We investigated 10 to 200 nm thick black phosphorus flakes on SiO2/Si and polyimide substrates by Angle-resolved Polarized Raman spectra (ARPRS) using 442 nm excitation wavelength. The results revealed that ARPRS with 442 nm excitation can provide unambiguous, convenient, non-destructive and fast determination of BP's crystallographic orientation. The substrate and thickness dependencies of Raman spectra and Raman tensor elements were studied. These dependencies were shown to be influenced by Raman excitation laser heating effect. By comparing with in-situ Raman measurements at elevated temperatures, we were able to quantify the laser-heating effect, which is significant and hard to avoid for Raman measurements of BP on polyimide substrates due to the poor thermal conductivity of the substrate. Thermal processing by substrate heating was shown to have a significant impact on BP on SiO2/Si substrate, but not for BP on polyimide due to smaller thermal expansion mismatch. Our results give important insights on Raman Spectroscopy characterizations of BP on different substrates.
Twisted stacked few layer black phosphorus heterostructures were successfully fabricated in this work. Abnormal blue shifts in their Ag1 and Ag2 Raman peaks and unique optical reflections were observed in these samples. The phonon behavior difference can be explained by our density functional theory calculations, which suggest that interlayer coupling has a significant effect in twisted bilayer black phosphorus. According to the calculations, the interlayer interactions are not simply van der Waals interactions. Additional interactions, such as weak valence bonding between the top and bottom f…
Probing the anisotropic behaviors of black phosphorus by transmission electron microscope, angular-dependent Raman spectra and electronic transports measurements
We report a study of the structural phase transitions induced by pressure in bulk black phosphorus by using both synchrotron x-ray diffraction for pressures up to 12.2 GPa and Raman spectroscopy up to 18.2 GPa. Very recently black phosphorus attracted large attention because of the unique properties of fewlayers samples (phosphorene), but some basic questions are still open in the case of the bulk system. As concerning the presence of a Raman spectrum above 10 GPa, which should not be observed in an elemental simple cubic system, we propose a new explanation by attributing a key role to the no…
In this work, we reported uniform layer-by-layer sublimation of black phosphorus under heating below 600 K. The uniformity and crystallinity of BP samples after thermal thinning were confirmed by Raman spectra and 2D Raman imaging. A uniform and crystalline bilayer black phosphorus flake with an area of 180 um^2 was prepared with this method. No micron scale defects were observed. The sublimation rate of BP was around 0.18 nm / min at 500 K and 1.5 nm / min at 550 K. Both room and high temperature Raman peak intensity ratio Si/A2g vs. BP thickness relations were established for in-situ thickne…
Black Phosphorus (BP) is unique among layered materials owing to its homonuclear lattice and strong structural anisotropy. While recent investigations on few layers BP have extensively explored the in-plane (a,c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE) and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. PL reveals an unexpected b-polarized emission occurring in the visible at 1.75 eV…
Exfoliated black phosphorus has recently emerged as a new two-dimensional crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have potentially important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and the behavior of phonons near different black phosphorus edges are …
The identification of the crystalline axis of anisotropic black phosphorus (BP) is key for the study of its physical properties and for its optical and electronic applications. Herein, we show that by applying an in-plane uniaxial strain and measuring the changes of Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on Raman shifts are angle-dependent, which can be expressed as a combination of Raman responses under zigzag and armchair strains. Different from previous polarized optical spectroscopic means, the proposed method does not rely on the laser pola…
Monolayer black phosphorus (MBP) is an interesting emerging electronic material with a direct band gap and relatively high carrier mobility. In this work we report a theoretical investigation of nonequilibrium spin injection and spin-polarized quantum transport in MBP from ferromagnetic Ni contacts, in two-dimensional magnetic tunneling structures. We investigate physical properties such as the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device con…
Power dissipation and electrical breakdown in black phosphorus
我们报告的工作温度和加热系数测量在多层黑磷设备作为注入的电功率的函数。通过结合显微拉曼光谱和电输运测量，我们观察到了高达600 K的线性温升，功耗率为0.896 K μm^3/mW。通过进一步提高偏压，确定了器件的击穿阈值功率和击穿温度，并利用扫描电镜和原子力显微镜对导致器件失效的黑磷层断裂进行了分析。研究结果将有利于基于新型二维材料的电子学和光电子学的研究与开发。
Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal X-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitr…
Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. …
The selection rule for angle-resolved polarized Raman (ARPR) intensity of phonons from standard group-theoretical method in isotropic materials would break down in anisotropic layered materials (ALMs) due to birefringence and linear dichroism effects. The two effects result in depth-dependent polarization and intensity of incident laser and scattered signal inside ALMs and thus make a challenge to predict ARPR intensity at any laser incidence direction. Herein, taking in-plane anisotropic black phosphorus as a prototype, we developed a so-called birefringence-linear-dichroism (BLD) model to qu…
Synthesis of thin-film black phosphorus on a flexible substrate
我们报告了一种可扩展的方法来合成一个大面积（高达4毫米）的薄黑磷（BP）薄膜上的柔性基板。我们首先在柔性聚酯基底上沉积一层赤磷（RP）薄膜，然后在室温下在高压多砧室中将其转化为BP。拉曼光谱和透射电子显微镜测量证实了形成的纳米晶体BP薄膜的厚度约为40 nm。光学表征表明，在转换BP的带隙约为0.28 eV，在剥离的薄膜中测得的带隙类似。薄膜BP晶体管表现出约0.5 cm 2/Vs的场效应迁移率，这可能会进一步提高在高温下的转换过程的优化。我们的工作为将来大规模、高质量的薄膜黑磷的示范开辟了道路。
Contrary to empirical observations, lowest-order $k\cdot\hat{p}$ theory predicts that monolayer black phosphorus ("phosphorene") is completely immune to zigzag-polarized optical excitation at the bandgap energy. Using symmetry arguments, we derive a $2\times2$ Hamiltonian under the $k\cdot\hat{p}$ formalism including higher-order corrections, which is used to show that the experimentally-measured band-gap transition with zigzag polarization is dominated by the third order $k\cdot\hat{p}$ perturbation in the interband optical matrix element, whereas the effects of spin-orbit interaction are neg…
Graphite intercalation compounds (GICs) with the geometrical anisotropy and strong electron-phonon coupling are in full swing and have shown their great potential for applications in nanodevices. I selected representative three elements in actinide group with valence electron arrangement: Thorium (Th) ([Rn]6d27s2), Uranium (U) ([Rn]5f36d17s2), Plutonium (Pu) ([Rn]5f67s2). I calculated their phonon spectra and demonstrated the atomic-scale microstructure identification of actinide graphite intercalation compounds by angle-resolved polarized Raman spectroscopy.
The polarized Raman spectroscopy is capable of giving confirmation regarding the crystalline phase as well as the crystallographic orientation of the sample. In this context, apart from crystallographic x-ray and electron diffraction tools, polarized Raman spectroscopy and corresponding spectral imaging can be a promising crystallographic tool for determining both crystalline phase and orientation. Sub-micron sized hexagonal AlGaN crystallites are grown by a simple atmospheric pressure chemical vapor deposition technique using the self catalytic vapor-solid process under N-rich condition. The …
In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g. Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e. structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature dependent struc…
Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model predicting ionic displacement suggested that the shift in ions is essentially along b-axis with very small amplitude (~10-5 Å). Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed polarized Raman spectroscopy study on epitaxial film of CuO, we have validated the theoretical stu…
Topological Weyl semimetals (WSMs) have attracted widespread interests due to the chiral Weyl fermions and surface Fermi arcs that enable unique optical and transport phenomena. In this work, we present angle-resolved Raman spectroscopy of TaP, a prototypical noncentrosymmetric WSM, for five excitation wavelengths ranging from 364 to 785 nm. The Raman active modes, $A_1$, $B_1^1$, and $B_1^2$ modes, exhibit two main unique features beyond the conventional Raman theory. First, the relative intensities of Raman active modes change as a function of the excitation wavelength. Second, angle-resolve…
The mid-infrared (MIR) spectral range is of immense use for civilian and military applications. The large number of vibrational absorption bands in this range can be used for gas sensing, process control and spectroscopy. In addition, there exists transparency windows in the atmosphere such as that between 3.6-3.8 $μ$m, which are ideal for free-space optical communication, range finding and thermal imaging. A number of different semiconductor platforms have been used for MIR light-emission. This includes InAsSb/InAs quantum wells, InSb/AlInSb, GaInAsSbP pentanary alloys, and intersubband trans…
We have systematically investigated the optical properties of the InP1-xBix ternary alloys with 0<x<2.46%, using high resolution polarized Raman scattering measurement. Both InP-like and InBi-like optical vibration modes (LO) were identified in all the samples, suggesting most of the Bi-atoms are incorporated into the lattice sites to substitute P-atoms. And the intensity of the InBi-like Raman modes increase exponentially as Bi-content increasing. Linearly red-shift of the InP-like longitudinal optical vibration modes was observed to be 1.1 cm-1 of percent Bi, while that of the InP-like optic…
Photoinduced topological phase transitions (TPTs) in black phosphorous (BP) under compressive strain were investigated by combining Floquet theory and first-principles calculations. Intriguing photo-dressed electronic states including Floquet-Dirac semimetals and Floquet topological insulators have been identified, which can be feasibly engineered by changing the direction, intensity and frequency of incident laser. Remarkably, tunable TPT from type-I to type-II Floquet-Dirac fermions can be realized by irradiating circularly polarized laser (CPL), with an origin rooted in optical Stark effect…
Polarized Temperature Dependent Raman Study of Bi2Te3-Cr2Ge2Te6 heterostructure and the Ferromagnetic Insulator Cr2Ge2Te6
Cr2 Ge 2 Te 6几十年来一直受到关注，因为它是少数几种铁磁绝缘体之一。最近，这种材料已被重新审视，由于其作为衬底的拓扑绝缘体的Bi 2 Te 3的潜力。这使得研究拓扑绝缘体中的反常量子霍尔效应成为可能，并为新型自旋电子器件提供了一条途径。为了探索这两种材料的兼容性，我们使用偏振变温拉曼显微镜研究Bi 2 Te 3-Cr2 Ge 2 Te 6异质结构以及Cr2 Ge 2 Te 6的声子动力学。我们发现的Cr2 Ge 2 Te 6声子的温度依赖性的结果主要从非谐性，虽然也观察到一个小的磁弹性耦合。我们的研究结果证实了潜在的Cr2 Ge 2 Te 6作为拓扑绝缘体的衬底。
Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudo-angular momenta. In circularly polarized Raman spectroscopy, the peak split of the $Γ_3$ mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudo-angular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe ch…
Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus
黑磷作为一种新型的二维材料，在电子学和光电子学领域有着潜在的应用前景。在这里，我们报告的各向异性面内导热系数的悬浮少层黑磷测量的显微拉曼光谱。对于厚度大于15 nm的黑磷膜，扶手椅型和锯齿型热导率分别为~20和~40 W m $^{-1}$ K $^{-1}$，并且随着膜厚度减小，热导率降低至~10和~20 W m $^{-1}$ K $^{-1}$，表现出显著的各向异性。发现对于厚的黑磷膜，热导率各向异性比为~2，对于最薄的9.5 nm厚的膜，热导率各向异性比下降到~1.5。理论模型表明，所观察到的各向异性主要与各向异性声子色散有关，而本征声子散射率被发现是类似的扶手椅和锯齿形方向。黑磷薄膜中的表面散射强烈抑制长平均自由程声学声子的贡献。
This study addresses high electric field transport in multilayer black phosphorus (BP) field effect transistors (FETs) with self-heating and thermal spreading by dielectric engineering. Interestingly, we found that multilayer BP device on a SiO2 substrate exhibited a maximum current density of 3.3 x 10E10 A/m2 at an electric field of 5.58 MV/m, several times higher than multilayer MoS2. Our breakdown thermometry analysis revealed that self-heating was impeded along BP-dielectric interface, resulting in a thermal plateau inside the channel and eventual Joule breakdown. Using a size-dependent el…
We find unusually large cross-polarized (and anti-symmetric) Raman signature of A$_{\rm g}$ phonon mode in CrI$_3$, in agreement with experiments. The signal is present only when the following three effects are considered in concert: ferromagnetism on Cr atoms, spin-orbit interaction, and resonant effects. Somewhat surprisingly, we find that the relevant spin-orbit interaction potential originates from iodine atoms, despite magnetism being mostly on chromium atoms. We analyze the Raman signature as a function of magnetic order, the direction of the magnetic moment, energy and polarization of l…
LiBC by polarized Raman spectroscopy: Evidence for lower crystal symmetry ?
本文介绍了偏振拉曼散射研究的几个微米大小的微晶LiBC与自然的面孔。对生长样品的实验显示出四种晶格模式，频率分别为1276 cm ^-1、830 cm ^-1、546 cm ^-1和170 cm ^-1。观察到的拉曼线的数量和它们的选择规则与假设的D6h对称性不相容。1276 cm ^-1和170 cm ^-1处的模式对应于预期的拉曼活动模式。与超导化合物MgB 2相比，B-C键伸缩模（在1276 cm ^-1处）具有相当小的阻尼。随后的热处理后，两个"禁戒"模（830 cm ^-1和546 cm ^-1）消失。
Resistivity measurements of a few-layer black phosphorus (bP) crystal in parallel magnetic fields up to 45 T are reported as a function of the angle between the in-plane field and the source-drain (S-D) axis of the device. The crystallographic directions of the bP crystal were determined by Raman spectroscopy, with the zigzag axis found within 5° of the S-D axis, and the armchair axis in the orthogonal planar direction. A transverse magneto-resistance (TMR) as well as a classically-forbidden longitudinal magneto-resistance (LMR) are observed. Both are found to be strongly anisotropic and non-m…
Quantum wells constitute one of the most important classes of devices in the study of 2D systems. In a double layer QW, the additional "which-layer" degree of freedom gives rise to celebrated phenomena such as Coulomb drag, Hall drag and exciton condensation. Here we demonstrate facile formation of wide QWs in few-layer black phosphorus devices that host double layers of charge carriers. In contrast to tradition QWs, each 2D layer is ambipolar, and can be tuned into n-doped, p-doped or intrinsic regimes. Fully spin-polarized quantum Hall states are observed on each layer, with enhanced Lande g…
The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, opening the possibility for manipulating material properties, but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edge…
In this work, we report a polarized Raman study on the vibrational properties of the InAs/InAs1-xSbx SLs as well as selected InAs1-xSbx alloys, all grown on GaSb substrates by either MBE or MOCVD, from both growth surface and cleaved edge.
Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode be…
