Infrared diode laser spectroscopy of the He-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the He-N₂O van der Waals complex has been recorded in the N₂O-monomer ν₁ region (∼1285 cm⁻¹) using an infrared tunable diode laser spectrometer in conjunction with a free supersonic jet expansion and an astigmatic multi-pass absorption cell. Twenty-two lines are assigned to the ν₂-band of He-N₂O. Rotational constants for the ν₂-excited state are derived. The band-origin of the spectrum is determined to be ν₀ = 1285.0666(7) cm⁻¹ and shows a blue-shift of 0.1633(8) cm⁻¹ compared with the N₂O ν₁-band origin.     

Rovibrational spectrum of the Ne-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the Ne-N₂O van der Waals complex has been recorded in the symmetric stretching mode region of the N₂O monomer (∼1285 cm⁻¹) using a tunable diode laser spectrometer in conjunction with an astigmatic multi-pass cell and a pulsed supersonic slit jet. The spectra of both ²⁰Ne-N₂O and ²²Ne-N₂O isotopomers are assigned and analyzed using a Watson S-reduced asymmetric-rotor Hamiltonian. The rotational and centrifugal constants for the excited vibrational state are accurately determined. The band-origin of the spectrum is determined to be ν₀ = 1285.12251(18) cm⁻¹ for ²⁰Ne-N₂O and 1285.12363(27) cm⁻¹ for ²²Ne-N₂O, which shows a blue-shift of 0.21921 cm⁻¹ for ²⁰Ne-N₂O and 0.22033 cm⁻¹ for ²²Ne-N₂O from that of the N₂O monomer, respectively.     

Direct FTIR high resolution probe of small and medium size Arn(CO2)m van der Waals complexes formed in a slit supersonic expansion

A slit nozzle continuous expansion of argon seeded with carbon dioxide is probed using a Bruker IFS 120 HR FTIR spectrometer operating at 0.05 cm⁻¹ or 0.01 cm⁻¹ spectral resolution. About 250 individual rovibrational transitions are assigned which belong to the asymmetric stretch of the CO₂ moiety in Ar-CO₂ and (CO₂)₂ complexes. This made it possible to refine the set of spectroscopic constants characterizing these van der Waals species. Analysis of the observed spectral features allowed for evaluation of the number densities of complexes formed in a jet. The manifold of rovibrational lines of van der Waals complexes (along with the monomer lines) sits on an unresolved pedestal, the shape of which varies dramatically as a function of reservoir pressure and initial CO₂-Ar mixing ratios. Thorough examination of these variations allows for the observed features to be assigned to Ar_n(CO₂)_m clusters formed in CO₂ seeded Ar expansion.     

Infrared diode laser spectroscopy of O2–N2O van der Waals complex in the ν1 symmetric stretch region of N2O

The rovibrational spectrum of O₂–N₂O van der Waals complex is measured in the v₁ symmetric stretch region of N₂O monomer using a tunable diode laser spectrometer. The complex is generated by a slit-pulsed supersonic expansion with gas mixtures of O₂, N₂O, and He. Both a- and b-type transitions are observed. The effective Hamiltonian for an open-shell complex consisting of a diatomic molecule in a ³Σ electronic state and a closed-shell partner is used to analyze the observed spectrum. Molecular constants in the vibrationally excited state are determined accurately. The band-origin of the spectrum is determined to be 1284.7504(25) cm^-1, red-shifted from that of the N₂O monomer by ～ 0.1529 cm^-1.     

Intermolecular hydrogen bonds, rotational spectrum, and structure of van der Waals complexes of (CH3)2O?CF2CH2 and (CH3)2O?CF2CHF

The results of molecular beam Fourier transform microwave (FTMW) investigations of the van der Waals complexes of dimethyl ether with 1,1-difluoroethene/trifluoroethene are reported. The rotational parameters of the complexes have been interpreted in terms of a C_s geometry with the two methyl groups lying out of the σ_v symmetry plane of complexes. The complexes are bound with three hydrogen bonds of which one is the stronger O?HC type and two are the weaker F?HC types. Some additional information on the structure and the hydrogen bond has been obtained from ab initio calculations.     

两步化学气相沉积法制备In2Se3/MoSe2范德华尔斯异质结

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research,but also for the application of next generation electronic and optoelectronic devices.Herein,we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In₂Se₃/MoSe₂ heterostructures.Transmission electron microscopy characterization reveals clearly that the In₂Se₃ has well-aligned lattice orientation with the substrate of monolayer MoSe₂.Due to the interaction between the In₂Se₃ and MoSe₂ layers,the heterostructure shows the quenching and red-shift of photoluminescence.Moreover,the current rectification behavior and photovoltaic effect can be observed from the heterostructure,which is attributed to the unique band structure alignment of the heterostructure,and is further confirmed by Kevin probe force microscopy measurement.The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.     

Infrared spectra of carbon monoxide-hydrogen sulfide van der Waals complexes in the C-O stretching region

The CO-stretching vibration-rotation spectra of CO-H₂S, CO-D₂S, and CO-HDS complexes have been studied in the 2150 cm⁻¹ region using a supersonic slit-jet expansion and a tunable diode laser spectrometer. The spectra were analyzed with the help of very recent microwave pure rotational studies of the same complexes. Two bands were assigned for each of the symmetric hydrogen sulfide isotopes, corresponding to the two nuclear spin modifications, para and ortho. The band origins were blue shifted, relative to the free CO molecule, by about 3.8 cm⁻¹ for CO-H₂S and 4.3 cm⁻¹ for CO-D₂S. These are considerably smaller shifts than exhibited by the related CO-water complexes, indicating that the intermolecular forces in CO-H₂S are weaker and more isotropic.     

Triply hydrogen bonded van der Waals complexes: CH2F2?CF2CH2 and CH2F2?CF2CHF

The results of molecular beam Fourier transform microwave (FTMW) investigations of the van der Waals complexes of difluoromethane with 1,1-difluoroethene (DFE) and trifluoroethene (TFE) are reported. The rotational parameters of the complexes have been interpreted in terms of a C_s geometry with the two H or F atoms of CH₂F₂ lying out of the σ_v symmetry plane of the complexes. The complexes are bound by three hydrogen bonds, of which one is the stronger C-F?H-C type, and two are the weaker C-H?F?H-C or C-F?H?F-C types for DFM-DFE or DFM-TFE, respectively. The most notable difference in the two complexes is that the out of plane atoms are two hydrogens for DFM-DFE, but are two fluorines for DFM-TFE. Additional information on the structure and hydrogen bonding has been obtained from ab initio calculations.     

Far-infrared hydrazine laser pumped by an N2O laser

We have used an N₂O laser to optically pump N₂H₄ molecules in a far-infrared cavity and observed 17 new laser lines in the wavelength range 93.0 to 374.2 μm, the 136.8 μm line pumped by 10P(16) being a doublet. We measured the frequencies of the laser lines by heterodyne mixing of the far-infrared radiation with radiation from two frequency-stabilized CO₂ lasers.     

高分辨的一氧化二氮光解离实验研究

利用"时间切片"离子速度成像技术研究了N₂O分子在134.20、135.20和136.43 nm波长下的真空紫外光解动力学. 实验中通过采集解离产物O(¹S_J=0)的离子影像来研究O(¹S_J=0)+N₂(X¹∑_g⁺)这一解离通道. 从各个波长下的实验影像可获得产物N₂(X¹∑_g⁺)的振动态分辨的结构,进而得到产物的总平动能谱和产物N₂的振动态布居. 实验结果表明在实验的光解波长下,产物N₂(X¹∑_g⁺)主要布居在v=2和v=3. 此外,还得到了产物N₂的振动态分辨的各向异性参数β,从中发现产物N₂的β值在三个解离波长下均表现出相似的特征,即随着振动量子数的增大,β值从趋近于2逐渐减小至1.4. 这一现象表明低振动态产物是通过一个以平行跃迁解离为主的解离过程产生的,而高振动态的产物来自于一个更加弯曲的中间构型的解离. 此推论与在平动能谱中所见到的最强转动态布居随着振动量子数的增大而出现的位移是相一致的.     

The structure of self-assembled N2O multilayers

Based on the structure of the monolayer, the structure of multilayered nitrous oxide (N₂O) was determined by applying the first-principle technique. Several potential structures of the N₂O multilayer were evaluated. The results showed that the multilayer consisted of many layers and that the distance between neighboring layers was equal to 4.1 Å.     

The characteristics of self-assemble N2O monolayer

The first-principle technique has been employed to determine the atomic structure of nitrous oxide (N₂O) monolayer. The potential structures of N₂O monolayer have been proposed. The calculation shows that the monolayer is a self-assembly system, in which the basic structure unit is the dislocated molecular chain.     

Prospects and Opportunities of 2D van der Waals Magnetic Systems

The existence of spontaneous magnetization in low dimensional magnetic systems has attracted intensive studies since the early 60s and research remains very active even now. Only recently, magnetic van der Waals (vdW) systems down to a few layers have been broadly discussed for their magnetic order ground states at finite temperature. The naturally inherited layered structure of the vdW magnetic systems possessing onsite magnetic anisotropy from band electrons can suppress the long-range fluctuations. This provides an excellent vehicle to study the transition of magnetism to 2D limits both theoretically and experimentally. Here the current status of 2D vdW magnetic system and its potential applications are briefly summarized and discussed.     

Rotational Spectrum, Dynamics, and Bond Energy of the Oxirane···Krypton van der Waals Complex

From the free jet millimeter-wave spectra of four isotopomers of the weakly bonded oxirane···Kr complex, information on the equilibrium conformation, dynamics, and dissociation energy has been deduced. A C_s symmetry is found for the complex, with Kr lying in the σ_v plane of symmetry of oxirane. The equilibrium distance of Kr with respect to the center of mass of bare oxirane is 3.67 Å, with Kr tilted 13.6° from the perpendicular to the center of mass of the ring toward the oxygen atom. The dissociation energy is estimated, from the centrifugal distortion constant D_J, to be ca. 3.1 kJ/mol.     

表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.     

Observation of magnetoresistance in CrI3/graphene van der Waals heterostructures

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI₃/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.     

N2- and O2-broadening coefficients and profiles for millimeter lines of N2O

For the purpose of atmospheric applications, we have measured N₂- and O₂-induced broadenings and shapes of rotational lines of N₂O in the 235-350 K temperature range, precisely the J=8←7, J=22←21, and J=23←22 lines, located near 201, 552, and 577 GHz, respectively. The analysis of experimental lineshapes shows up significant deviations from the Voigt profile, which are characteristic of line narrowing processes. In a first step, the Voigt profile was considered for the determination of pressure broadening parameters and of their temperature dependencies. Results are in good agreement with the dependence from rotational quantum number previously observed for other rotational and rovibrational lines. They are well explained by calculations based on a semiclassical formalism that includes the atom-atom Lennard-Jones potential in addition to electrostatic interactions up to hexadecapolar contributions. In a second step, observed lineshapes were analyzed by using the Galatry profile and a speed-dependent Voigt profile. The nonlinear pressure behavior observed for the diffusion rate β involved in the Galatry profile leads to rule out the possible role of velocity/speed changing collisions, and to infer that discrepancies from the Voigt profile result from the dependence of relaxation rates on molecular speeds. This interpretation is supported by the comparison of optical and kinetic radii and confirmed by theoretical calculations of relaxation rates. Finally, it can be claimed that, for the N₂O-N₂ and N₂O-O₂ systems, deviations from the Voigt profile are explained by a speed-dependent Voigt profile.     

Spin freezing in the van der Waals material Mn2Ga2S5

Geometrical frustration in low-dimensional magnetic systems has been an intriguing research aspect, where the suppression of conventional magnetic order may lead to exotic ground states such as spin glass or spin liquid. In this work we report the synthesis and magnetism study of the monocrystalline Mn$_2$Ga$_2$S$_5$, featuring both the van der Waals structure and a bilayered triangular Mn lattice. Magnetic susceptibility reveals a significant antiferromagnetic interaction with a Curie-Weiss temperature $\theta_{\rm w}\sim-260$ K and a high spin $S=5/2$ Mn$^{2+}$ state. However, no long range magnetic order has been found down to 2 K, and a spin freezing transition is found to occur at around 12 K well below its $\theta_{\rm w}$. This yields a frustration index of $f = -\theta_{\rm w}/T_{\rm f} \approx 22$, an indication that the system is highly frustrated. The absence of a double-peak structure in magnetic specific heat compared with the $TM_2$S$_4$ compounds implies that the spin freezing behavior in Mn$_2$Ga$_2$S$_5$ is a result of the competition between exchange interactions and the 2D crystalline structure. Our results suggest that the layered Mn$_2$Ga$_2$S$_5$ would be an excellent candidate for investigating the physics of 2D magnetism and spin disordered state.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.