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Uniaxial Strain Engineering of Anisotropic Phonon in Few-Layer Violet Phosphorus with High Flexibility for Polarized Sensitive Photodetector

Conghui Shang,# Weiwei Wang,# Jianbin Zhang, Yi Zhao, Jiaxuan Li, Guohui Jia, Nan Zhou, Ge Liu, Mantang Hui, Hongyang Huang, Guangzhi Dong, Hua Xu, Jinying Zhang, Xiaobo Li*, Rusen Yang*
尚聪辉, # 王薇薇, # 张建斌, 赵毅, 李佳轩, 贾国辉, 周楠, 刘革, 许满堂, 黄红阳, 董光志, 徐华, 张金英, 李晓波*, 杨汝森*

C. H. Shang, Prof. W. W. Wang, J. B. Zhang, Y. Zhao, J. X. Li, G. H. Jia, Prof. N. Zhou, G. Liu, M. T. Hui, Prof. G. Z. Dong, Prof. X. B. Li, Prof. R. S. Yang
尚昌华, 王伟华教授, 张俊斌, 赵彦, 李俊晖, 贾国辉, N. 周教授, 刘国辉, 许明潭, 董志强教授, 李旭斌教授, 杨瑞善教授

Shaanxi Joint Key Laboratory of Graphene, Shaanxi Key Laboratory of High-Orbits-Electron Materials and Protection Technology for Aerospace

School of Advanced Materials and Nanotechnology

Xidian University, Xi'an, 710126, P. R. China
西安电子科技大学, 习安, 710126, 中国

E-mail: xbli@xidian.edu.cn rsyang@xidian.edu.cn
电子邮件: xbli@xidian.edu.cn rsyang@xidian.edu.cn

Prof. H. Xu

Key Laboratory of Applied Surface and Colloid Chemistry

Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices

Shaanxi Engineering Lab for Advanced Energy Technology

School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710119, P. R. China
陕西师范大学材料科学与工程学院, 习安710119, 中国

H. Y. Huang, Prof. J. Y. Zhang
H. Y. Huang, J. Y. Zhang教授

State Key Laboratory of Electrical Insulation and Power Equipment, Center of Nanomaterials for Renewable Energy, School of Electrical Engineering, Xian Jiaotong University, Xian, 710049, China
西安交通大学电气工程学院, 可再生能源纳米材料中心, 电绝缘与电力设备国家重点实验室, 西安710049

Prof. X. B. Li, Prof. N. Zhou
李旭斌教授, 周恩教授

Guangzhou Institute of Technology, Xidian University, Guangzhou, 710068, P. R. China
西安电子科技大学广州理工学院, 广州, 710068, 中国


The manifestation of quantum mechanical phenomena in quantum materials at the macroscopic level is intricately linked to pronounced electron-electron interactions within their lattices, a relationship that becomes especially evident in materials characterized by reduced dimensionality. Violet phosphorous (VP), a nascent two-dimensional material distinguished by its unique vertically aligned tubular structures across layers, has garnered considerable attention owing to its layer-specific electronic bandgap variability, exceptional carrier mobility, and robust air stability. Herein, we have undertaken a comprehensive Raman spectroscopic analysis of few-layer VP subjected to in-plane uniaxial tensile strains along the a, b, and <110> directions, reaching up to 0.75%. The observed plethora of Raman modes reveals significantly differentiated shifts contingent upon the direction of applied strain. Notably, when strain is applied along the <110> direction, the rate of Raman shift for the mode at 444.5 cm-1 achieves an extraordinary figure of approximately -8.51 cm-1/% strain. Moreover, the calculated Grüneisen parameter, approximately 2.4, surpasses that of other well-documented two-dimensional materials, showcasing a greater magnitude. Density functional theory (DFT) calculations highlight that the applied strain induces modifications not only in bond lengths but also in bond angles within the ultra-thin VP, culminating in a pronounced anisotropic Raman response. Moreover, a polarized sensitive photodetector leveraging flexible VP has been developed, demonstrating a swift response time of 18.2 ms alongside an anisotropic ratio of 2.26. This investigation not only confirms the superior stretchability and impact resistance properties of cross-structured VP but also lays the groundwork for exploring the strain-induced anisotropic electrical conductance properties inherent to VP.
量子力学现象在宏观层面上在量子材料中的表现与其晶格内明显的电子-电子相互作用有着错综复杂的联系,这种关系在以降维为特征的材料中变得尤为明显。紫磷 (VP) 是一种新兴的二维材料,以其独特的跨层垂直排列的管状结构而著称,由于其特定于层的电子带隙可变性、出色的载流子迁移率和强大的空气稳定性,引起了相当大的关注。在此,我们对沿a、b和<110>方向的面内单轴拉伸应变的几层VP进行了全面的拉曼光谱分析,最高可达0.75%。观察到的大量拉曼模式揭示了根据施加应变方向的显着差异化位移。值得注意的是,当沿<110>方向施加应变时,444.5 cm -1 处模式的拉曼位移速率达到了大约-8.51 cm -1 /%应变的非凡数字。此外,计算出的Grüneisen参数约为2.4,超过了其他有据可查的二维材料,显示出更大的量级。密度泛函理论 (DFT) 计算强调,施加的应变不仅会引起键长的修饰,还会引起超薄 VP 内键角的修饰,最终导致明显的各向异性拉曼响应。此外,还开发了一种利用柔性VP的偏振灵敏光电探测器,其快速响应时间为18.2 ms,各向异性比为2.26。 本研究不仅证实了跨结构VP优异的拉伸性和抗冲击性能,还为探索VP固有的应变诱导各向异性电导性能奠定了基础。

Key Words:

Violet Phosphorus, Raman shift, Strain, Polarized, Flexible


Strain engineering has emerged as a potent strategy for fine-modulating the electronic, transport, piezo and optical characteristics of semiconductor materials over the past decades, enabling a nuanced control over their fundamental properties and paving the way for advancements in device functionality. The inherent mechanical stretchability of two-dimensional (2D) crystalline materials facilitates the application of strain to continuously and reversibly tune their lattice constants and electronic properties, as these reduced-dimensional structures possess an exceptional capacity to endure substantially elevated levels of strain compared to their bulk counterparts. This resilience augments the prospects for the modulation of properties through strain engineering, thereby extending the frontiers of material innovation and functionalization. For instance, theoretical calculations have forecasted that the imposition of a modest uniaxial strain of approximately 2% to monolayer MoS2 will precipitate a transformation from a direct to an indirect bandgap. Moreover, the exertion of a biaxial strain approaching 10% is anticipated to catalyze a more dramatic metamorphosis, which culminate a semiconductor-to-metal transition within the material. In the case of low-symmetry 2D material, they exhibit more pronounced property-dependent strain sensitivity wherein the intrinsic properties of these materials are contingent upon the specific crystalline lattice orientations. The applied strain will substantially alter the anisotropy ratio of these low-symmetry materials and then affect its detection performance of polarized-sensitive light-emitting diodes and quantum photonics.
在过去的几十年中,应变工程已成为一种有效的策略,用于微调半导体材料的电子、传输、压电和光学特性,从而能够对其基本特性进行细致入微的控制,并为器件功能的进步铺平道路。二维(2D)晶体材料固有的机械拉伸性有助于应用应变来连续和可逆地调整其晶格常数和电子特性,因为与块状结构相比,这些降维结构具有承受大幅升高的应变水平的特殊能力。这种弹性增强了通过应变工程调节性能的前景,从而扩展了材料创新和功能化的前沿。例如,理论计算预测,对单层MoS 2 施加约2%的适度单轴应变将促进从直接带隙到间接带隙的转变。此外,预计接近10%的双轴应变将催化更剧烈的,最终导致材料内的半导体到金属的转变。在低对称性二维材料的情况下,它们表现出更明显的属性相关应变敏感性,其中这些材料的内在特性取决于特定的晶格取向。施加的应变将显著改变这些低对称性材料的各向异性比,进而影响其对偏振敏感发光二极管和量子光子学的检测性能。

The polymorphic complexity of phosphorus, characterized by an ever-unfolding phase diagram, has captivated the scientific community's interest. Among the allotropic manifestations of this element, the layered metastable variant known as violet phosphorus (VP), alternatively referred to as Hittorf's phosphorus, has emerged as a focal point for research endeavors for a much better thermal and chemical stability performance. The distinctive crystallographic arrangement (rhombohedral structure) connecting with two vertically arranged tubular structures for each layer, endows it with a low-symmetry configuration and concomitant electronic or mechanical attributes that set it apart from other phosphorus forms. Previous work loading strain by atomic force microscopy (AFM) probe has elucidated the remarkable capacity of VP to withstand deformation, with its monolayer exhibiting a 2D Young's modulus of 1512±76 N m-1. This value represents a 4.4-fold enhancement over the corresponding modulus of graphene, thereby highlighting VP's exceptional mechanical robustness among two-dimensional materials. Furthermore, Zhong et al. investigated the in-plane uniaxial tension of VP by first-principles calculations and found an extremely large Poissons ratio in the in-plane direction while a negative Poissons ratio in the out-of-plane direction. Although previous theoretical and experimental work demonstrated the distinctive mechanical properties of VP, the influence of the strain engineering, with particular emphasis on the phonons, electrons related properties and potential applications under in-plane uniaxial strain conditions, remains uninvestigated.
磷的多态复杂性,其特征是不断展开的相图,引起了科学界的兴趣。在这种元素的同素异形体表现中,被称为紫磷 (VP) 的层状亚稳态变体,也称为 Hittorf 磷,已成为研究工作的焦点,以获得更好的热稳定性和化学稳定性性能。独特的晶体排列(菱面体结构)与每层的两个垂直排列的管状结构相连,赋予其低对称性配置和伴随的电子或机械属性,使其与其他磷形式区分开来。先前通过原子力显微镜 (AFM) 探针加载应变的工作阐明了 VP 承受变形的显着能力,其单层表现出 1512±76 N m -1 的 2D 杨氏模量。该值比石墨烯的相应模量提高了 4.4 倍,从而突出了 VP 在二维材料中卓越的机械鲁棒性。此外,Zhong等人通过第一性原理计算研究了VP的面内单轴张力,发现面内方向的泊松比非常大,而面外方向的泊松比为负。尽管先前的理论和实验工作证明了VP独特的力学性能,但应变工程的影响,特别是强调声子、电子相关性质和在平面内单轴应变条件下的潜在应用,仍未得到研究。

The Raman spectroscopy, intrinsically linked to electron-phonon coupling, has ascended to the forefront as a quintessential characterization tool for the examination of 2D materials, owing to its non-destructive nature and exceptional sensitivity to layer thickness. This technique has been extensively leveraged to unravel a myriad of material attributes, delving into the complex interplay of strain, doping, structural imperfections, and other critical elements that govern the intrinsic dynamics and functional capabilities of these ultrathin architectures. In this work, we undertake a comprehensive exploration of the phonon modes exhibited by a few-layer VP through an integrated experimental-theoretical approach, focusing on the modulation of their Raman response under the application of uniaxial mechanical strain in three different directions of a-axis, b-axis and <110> direction, respectively. The modes designated as Sq[P8], S1[P8], S2[P8], Tg, and Ptub exhibit a distinct redshift as a consequence of increasing tensile strain. Notably, the modes of Sq[P8], S2[P8], Tg and Ptub demonstrate a marked sensitivity to strain along the tube axis (<110> direction), characterized by a substantial linear strain coefficient, surpassing those observed in other two-dimensional materials due to its cross-structure. Density Functional Theory (DFT) elucidates that the applied strain precipitates alterations not merely in bond lengths but also significantly impacts the bond angles within the ultrathin VP, leading to a pronounced anisotropic Raman response. Moreover, the fabricated flexible VP-based photodetector demonstrates exceptional polarization-dependent optoelectrical characteristics, with an anisotropy ratio of 2.26. Notably, it exhibits a responsibility of 0.865 mA/W along the b axis direction, coupled with a swift response time of 18.2 ms. These outstanding properties underscore the VP device's considerable potential for integration within the burgeoning landscape of flexible electronic devices.
拉曼光谱与电子-声子耦合有着内在的联系,由于其非破坏性和对层厚的异常敏感性,它已成为研究二维材料的典型表征工具。该技术已被广泛用于解开无数的材料属性,深入研究应变、掺杂、结构缺陷和其他控制这些超薄架构的内在动力学和功能能力的关键因素的复杂相互作用。本文通过一体化的实验理论方法,对单轴机械应变作用下声子响应的调制进行了全面探索,重点研究了单轴力学应变作用下声子响应在a轴、b轴和<110>方向三个不同方向上的声子响应调制。被指定为Sq [ P 8] 、S、S 1 [P8] 2 [P8] 、T g 和P的模态由于拉伸应变的增加而 tub 表现出明显的红移。值得注意的是,Sq [ P 8] 、 S 2 [P8] 、 T g 和 P tub 模态对沿管轴(<110>方向的应变具有显著的敏感性,其特点是具有显著的线性应变系数,由于其交叉结构,超过了在其他二维材料中观察到的应变系数。密度泛函理论 (DFT) 阐明,施加的应变不仅会促进键长的改变,还会显着影响超薄 VP 内的键角,从而导致明显的各向异性拉曼响应。 此外,基于VP的柔性光电探测器具有出色的偏振相关光电特性,各向异性比为2.26。值得注意的是,它在 b 轴方向上表现出 0.865 mA/W 的载荷,以及 18.2 ms 的快速响应时间。这些出色的性能凸显了VP器件在蓬勃发展的柔性电子设备领域中的巨大集成潜力。

Results and Discussion

Figure 1. Crystal structure and characterization of 2D VP. (a,b) Schematic crystal structure of VP at top and side view, respectively. (c,d) Low-magnification TEM image of 2D VP and EDS mapping of P element. (e) HRTEM and (f) a partial enlargement HRTEM image of 2D VP. (g) SAED pattern corresponding to (c) with zone axis of [1,1,0]. (h) XRD and (i) Raman spectrum of 2D VP at room temperature.
图 1.2D VP的晶体结构和表征。(a,b)VP的晶体结构示意图分别为顶视图和侧视图。(c,d)P元素二维VP和EDS映射的低倍率TEM图像。(e) HRTEM 和 (f) 2D VP 的部分放大 HRTEM 图像。(g) 对应于 (c) 的 SAED 模式,区域轴为 [1,1,0]。(h) 室温下二维VP的XRD和(i)拉曼光谱。

The violet phosphorus (VP) crystallizes within a monoclinic lattice structure, characterized by the space group C2h, is composed of two intersecting nanorods, with the repeating unit delineated as P2-[P8]-P2-[P9], as illustrated in the top-view representation of the VP crystal depicted in Figure 1a. The layers of this structure are interconnected by phosphorus atoms through covalent bonding, as elucidated in Figure 1b, in which the purple atoms denote phosphorus, and the yellow semitransparent tubes accentuate the cross-structural configuration of VP. Additionally, the interlayer interactions are mediated by van der Waals forces, with a-b surface orientations. Figure 1c presents a representative low-magnification transmission electron microscopy (TEM) image of mechanically exfoliated VP with a regular rectangular morphology. Elemental analysis of the VP nanosheet was conducted using an Energy Dispersive Spectrometer (EDS), the results of which are depicted in the EDS spectra of Figure 1d, confirming the sample's composition to be predominantly phosphorus with a high degree of purity. The crystalline architecture of VP was further examined by high-resolution transmission electron microscopy (HRTEM), as illustrated in Figure 1e, which reveals that the <110> direction constitutes the preferential cleavage edge of VP, aligning with the orientation of the tube. The (110) crystal plane is identified as corresponding to the selected area electron diffraction pattern in Figure 1g with a 0.64 nm spacing (Figure 1f). Moreover, the X-ray diffraction (XRD) analysis of high crystallinity VP demonstrated in Figure 1h exhibits three sharp peaks corresponding to (004), (006) and (008) plane. In addition, the increase in photoluminescence (PL) peak and intensity in Figure S1 with the thickness of VP suggests an enlargement of the band gap and a higher probability of exciton generation and recombination.
紫磷 (VP) 在单斜晶格结构中结晶,以空间群 C 2h 为特征,由两个相交的纳米棒组成,重复单元描述为 P2-[P8]-P2-[P9],如图 1a 中描绘的 VP 晶体的俯视图所示。如图1b所示,该结构的层通过共价键通过磷原子相互连接,其中紫色原子表示磷,黄色半透明管突出了VP的交叉结构构型。此外,层间相互作用由范德华力介导,具有a-b表面取向。图1c显示了具有规则矩形形态的机械剥离VP的代表性低倍率透射电子显微镜(TEM)图像。使用能量色散光谱仪(EDS)对VP纳米片进行元素分析,其结果如图1d的EDS光谱所示,证实样品的成分主要是磷,纯度很高。如图1e所示,通过高分辨率透射电子显微镜(HRTEM)进一步检查了VP的晶体结构,该结果显示<110>方向构成了VP的优先切割边缘,与管的取向一致。(110)晶平面被标识为对应于图1g中选定的区域电子衍射图,间距为0.64nm(图1f)。此外,图1h所示的高结晶度VP的X射线衍射(XRD)分析显示出对应于(004)、(006)和(008)平面的三个尖峰。 此外,图S1中光致发光(PL)峰和强度随着VP厚度的增加而增加,表明带隙扩大,激子产生和复合的可能性更高。

Figure 1i shows a typical Raman spectrum of the ultrathin VP on the SiO2/Si substrate. Plethora of Raman vibrational modes including those of Ag and Bg symmetries can be observed due to the reduced structural symmetry relative to orthorhombic black phosphorous, which are in good agreement with the literature. Nevertheless, the significant overlap of the Ag and Bg Raman peaks in VP necessitates the selection of typical and predominant vibrational modes to elucidate the subsequent strained phonon behaviors. The atomic displacements associated with these characterized Raman modes are detailed in Table S1.
图1i显示了SiO 2 /Si衬底上超薄VP的典型拉曼光谱。由于相对于正交黑磷的结构对称性降低,可以观察到大量的拉曼振动模式,包括A g 和B g 对称性,这与文献非常吻合。然而,VP中A g 和B g 拉曼峰的显著重叠需要选择典型和主要的振动模式来阐明随后的应变声子行为。表S1中详细说明了与这些表征拉曼模式相关的原子位移。

Figure 2. Polarized Raman and PL spectra at different polarization configuration. (a) Schematic diagrams of the angle-resolved polarized Raman spectroscopy setup containing parallel (∥) and perpendicular (⊥) polarization configurations. (b, c) Polarization Raman intensity contour maps in perpendicular (cross) and parallel configurations, respectively. (e,f) PL intensity variation as a function of polarized angle in perpendicular and parallel configurations, respectively. (f) The Raman spectra of the same VP nanoplate obtained by rotating 0°, 45° and 135° in the direction of <110>, respectively.
图2.不同偏振配置下的偏振拉曼光谱和PL光谱。(a) 包含平行(∥)和垂直(⊥)偏振配置的角度分辨偏振拉曼光谱装置示意图。(二、三)偏振拉曼强度等值线图分别采用垂直(交叉)和平行配置。(五、女)PL强度变化分别作为垂直和平行配置中偏振角的函数。(f) 通过分别沿<110>方向旋转0°、45°和135°获得的相同VP纳米板的拉曼光谱。

Distinct from other low-symmetry materials, the <110> direction in VP, which corresponds to the tubular structure orientation, does not align with either its a-axis or b-axis. Consequently, to investigate the strain behavior of phonons along various directions, it is imperative to first ascertain the orientation of the a and b-axis of the sample, thereby establishing a fundamental prerequisite for this study. Figure 2a shows a schematic measurement of the parallel and perpendicular configurations of the angle-resolved polarized Raman spectroscopy (ARPRS). Initially, a 0° rotating polarizer aligns the incident and collected linearly polarized light for parallel ARPRS measurement. Next, a 45°-rotated half-wave plate in the incident path facilitates 90° light rotation for perpendicular-polarized Raman spectra acquisition. Then, sample rotation enables ARPRS measurements in both parallel and perpendicular configurations. The comprehensive polarization Raman tensors pertaining to various modes at the two configurations are delineated in Table S2 and S3 of the supplementary material. Figure 2b and Table S2 illustrates an ARPRS contour map of Raman intensity in the perpendicular configuration, exhibiting a four-lobed shape characteristic across all modes, whereas Figure 2c and Table S3 reveal a predominant two-lobed shape in the parallel configuration. These observations substantiate the in-plane phonon anisotropy characteristics of VP. Naturally, the anisotropy of exciton can also be discernible in angle-resolved PL spectra, manifesting distinctively in perpendicular (Figure 2d) and parallel (Figure 2e) configurations. Specifically, the perpendicular configuration exhibits a four-cycle profile, while the parallel configuration presents a two-cycle shape characteristic. Based on the anisotropic of VP and polarized Raman result, the orientation of VP can be readily ascertained by rotating the sample at 45° or 135° along the <110> direction, as shown in Figure 2f. When the polarization of the incident laser aligns parallel to the a-axis, the Raman intensities at 109, 275, 471, and 491 cm-1 are observed to attain their maxima. Conversely, when the polarization of the incident laser is parallel to the b-axis, the Raman intensities at 119 and 225 cm-1 reach their peak, and the 495 cm-1 peak vanishes.
与其他低对称材料不同,VP 中的 <110> 方向(对应于管状结构方向)与其 a 轴或 b 轴不对齐。因此,为了研究声子沿各个方向的应变行为,必须首先确定样品a轴和b轴的取向,从而为本研究奠定基础前提。图2a显示了角度分辨偏振拉曼光谱(ARPRS)的平行和垂直构型的示意图。最初,0°旋转偏振器对准入射光和收集的线性偏振光,用于平行ARPRS测量。接下来,入射路径中旋转 45° 的半波板有助于 90° 光旋转,用于垂直偏振拉曼光谱采集。然后,样品旋转可在平行和垂直配置下进行 ARPRS 测量。在补充材料的表S2和S3中描述了两种配置下各种模式的综合极化拉曼张量。图2b和表S2显示了垂直构型中拉曼强度的ARPRS等值线图,在所有模式中都表现出四瓣形状特征,而图2c和表S3显示了平行构型中占主导地位的两瓣形状。这些观察结果证实了VP的面内声子各向异性特性。当然,激子的各向异性也可以在角度分辨PL光谱中辨别出来,在垂直(图2d)和平行(图2e)配置中明显表现出来。具体而言,垂直配置呈现四循环轮廓,而平行配置呈现两循环形状特征。 根据VP的各向异性和偏振拉曼结果,通过沿<110>方向旋转样品45°或135°,可以很容易地确定VP的取向,如图2f所示。当入射激光的偏振平行于a轴对齐时,观察到109、275、471和491 cm -1 处的拉曼强度达到最大值。相反,当入射激光的偏振平行于b轴时,119和225 cm -1 处的拉曼强度达到峰值,而495 cm -1 的峰值消失。

Figure 3. Raman spectral alterations in few-layer VP samples throughout the coating process and under a-axis strain. (a-b) Encapsulation process of VP samples for strain Raman testing. (c) Depiction of uniaxial strain implementation along various orientations of the VP. (d) Raman spectra of few-layer VP at different strain along a-axis with increments of 0.15 %. (e-f) Raman shift of the main modes when the uniaxial tensile strain was applied to the a-axis.
图3.在整个镀膜过程中和在a轴应变下,少层VP样品中的拉曼光谱变化。(A-B)用于应变拉曼测试的VP样品的封装过程。(c) 沿VP不同方向的单轴应变实现的描述。(d) 沿a轴不同应变的几层VP的拉曼光谱,增量为0.15 %。(E-F)当单轴拉伸应变施加到a轴时,主要模态的拉曼偏移。

To investigate the phonon strain characteristics of VP, we employed the encapsulation technique as documented in the extant literature as a reference, subsequently integrating the material onto the surface of a polyethylene terephthalate (PET) flexible substrate to substantially enhance the efficiency of stress transfer, as illustrated in Figure 3a and b. Briefly, few-layer VP were mechanically exfoliated onto SiO2/Si substrates, then transferred to a 188 μm thick PET substrate using polyvinyl alcohol (PVA) and adhesive glue, resulting in a PVA/glue/PET layer structure in which the VP sample was embedded in PVA. Then, the strain applied to the sample can be calculated by the following equation:
为了研究VP的声子应变特性,我们采用了现有文献中记录的封装技术作为参考,随后将材料集成到聚对苯二甲酸乙二醇酯(PET)柔性基板的表面上,以显着提高应力传递的效率,如图3a和b所示。简而言之,将几层VP机械剥离到SiO 2 /Si基板上,然后使用聚乙烯醇(PVA)和粘合剂胶转移到188 μm厚的PET基板上,从而形成PVA/胶水/PET层结构,其中VP样品嵌入PVA中。然后,施加到样品上的应变可以通过以下公式计算:

ε=(D+d/2)−DⅆθDⅆθ=d2D (1)
ε=(D+d/2)−DⅆθDⅆθ=d2D (一)

Where d, D, and θ are substrate thickness, curvature radius and bending angle, respectively. The efficacy of the applied strain on the specimen was rigorously evaluated, and the resultant estimation approximated a 95% efficiency. Then, the Raman spectra of few-layer VP were systematically measured under uniaxial tensile strain, reaching a maximum of 0.75 %, with increments of 0.15 % applied along both the a- and b-axIs as well as the <110> direction, as depicted in Figure 3c and Figure S2. It is pertinent to highlight that the Raman signals emanating from few-layer VP deposited on a flexible substrate exhibited diminished intensity compared to those on a SiO2/Si substrate, attributable to the reduced reflectivity of the polyimide substrate. In light of the substantial overlap of the Ag and Bg Raman peaks in VP, the typical and main vibrational modes were employed to characterize the strained phonon behaviors. Initially, we examine the scenario of tensile strain applied along the a-axis in a nonpolarized configuration, as shown in Figure 3d. With the progressive escalation of strain, the most phonon modes of T1, Sq[P9], Sq[P8], S2[P9], S1[P8], S2[P8], Tg and PTub exhibit discernible softening with different change rates, thereby suggesting a reduction in the energy of the corresponding phonon modes, which consequently diminishes the associated restoring force within these vibrational modes. This observation is more distinctly elucidated by the alteration in the peak positions fitted by the Lorentzian function, as depicted in Figure 3e-g and Figure S3. Additionally, the softening of these phonons is accompanied by an increase in the half-peak width, which can be explicable by the modulation of polarizability and electron-phonon coupling under strain conditions. It is worth noting that the two torsional vibrational (T1 and T2) modes of the tubular structure located at 77.7 and 107.1 cm-1 exhibit different strain response behavior. The intensity of the T1 mode exhibits a pronounced decrease at high strain condition, and the half-peak width is substantially broadened due to its coalescence with the peak at 75.5 cm-1. In contrast, the peak position, intensity, and half-peak width of the T2 mode undergo negligible alterations. This disparity can be ascribed to the principal torsion of T2 being situated within the [P9] cages, which diminishes the sensitivity to strain when juxtaposed with the torsion of T1, located at the [P2]-P9-[P2] juncture.
W 这里 d、D 和 θ 分别是基板厚度、曲率半径和弯曲角。对施加应变对试样的功效进行了严格评估,结果估计效率约为 95%。然后,在单轴拉伸应变下系统地测量了少层VP的拉曼光谱,最大值达到0.75 %,沿a轴和b轴以及<110>方向施加0.15 %的增量,如图3c和图S2所示。需要强调的是,与SiO 2 /Si衬底上的拉曼信号相比,沉积在柔性衬底上的几层VP发出的拉曼信号表现出减弱的强度,这归因于聚酰亚胺衬底的反射率降低。鉴于VP中A g 和B g 拉曼峰的大量重叠,采用典型和主要振动模式来表征应变声子行为。首先,我们研究了在非极化配置中沿 a 轴施加拉伸应变的场景,如图 3d 所示。随着应变的逐渐增加,T 1 、Sq [P9] 、Sq [P8] 、S 2 [P9] 1 [P8] 2 [P8]g T和P Tub 等声子模态表现出明显的软化,变化率不同,表明相应声子模态的能量降低,从而降低了这些振动模态内相关的恢复力。如图3e-g和图S3所示,洛伦兹函数拟合的峰位置变化更清楚地阐明了这一观察结果。 此外,这些声子的软化伴随着半峰宽度的增加,这可以通过应变条件下极化率和电子-声子耦合的调制来解释。值得注意的是,位于77.7 cm和107.1 cm -1 的管状结构的两种扭转振动(T 1 和T 2 )模式表现出不同的应变响应行为。在高应变条件下,T 1 型的强度明显降低,并且由于其与75.5 cm -1 处的峰合并,半峰宽度显著变宽。相比之下,T 2 模式的峰位置、强度和半峰宽度的变化可以忽略不计。这种差异可以归因于T 2 的主扭转位于[P9]笼内,当与位于[P2]-P9-[P2]交界处的T 1 的扭转并列时,这降低了对应变的敏感性。

Furthermore, under the a-axis strain condition, the asymmetric radial breathing (RB) mode of the tubular structure located at a frequency of 276.2 cm-1, experiences a slight blue-shift with a change rate of 1.10. This shift is indicative of a reduction in tubular diameter and an increase in stiffness consequent to tension along the a-axis, attributable to the tubular configuration akin to that observed in carbon nanotubes. Notably, the Raman intensity of both the tangential stretching mode (Tg) along the tubular axis situated at 471 cm-1 and the perimetrical vibration mode (Ptub) at 475.5 cm-1 exhibits a discernible enhancement, attributable to the escalation in anharmonicity.
此外,在a轴应变条件下,频率为276.2 cm -1 的管状结构的非对称径向呼吸(RB)模式发生轻微的蓝移,变化率为1.10。这种变化表明管状直径减小,刚度增加,这是由于沿a轴的张力引起的,这归因于类似于在碳纳米管中观察到的管状结构。值得注意的是,沿管轴的切向拉伸模式(T g-1 和位于475.5 cm -1 的周边振动模式(P tub )的拉曼强度均表现出明显的增强,这归因于不和谐度的升级。

Figure 4. The experimental Raman shifts as a function of the uniaxial tensile strain applied to few-layer VP with their corresponding atom vibration. (a-f) Raman shift of the main modes when the uniaxial tensile strain was applied to the three direction,respectively. (g) Atoms vibrations of the typical Raman modes.
图4.实验拉曼的位移是施加在几层VP上的单轴拉伸应变及其相应的原子振动的函数。(A-F)分别在三个方向施加单轴拉伸应变时主要模态的拉曼位移。(g) 典型拉曼模式的原子振动。

To elucidate the disparities in the influence of strain along the distinctive orientation of VP on its phonon characteristics, we undertake an analysis and comparison of the strain-dependent behavior of its Raman modes across the three principal axes of a, b and <110> direction, shown in Figure 4 and Figure S4. In general, the broadening of the T1, Sq[P9], Sq[P8], S2[P9], S1[P8], S2[P8], Tg and PTub modes is observed to increase concomitantly with the escalation of strain, irrespective of the orientations of the applied strain. Excluding the negligible alterations observed at T2 mode, the remaining modes demonstrated linear relationships between Raman shifts and strain, as typically articulated by the following equation:
为了阐明应变沿VP独特取向对其声子特性的影响的差异,我们分析并比较了其拉曼模式在a、b和<110>方向三个主轴上的应变依赖行为,如图4和图S4所示。一般来说,观察到T 1 、Sq [P9] 、Sq [P8] 、S、 1 S [P8] 、S 2 [P9] 、T 2 [P8] g 和P Tub 模式的拓宽随着应变的增加而增加,而不管施加的应变的方向如何。除了在T 2 模式下观察到的微不足道的变化外,其余模式显示出拉曼位移和应变之间的线性关系,通常由以下公式表示:

Δw=k×ε (2)
Δw=k×ε (2)

in which ∆ω, k and ɛ are the Raman frequency shift, corresponding linear coefficients and strain. The linear coefficients k along three directions are shown in Table 1 as well, and the anisotropic strain response behavior exhibited in these distinct directions can be elucidated by the specific orientations of atomic vibrations inherent to each direction. As in the instance of the a-axis, the T2 mode also exhibits weak sensitivity to other axial strains shown in Figure 3a, attributable to its vibrational confinement within the P9 cage (Figure 3g). Furthermore, the Sq[P8], S1[P8], S2[P8], Tg, and Ptub modes undergo a redshift in response to the escalation of tensile strain, albeit with disparate linear coefficients. Specifically, the Sq[P8], S2[P8], Tg and Ptub modes (Figure 3b-d) modes exhibit pronounced sensitivity to strain in the direction of the tube axis (<110> direction). This is because that the squeezing vibration of [P8] cage corresponding to Sq[p8] mode, the P5-P5 atom stretching direction for the [P8] cages of S2[P8] mode, tangential stretching mode (Tg) of [P9] cage and the perimetrical vibration ascribing to Ptub mode dominant the P-P bond vibrations along the direction parallel to the <110> axis. Of particular interest is the S1[P8] mode (Figure 3e), which uniquely manifests a conspicuous strain sensitivity with respect to the b-axis, and it can be attributed to the fact that the atomic displacement of in the S1[P8] mode is mainly along the b-axis orientation. With regard to the RB mode (Figure 3f), although the imposition of tensile strain across diverse axes engenders a blue shift within the Raman spectra due to the reduced diameter at tensile strain, the frequency of this spectral shift manifests an augmented sensitivity to strain applied along the b-axis due to the asymmetry radial breathing vibration of [P8] and [P9] cages with stiffing [P2] engender a pronounced asymmetry along the b axis. In addition, in the context of the b-axis tensile configuration, only a singular peak emerges at 450 and 470 cm-1, respectively, attributable to the superposition of the Ag and Bg modes. Analogous trends of strain anisotropy above are discernible in the VP samples of varying thicknesses (9.68 nm, 11.2 nm, and 42.9 nm), as depicted in Figure S5 and 6.
其中∆ω、k和ɛ为拉曼频移,对应的线性系数和应变。表1也显示了沿三个方向的线性系数k,并且可以通过每个方向固有的原子振动的特定方向来阐明在这些不同方向上表现出的各向异性应变响应行为。与a轴的情况一样,T 2 模式对图3a所示的其他轴向应变也表现出较弱的敏感性,这归因于其在P9笼内的振动限制(图3g)。此外,Sq [ P 8] 、 S 1 [P8] 、 S 2 [P8] 、 T g 和 P tub 模态会随着拉伸应变的升级而发生红移,尽管线性系数不同。具体而言,Sq [ P 8] 、S 2 [P8] 、T g 和P tub 模式(图3b-d)模式在管轴方向(<110>方向上表现出明显的应变敏感性。这是因为对应于Sq [p8] 模式的[P8]笼的挤压振动,S 2 [P8] 模式的[P8]保持架的P5-P5'原子拉伸方向,[P9]保持架的切向拉伸模式(T g )以及归因于P tub 模式的周长振动主导了沿平行于<110>轴方向的P-P键振动。特别令人感兴趣的是S 1 [P8] 模式(图3e),它独特地表现出相对于b轴的明显应变灵敏度,这可以归因于S 1 [P8] 模式中的原子位移主要沿b轴方向。 关于RB模式(图3f),尽管由于拉伸应变处的直径减小,在不同轴上施加拉伸应变会在拉曼光谱内产生蓝移,但由于[P8]和[P9]保持架的不对称径向呼吸振动与刚性[P2]沿b轴产生明显的不对称性,这种光谱偏移的频率表现出对沿b轴施加的应变的敏感性增强。此外,在 b 轴拉伸构型的背景下,仅分别在 450 和 470 cm -1 处出现一个奇异峰,这归因于 A g 和 B g 模式的叠加。在不同厚度(9.68 nm、11.2 nm 和 42.9 nm)的 VP 样品中可以辨别出上述应变各向异性的类似趋势,如图 S5 和 6 所示。

Table 1. The linear coefficients k of Raman shift obtained from VP along three directions.
表 1.从VP沿三个方向获得的拉曼位移的线性系数k。

Raman shift (cm-1)
拉曼位移 (cm -1

ε <110>
电∥ <110>

ε a-axis
ε∥ A 轴

ε b-axis
ε∥ b 轴





























Figure 5. Theoretical-calculation mechanical properties of VP and the displacement of atomic structural parameters under strain. (a) Comparison of the Raman shift linear coefficient between VP and other 2D material. Calculated (b) Young's modulus and (c) Poissons ratio of VP. (d-f) DFT calculated bond lengths R1-R13 as a function of uniaxial tensile strain along <110>, b and a-axis, respectively. (g-i) DFT calculated bond lengths θ1-θ6 as a function of uniaxial tensile strain along <110>, b and a-axis, respectively.
图5.VP力学性能的理论计算及应变作用下原子结构参数的位移.(a) VP与其他二维材料之间的拉曼位移线性系数比较。计算 (b) 杨氏模量和 (c) VP 的泊松比。(D-F)DFT 计算了键长 R1-R13 分别作为沿 <110>、b 和 a 轴的单轴拉伸应变的函数。(G-I)DFT 计算了键长 θ1-θ6 分别作为沿 <110>、b 和 a 轴的单轴拉伸应变的函数。

It is pertinent to highlight that the peak values of the derived shift rates for the 444.5 cm-1 modes attain a magnitude of 8.51 cm−1/%, significantly exceeding those documented in the literature, as summarized in Table S4. Concurrently, as depicted in Figure 5a, the VP sheets exhibit pronouncedly anisotropic and tunable phonon properties, with a range that substantially surpasses the previously reported literature values. The vibrational characteristics of a monolayer VP were computationally assessed via the Density Functional Theory (DFT) method to elucidate the strain anisotropy exhibited by few-layer VP systems. Despite the computational model's thickness being less than that of the actual experimental samples, experimental evidence has substantiated that the frequency discrepancies between different-layer VP for the aforementioned optical modes are marginal, amounting to less than 1 cm-1 (Figure S7). Prior to delving into the behaviors of the strain phonon behavior, we commence with a succinct overview of the mechanical properties of the VP systems under investigation. Figure 5a and 5b delineate the calculated Young's modulus (E) and Poissons ratio (ʋ) values along the periodic strain directions within the in-plane orientation, revealing that the E value along the a-axis (158 GPa) and b-axis (147 GPa) direction are notably lower than those of other monolayer 2D materials such as MoS2 (295.57 GPa) and black phosphorus (407.62 GPa). This observation accentuates the remarkably pliant character of VP structures when subjected to uniaxial tension.
需要强调的是,444.5 cm -1 模式的推导移位率峰值达到8.51 cm −1 /%,大大超过了文献中记录的峰值,如表S4所示。同时,如图5a所示,VP片表现出明显的各向异性和可调谐声子特性,其范围大大超过了先前报道的文献值。通过密度泛函理论(DFT)方法对单层VP的振动特性进行计算评估,以阐明少层VP系统表现出的应变各向异性。尽管计算模型的厚度小于实际实验样品的厚度,但实验证据证实,上述光学模式下不同层VP之间的频率差异很小,小于1 cm -1 (图S7)。在深入研究应变声子行为的行为之前,我们首先简要概述了所研究的VP系统的机械性能。图5a和图5b描绘了沿面内方向周期应变方向计算的杨氏模量(E)和泊松比(ʋ)值,表明沿a轴(158 GPa)和b轴(147 GPa)方向的E值明显低于其他单层二维材料,如MoS 2 (295.57 GPa)和黑磷(407.62 GPa)。这一观察结果强调了VP结构在受到单轴拉伸时非常柔顺的特性。

The coefficients extracted as mentioned above for the phonon shift under strains facilitate the calculation of the Grüneisen parameter, denoted by γ, through the following equation involving υ.
如上所述,应变下声子位移的系数有助于通过以下涉及 υ 的方程计算 Grüneisen 参数(用 γ 表示)。

γ=−1ω0(1−ϑ)∂ω∂ε (3)
γ=−1ω0(1−ϑ)∂ω∂ε (3)

This parameter, essential for describing thermodynamic properties, has been extensively debated in the context of anisotropic 2D materials such as black phosphorus and PdSe2. The Grüneisen parameters corresponding to different vibrational modes, as presented in Table 2, demonstrate a significant discrepancy when subjected to strain along different orientations, which further underscores the evident intrinsic anisotropy in the phononic properties within VP.
该参数对于描述热力学特性至关重要,在各向异性 2D 材料(如黑磷和 PdSe)的背景下进行了广泛的争论 2 。如表2所示,不同振动模式对应的Grüneisen参数在沿不同方向施加应变时表现出显著差异,这进一步强调了VP中声子特性的明显内在各向异性。

Table 2. The mode Grüneisen parameters of VP.
表 2.VP. 的模式 Grüneisen 参数。

Mode Grüneisen parameters γ


























To investigate the origin of the pronounced anisotropic Raman response under uniaxial strain, we conducted further calculations to assess the alterations in the structural parameters of VP subjected to uniaxial strain, as depicted in the Figure 5d, 19 relevant structural parameters are considered including 13 P-P distances (R1, R2, and R3...) and 6 bond angles (θ1, θ2 and θ3...), as summarized in Table 3. As shown in Figure 4g, the Sq[P8], S1[P8], S2[P8], Tg, and Ptub modes corresponds to atomic vibration mostly along the tubular direction. The <110> orientation depicted in Figure 5d-i demonstrates that the applied strain visibly extends R11 and significantly augments θ1. Specifically, a 1.5% strain enhances R11 by 0.015 Å and elevates θ1 by 2.32°; in contrast, the alterations observed in R4, R6, R9, and θ5 are relatively nominal. The pronounced alteration in R1 and θ1 instigates an expanded separation of atoms across the x and y axes within individual sublayers, thereby diminishing the restorative force along the tubular axis, culminating in marked redshifts of these vibrational modes. In the context of the S1[P8] mode, the core vibration is primarily attributed to incremental shifts in the P3-P3’ bond length named R6. These shifts manifest as a large increase along the b-axis and display heightened susceptibility to b-axis orientational strain. Analogously, under the imposition of uniaxial strain along the b-axis, notable contractions of R6 and R9 within the [p8] and [p9] cages are observed, respectively, coupled with a slight diminishment of θ2 and θ5, which collectively contribute to the stiffening of the RB mode.
为了研究单轴应变下显著各向异性拉曼响应的起源,我们进行了进一步的计算,以评估VP在单轴应变作用下的结构参数的变化,如图5d所示,考虑了19个相关结构参数,包括13个P-P距离(R1,R2和R3...)和6个键角(θ1, θ2 和 θ3...),如表 3 所示。如图4g所示,Sq [ P 8] 、S、S 1 [P8] 2 [P8] 、T g 和P tub 模式主要对应于管状方向的原子振动。图5d-i中描绘的<110>方向表明,施加的应变明显延长了R11并显著增加了θ1。具体而言,1.5% 的应变使 R11 增强 0.015 Å,使 θ1 升高 2.32°;相比之下,在 R4、R6、R9 和 θ5 中观察到的蚀变是相对标称的。R1 和 θ1 的明显变化引发了各个子层内 x 轴和 y 轴上的原子分离扩大,从而削弱了沿管轴的恢复力,最终导致这些振动模式的显着红移。在 S 1 [P8] 模式的背景下,磁芯振动主要归因于名为 R6 的 P3-P3' 键长的增量位移。这些偏移表现为沿 b 轴的大幅增加,并表现出对 b 轴取向应变的敏感性增强。类似地,在沿b轴施加单轴应变的情况下,分别观察到[p8]和[p9]笼内R6和R9的显着收缩,θ2和θ5略有减少,这共同有助于RB模式的刚化。

Table 3. The atomic displacements associated with the main characterized Raman modes.
表 3.与主要特征拉曼模式相关的原子位移。
















































































In a nutshell, as previously calculated, VP demonstrates a relatively low in-plane Young's modulus along with a high phonon anisotropic response to strain. When paired with the higher Young's modulus in the vertical direction, as documented in prior literature, these characteristics suggest that VP is poised for application in high-performance flexible polarized electronic devices due to its expected stretchability and impact resistance properties.

Figure 5. Optoelectric performence of flexible VP photodetector. (a) Optical photograph of VP flexible device. (b) Schematic diagram of optoelectronic device for in-plane anisotropic VP. (c) Transfer curve of the VP device. (d) Photoresponse of VP device illumination under the light power from 0.009 to 8.4 μW. (e,f) Rise time and fall time of VP photodetector at λ = 532nm. (g) Responsivity and photocurrent of VP devices as function of variation light power. (h, i) Photocurrent and polar coordinates at different polarization angles of the VP devices.
图5.柔性VP光电探测器的光电性能。(a) VP柔性装置的光学照片。(b) 面内各向异性VP光电器件示意图。(c) VP设备的传输曲线。(d) VP器件照明在0.009至8.4μW光功率下的光响应。(五、女)VP光电探测器在λ = 532nm时的上升时间和下降时间。(g) VP器件的响应度和光电流随光功率的变化而变化。(h,i)VP器件不同偏振角下的光电流坐标和极坐标。

To explore the photoelectric detection performance of flexible VP devices, we prepared flexible devices on PET substrate. Figure 6a is the optical photograph of VP flexible device. Figure 6b is a schematic diagram of the VP crystal optoelectronic device (electrode position along both tube directions and a, b-axis). It can be seen from the transfer characteristic curve of previous devices prepared on SiO2/Si substrate that VP is a typical p-type semiconductor (Figure S8). The output characteristic curves along different electrode directions in Figure 6c shows that the b-axis direction has significantly higher conductivity. Therefore, the subsequent research on photoelectric properties mainly focuses on the conductive channel in the b-axis direction. As illustrated in Figure 6d, the VP device manifests a minimal dark photocurrent, and its photoresponse escalates in tandem with the increase of laser power at a bias voltage of 2 V, thereby exemplifying a typical photoconductive behavior. Additionally, the response time under 532 nm laser illumination was quantified, yielding a rise time of 18.2 ms and a decay time of 37.0 ms, as respectively depicted in Figure 6e and f. Subsequently, the efficacy of the VP photodetector can be appraised by the magnitude of responsivity (R), which is calculated using the following equation:
为了探究柔性VP器件的光电检测性能,我们在PET基板上制备了柔性器件。图6a是VP柔性器件的光学照片。图6b是VP晶体光电器件的示意图(电极沿管子方向和a,b轴的位置)。从先前在SiO 2 /Si衬底上制备的器件的转移特性曲线可以看出,VP是典型的p型半导体(图S8)。图6c中沿不同电极方向的输出特性曲线显示,b轴方向的电导率明显更高。因此,后续对光电性能的研究主要集中在b轴方向的导电通道上。如图6d所示,VP器件表现出最小的暗光电流,并且在2 V偏置电压下,其光响应随着激光功率的增加而增加,从而体现了典型的光电导行为。此外,还量化了532 nm激光照射下的响应时间,产生了18.2 ms的上升时间和37.0 ms的衰减时间,分别如图6e和f所示。随后,VP光电探测器的功效可以通过响应度(R)的大小来评估,该响应度使用以下公式计算:

R=IpℎPA (4)
R=IpℎPA (4)

where Iph is photocurrent (Iph = Ilight - Idark), P is laser power density, and A is effective laser area.
其中 I ph 是光电流 (I ph = I light - I dark ),P 是激光功率密度,A 是有效激光面积。

Figure 6g delineates the photocurrent and the corresponding R as a function of incident light power. The photocurrent exhibited by the device demonstrates a near-linear relationship with the escalation of light power, achieving a peak R of 0.865 mA/W and detectivity of 1.1*1010 under a light power of 0.009 μW. To elucidate the response characteristics of VP to linearly polarized light, a 532 nm laser was employed in conjunction with a manual rotation of the half-wave plate positioned in the laser's incident light path, thereby modulating the laser's polarization angle. As depicted in Figure 6h and g, the photocurrent undergoes two complete cycles in response to alterations in the laser's polarization angle. Notably, the photocurrent attains its nadir when the laser's polarization direction aligns parallel to the a-axis, and conversely, it achieves its zenith when the laser's polarization direction is parallel to the b-axis. The polarization dichroism ratio at 532 nm is quantified as 2.26. The pronounced anisotropy and expeditious response time collectively underscore the exceptional suitability of VP as a high-performance polarized photodetector.
图6g描绘了光电流和相应的R作为入射光功率的函数。该器件所展示的光电流与光功率的递增呈近乎线性关系, 10 在0.009 μW的光功率下,峰值R为0.865 mA/W,检测率为1.1*10。为了阐明VP对线偏振光的响应特性,将532 nm激光器与位于激光入射光路中的半波板的手动旋转相结合,从而调制了激光的偏振角。如图6h和g所示,光电流会随着激光偏振角的变化而经历两个完整的周期。值得注意的是,当激光的偏振方向平行于 a 轴对齐时,光电流达到最低点,相反,当激光的偏振方向平行于 b 轴时,光电流达到顶峰。532 nm处的偏振二色性比量化为2.26。显著的各向异性和快速的响应时间共同凸显了VP作为高性能偏振光电探测器的出色适用性。


In conclusion, our investigation has meticulously examined the anisotropic strain response observed in the Raman modes of few-layer VP. The modes designated as Sq[P8], S1[P8], S2[P8], Tg, and Ptub exhibit a distinct redshift as a consequence of increasing tensile strain. Notably, the modes of Sq[P8], S2[P8], Tg and Ptub demonstrate a marked sensitivity to strain along the tube axis (<110> direction), characterized by a substantial linear strain coefficient, surpassing those observed in other two-dimensional materials due to its cross-structure. Theoretical analysis using Density Functional Theory (DFT) elucidates that the applied strain precipitates alterations not merely in bond lengths but also significantly impacts the bond angles within the ultrathin VP, leading to a pronounced anisotropic Raman response. Furthermore, the development of a flexible polarimetric sensitive detector utilizing VP reveals an intrinsic anisotropic photoresponse ratio of 2.26. Our findings illuminate the complex interplay between the vibrational attributes of this layered material and the external stress conditions, thereby substantially enriching our understanding of its mechanical and electronic properties.
总之,我们的研究仔细检查了在少层VP的拉曼模式中观察到的各向异性应变响应。被指定为Sq [ P 8] 、S、S 1 [P8] 2 [P8] 、T g 和P的模态由于拉伸应变的增加而 tub 表现出明显的红移。值得注意的是,Sq [ P 8] 、 S 2 [P8] 、 T g 和 P tub 模态对沿管轴(<110>方向的应变具有显著的敏感性,其特点是具有显著的线性应变系数,由于其交叉结构,超过了在其他二维材料中观察到的应变系数。使用密度泛函理论 (DFT) 的理论分析表明,施加的应变不仅会沉淀键长的变化,还会显着影响超薄 VP 内的键角,从而导致明显的各向异性拉曼响应。此外,利用VP的柔性偏振灵敏探测器的开发揭示了2.26的固有各向异性光响应比。我们的研究结果阐明了这种层状材料的振动属性与外部应力条件之间的复杂相互作用,从而大大丰富了我们对其机械和电子性能的理解。

4. Experimental section

Characterization:Violet phosphorus thin films were prepared on SiO2/Si substrates by mechanically exfoliated method. and the optical, Raman, AFM, and TEM measurements were carried out. Optical images were obtained by optical microscopy (LV100ND). Raman (Renishaw) test is based on the point-sweep mode at 0.076 mW laser power measured Raman spectra. The sample thickness was scanned by AFM (Bruker) for AC air Topography mode. HRTEM、EDS and SAED measured using TEM(FEI Talos F200X).
表征:采用机械剥离法在SiO 2 /Si衬底上制备紫磷薄膜。并进行了光学、拉曼、AFM 和 TEM 测量。光学图像是通过光学显微镜(LV100ND)获得的。拉曼(雷尼绍)测试基于点扫描模式,在0.076 mW激光功率下测得拉曼光谱。通过AFM(布鲁克)扫描样品厚度,以交流空气形貌模式。使用TEM(FEI Talos F200X)测量HRTEM、EDS和SAED。

VP Crystal Orientation identification: VP Crystal orientation is determined by measuring Angle-Resolved Polarized Raman in parallel and cross configurations. The parallel polarization is measured by rotating the polarizer to 0 ° in the collecting light path, and the vertical polarization is measured by rotating the half-wave plate to 45 ° in the incident light path.

Sample PreparationThe violet phosphorus sample was transferred a flexible substrate PET with a thickness of around 188μm. Firstly, violet phosphorus thin films were prepared on SiO2/Si substrates by mechanically exfoliated method. Next, 10wt% polyvinyl alcohol (Alfa Aesar, 98-99% hydrolyzed, high molecular weight) solution is spin-coated on the VP/SiO2/Si at 1000rpm for 40 s, the samples were then placed on a heating table at 70 ° C and heated for 1 minute. PVA and PET were bonded using an adhesive glue and left to rest overnight. Next, the silicon was gently torn off with tweezers. The sample was prepared and the layer structure was PVA/glue/PET, the overall thickness is 230 μm.
样品制备:将紫磷样品转移到厚度约为188μm的柔性基材PET中。首先,采用机械剥离法在SiO 2 /Si衬底上制备紫磷薄膜;接下来,将10wt%聚乙烯醇(Alfa Aesar,98-99%水解,高分子量)溶液以1000rpm旋涂在VP/SiO 2 / Si上40 s,然后将样品置于70°C的加热台上并加热1分钟。PVA和PET使用粘合剂粘合并静置过夜。接下来,用镊子轻轻撕下硅。制备样品,层结构为PVA/胶水/PET,总厚度为230 μm。

Device Fabrication and photoelectric measurement: violet phosphorus thin films were mechanically exfoliated on SiO2/Si, and the electrodes were fabricated by maskless lithography. The thickness of the electrodes was 5 nm Cr/60 nm Au. After the photoresist is removed, the device was annealed in nitrogen at 120 ° C for 6 h to improve the contact quality. The electrical and photoelectric properties of the materials were measured using a semiconductor tester (Keithley 4200) and a Lakeshore probe. Using 532nm laser source, laser power measured by power meter.
器件制造和光电测量:紫磷薄膜在SiO2/Si上机械剥离,电极采用无掩模光刻法制备。电极厚度为5 nm Cr/60 nm Au。去除光刻胶后,将器件在120°C的氮气中退火6 h,以提高接触质量。使用半导体测试仪(Keithley 4200)和Lakeshore探针测量材料的电学和光电性能。使用532nm激光源,激光功率计测量。

5. Acknowledgements
5. 致谢

C. H. Shang, and W. W. Wang contributed equally to this work. This work was supported by Fundamental Research Funds for the Central Universities (Nos. 10251210015, 20101237327, ZYTS23089) and the Innovation Fund of Xidian University, Guangdong Basic and Applied Basic Research Foundation (Nos.2021A1515110013), National Natural Science Foundation of China (Nos. 22305182, 52192613, 52192610, 51972204, 22222505, 22003049) and Natural Science Basic Research Program of Shaanxi (Nos. 2023-JC-QN-0508).
C. H. Shang 和 W. W. Wang 对这项工作做出了同样的贡献。本研究得到了中央高校基础研究基金(10251210015、20101237327、ZYTS23089)和西安电子科技大学创新基金、广东省基础与应用基础研究基金(编号:2021A1515110013)、国家自然科学基金(22305182、52192613、52192610、51972204、22222505、22003049)和陕西省自然科学基础研究计划(编号:2023-JC-QN-0508)的支持。

Conflict of Interest

The authors declare no conflict of interest.