Second-Order Raman Scattering in Exfoliated Black Phosphorus
Alexandre Favron, Félix Antoine Goudreault, Vincent Gosselin, Julien Groulx, Michel Côté, Richard Leonelli, Jean-Francis Germain, Anne-Laurence Phaneuf-L'Heureu… 展开作者
Alexandre Favron, Félix Antoine Goudreault, Vincent Gosselin, Julien Groulx, Michel Côté, Richard Leonelli, Jean-Francis Germain, Anne-Laurence Phaneuf-L'Heureux, Sébastien Francoeur, Richard Martel
Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, \emph{e.g.} nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, etc. The sp$^2$-hybridized carbon systems are, however, the exception among most nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named $D_1$, $D_1'$, $D_2$ and $D_2'$, in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the $A^1_g$ and $A^2_g$ modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly-resonant Raman process. \emph{Ab initio} simulations of a monolayer reveal that the $D'$ and $D$ modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these $D$ modes to disorder helps explaining several discrepancies found in the literature.
Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation
发现未包封的剥离的黑磷（BP）薄片在暴露于环境条件时发生化学降解。原子力显微镜，静电力显微镜，透射电子显微镜，X-射线光电子能谱学，和傅里叶变换红外光谱表征的结构和化学的降解过程中，表明O2饱和H2O不可逆地与BP反应，形成氧化磷物种。这一解释得到了以下观察结果的进一步支持：与亲水性SiO2相比，疏水性十八烷基三氯硅烷自组装单层和H-Si（111）上的BP降解发生得更快。对于未封装的BP场效应晶体管，环境退化导致在环境中6小时后阈值电压大幅增加，随后在48小时后FET电流开/关比和迁移率降低约10^3。原子层沉积的AlOx覆盖层有效地抑制了环境退化，允许封装的BP FET在环境中保持约10^3的高开/关比和约100 cm 2/（V*s）的迁移率超过两周。这项工作表明，BP的环境降解，可以有效地管理时，薄片充分钝化。反过来，我们提高BP环境稳定性的战略将加速BP在电子和光电应用中的实施。
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…
2H-TaSe2 is a model transition metal dichalcogenide material that develops charge density waves (CDWs).Here we present variable-temperature Raman spectroscopy study on both incommensurate charge density waves (ICDW) and optical phonon modes of 2H-TaSe2 thin layers exfoliated onto SiO2 substrate. Raman scattering intensities of all modes reach a maximum when the sample thickness is about 11 nm. This phenomenon can be explained by optical interference effect between the sample and the substrate. The E2gICDW amplitude modes experience redshift as temperature increases. We extract ICDW transition …
Probing the anisotropic behaviors of black phosphorus by transmission electron microscope, angular-dependent Raman spectra and electronic transports measurements
We demonstrate the large scale exfoliation of bulk black phosphorus into few-layers black phosphorus. We use wet-jet mill (WJM) as an industrially scalable exfoliation technique. In order to avoid the degradation by oxidation of the exfoliated flakes, the few-layers black phosphorus is protected by functionalisation (with 4-bromobenzene-diazonium tetrafluoroborate) during the exfoliation process. We used the exfoliated and functionalised flakes as fillers in a polymethyl methacrylate matrix to test the mechanical endurance, obtaining an improvement in the Young modulus of 106% compared to the …
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…
Detailed high pressure Resonance Raman ($RR$) Spectroscopy and X-ray diffraction ($XRD$) studies are carried out on 3-4 layered $MoSe_2$ obtained by liquid exfoliation. Analysis of ambient $XRD$ pattern and $RR$ spectra indicate the presence of a triclinic phase along with its parent hexagonal phase. Pressure evolution of prominent Raman modes and their full width at half maximum ($FWHM$) show slope changes at about 13 GPa and 33 GPa, respectively. Slope change in the linear behavior of reduced pressure ($H$) with respect to Eulerian strain ($f_E$) is observed at about 13 GPa. A minimum in the…
We investigate the thermal properties of these Zirconium based materials using confocal Raman spectroscopy. We observed 2 different and distinctive Raman signatures for exfoliated ZrX2 (where X = S or Se). These Raman modes generally depend on the shape of the exfoliated nanosheets, regardless of the incident laser polarization. These 2 shapes are divided into 2D- ZrX2 and quasi 1D- ZrX2. For 2D- ZrX2, Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX2, we show that Raman modes are identical to exfoliated ZrX3 nanosheets, indicating a major lattice trans…
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。
The deformation and fracture behaviour of one-atom-thick mechanically exfoliated graphene has been studied in detail. Monolayer graphene flakes with different lengths, widths and shapes were successfully prepared by mechanical exfoliation and deposited onto poly(methyl methacrylate) (PMMA) beams. The fracture behaviour of the monolayer graphene was followed by deforming the PMMA beams. Through in-situ Raman mapping at different strain levels, the distributions of strain over the graphene flakes were determined from the shift of the graphene Raman 2D band. The failure mechanisms of the exfoliat…
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…
The liquid-phase exfoliation (LPE) of black phosphorus (BP) is a strategic route for the large-scale production of phosphorene and few-layer BP (FL-BP) flakes. The exploitation of this exfoliated material in cutting-edge technologies, e.g., in flexible electronics and energy storage, is however limited by the fact that the LPE of BP is usually carried out at a high boiling point and in toxic solvents. In fact, the solvent residual is detrimental to device performance in real applications; thus, complete solvent removal is critical. Here, we tackle these issues by exfoliating BP in different lo…
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…
Elemental phosphorus nanostructures are notorious for a large number of allotropes, which limits their usefulness as semiconductors. To limit this structural diversity, we synthesize selectively quasi-1D phosphorus nanostructures inside carbon nanotubes (CNTs) that act both as stable templates and nanoreactors. Whereas zigzag phosphorus nanoribbons form preferably in CNTs with an inner diameter exceeding 1.4 nm, a previously unknown square columnar structure of phosphorus is observed to form inside narrower nanotubes. Our findings are supported by electron microscopy and Raman spectroscopy obs…
The discovery of graphene and the related fascinating capabilities have triggered an unprecedented interest in inorganic two-dimensional (2D) materials. Despite the impressive impact in a variety of photonic applications, the absence of energy gap has hampered its broader applicability in many optoelectronic devices. The recent advance of novel 2D materials, such as transition-metal dichalcogenides or atomically thin elemental materials, (e.g. silicene, germanene and phosphorene) promises a revolutionary step-change. Here we devise the first room-temperature Terahertz (THz) frequency detector …
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…
We study surface charge transfer doping of exfoliated black phosphorus (bP) flakes by copper using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. The tunneling spectra reveal a gap in correspondence of Cu islands, which is attributed to Coulomb blockade phenomena. Moreover, using line spectroscopic measurements across small copper islands, we exploit the potential of the local investigation, showing that the n-type doping effect of copper on bP is short-ranged. These experimental results are substantiated by first-principles simulations, which quantify the role…
Environmental instability of few-layer black phosphorus
研究了机械剥离少层黑磷（BP）的环境不稳定性。从连续几天的测量薄片地形，我们观察到超过200%的体积增加，由于从空气中的水分冷凝。我们发现，长期暴露于环境条件下的结果在一个逐层的蚀刻过程中的BP薄片。有趣的是，薄片可以被蚀刻到单层（磷烯）厚度。BP对水的强烈亲和力极大地改变了在环境条件下测量的制造场效应晶体管（FET）的性能。在暴露于空气中时，我们区分两个时间尺度的掺杂BP FET transfer characteristics：一个短的时间尺度（分钟），其中发生的阈值电压的偏移由于物理吸附的氧和氮，和一个长的时间尺度（小时），其中发生的p型掺杂从水的吸收。在空气中的BP FET的连续测量揭示了由于逐层蚀刻过程，几天后沟道材料的最终退化和击穿。
Orhorhombic $α$-MoO$_3$ is a layered oxide with various applications and with excellent potential to be exfoliated as a 2D ultrathin film or monolayer. In this paper, we present a first-principles computational study of its vibrational properties. Our focus is on the zone center modes which can be measured by a combination of infared and Raman spectroscopy. The polarization dependent spectra are simulated. Calculations are also performed for a monolayer form in which "double layers" of Mo$_2$O$_6$ which are weakly van der Waals bonded in the $α$-structure are isolated. Shift in phonon frequenc…
Black phosphorus (bP) has been recently investigated for next generation nanoelectronic multifunctional devices. However, the intrinsic instability of exfoliated bP (the bP nanoflakes) towards both moisture and air has so far overshadowed its practical implementation. In order to contribute to fill this gap, we report here the preparation of new hybrid polymer-based materials where bP nanoflakes exhibit a significantly improved stability. The new materials have been prepared by different synthetic paths including: i) the mixing of conventionally liquid-phase exfoliated bP (in DMSO) with PMMA s…
We investigated interlayer phonon modes of mechanically exfoliated few-layer 2H-SnS2 samples by using room temperature low-frequency micro-Raman spectroscopy. Raman measurements were performed using laser wavelength of 441.6, 514.4, 532 and 632.8 nm with power below 100 uW and inside a vacuum chamber to avoid photo-oxidation. The intralayer Eg and A1g modes are observed at ~206 cm-1 and ~314 cm-1, respectively, but the Eg mode is much weaker for all excitation energies. The A1g mode exhibits strong resonant enhancement for the 532 nm (2.33 eV) laser. In the low-frequency region, interlayer vib…
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。
We report high pressure X-ray diffraction and a detailed systematic Raman measurements on $ReS_2$ sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted $1T$ to distorted $1T$$^{\prime}$ in triclinic structure. The same is corroborated by appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to $Re$ atom vibrations are observed in the distorted $1T$$^{\prime}$ phase in the pressure range 15-25 GPa. In the same pressure range the anom…
