation. At a low-irradiation intensity, 4.5-monolayer NPLs exhibit blue-shifted photoluminescence peaks due to the migration of iodide ions. Under higher laser fluence, a new photoluminescence component appears in the long wavelength region after the spectral blue shift, which is attributed to the coalescence of NPLs according to transmission electron microscopy analysis. A similar spectral evolution is also observed in 8-monolayer NPLs, while only the spectral blue shift caused by ion migration is detected in cuboidal CsPbBr2.7I0.3 nanocrystals. The use of strong bonding ligands can inhibit the fusion process of the NPLs, but not to impede ion migration, suggesting that fusion requires ligand detachment rather than ion migration. Similar suppression effects can be achieved in a vacuum atmosphere. Moreover, we demonstrate that mixed-halide NPLs can be used to realize anti-counterfeiting applications with superior photosensitivity.

Coupling interaction between the bottom and top surface electronic states and the influence on transport and optical properties of Bi2Se3 thin films with 1–8 quintuple layers (QLs) have been investigated by first principles calculations. Obvious spatial and thickness dependences of coupling interaction are found by analyzing hybridization of two surface states. In the thin film with a certain thickness, from the outer to inner atomic layers, the coupling interaction exhibits an increasing trend. On the other hand, as thickness increases, the coupling interaction shows a disproportionate decrease trend. Moreover, the system with 3 QLs exhibits stronger interaction than that with 2 QLs. The presence of coupling interaction would suppress destructive interference of surface states and enhance resistance in various degrees. In view of the inversely proportional relation to transport channel width, the resistance of thin films should show disproportionate thickness dependence. This prediction is qualitatively consistent with the transport measurements at low temperature. Furthermore, the optical properties also exhibit obvious thickness dependence. Especially as the thickness increases, the coupling interaction results in red and blue shifts of the multiple-peak structures in low and high energy regions of imaginary dielectric function, respectively. The red shift trend is in agreement with the recent experimental observation and the blue shift is firstly predicted by the present calculation. The present results give a concrete understanding of transport and optical properties in devices based on Bi2Se3 thin films with few QLs.

In this paper, a linear lumped-element equivalent circuit model (ECM) for ultrasonic laterally transduced electrostatic bulk-mode air-coupled resonant micro-electro-mechanical systems (MEMS) is described. A single-crystal silicon (SCS) square plate with T-shaped tethers is considered as the geometry of interest with a one-sided electrostatic actuation. This type of sensor can be used for sensitive mass sensing of airborne particles and possesses a large active surface with in-plane vibration modes in the ultrasonic frequency range. Firstly, the eigensolutions and eigenvectors of the problem are obtained using analytical equations and compared with finite-element modeling (FEM) solutions. Secondly, using modal analysis, the number of degrees of freedom is reduced and individual solutions are provided for each vibration mode, leading to various effective masses, stiffnesses and dampings. The first order Taylor expansion of both the electrical current equation and the electrostatic force applied on the resonator allows one to obtain expressions for the additional stiffness and the electro-mechanical transformation coefficient linked to the membrane actuation. Based on theses results, single-input single output (SISO) equivalent circuits are established using electro-mechanical and Butterworth-Van Dyke (BVD) approaches. Electrical admittance simulations resulting from different in-plane vibration modes are proven to be in excellent agreement with FEM simulations. Finally, a numerical mass sensing application is described to evaluate the relevance of both the model and the resonator design to act as a microbalance. The proposed model can be used to design, predict, analyze and optimize the behavior of highly sensitive air-coupled ultrasonic bulk-mode SCS MEMS for various physical applications.

Dispersed fluorescence spectra of jet-cooled adenine were obtained by exciting the vibronic bands observed in the fluorescence excitation spectrum from an earlier study (Kim et al., J. Chem. Phys., 2000, 113, 10 051). The dispersed fluorescence spectra reveal sufficiently well-resolved vibrational bands of the ground electronic state. The excitation of the C′ band at 36 062 cm−1 and the D′ band at 36 105 cm−1 results in emissions that are associated with out-of-plane and in-plane vibrational modes in the ground state, respectively, which suggests that these bands belong to different electronic states, as first proposed by us and later supported by others. A vibrational analysis yielded an assignment of the C′ band as an out-of-plane vibrational mode of the nπ* state and the D′ band as the 0–0 band of the ππ* state. Combination bands of the out-of-plane and in-plane vibrational modes are observed in the dispersed fluorescence spectrum of the C′ band, which seems to support that there is indeed a strong vibronic coupling between the nπ* and ππ* excited states of adenine.