Browsing by Author "Zhang, Xiang"
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Item Atomistic measurement and modeling of intrinsic fracture toughness of two-dimensional materials(PNAS, 2022) Zhang, Xu; Nguyen, Hoang; Zhang, Xiang; Ajayan, Pulickel M.; Wen, Jianguo; Espinosa, Horacio D.Quantifying the intrinsic mechanical properties of two-dimensional (2D) materials is essential to predict the long-term reliability of materials and systems in emerging applications ranging from energy to health to next-generation sensors and electronics. Currently, measurements of fracture toughness and identification of associated atomistic mechanisms remain challenging. Herein, we report an integrated experimental–computational framework in which in-situ high-resolution transmission electron microscopy (HRTEM) measurements of the intrinsic fracture energy of monolayer MoS 2 and MoSe 2 are in good agreement with atomistic model predictions based on an accurately parameterized interatomic potential. Changes in crystalline structures at the crack tip and crack edges, as observed in in-situ HRTEM crack extension tests, are properly predicted. Such a good agreement is the result of including large deformation pathways and phase transitions in the parameterization of the inter-atomic potential. The established framework emerges as a robust approach to determine the predictive capabilities of molecular dynamics models employed in the screening of 2D materials, in the spirit of the materials genome initiative. Moreover, it enables device-level predictions with superior accuracy (e.g., fatigue lifetime predictions of electro- and opto-electronic nanodevices).Item Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser(AIP Publishing, 2021) Britz, Alexander; Attar, Andrew R.; Zhang, Xiang; Chang, Hung-Tzu; Nyby, Clara; Krishnamoorthy, Aravind; Park, Sang Han; Kwon, Soonnam; Kim, Minseok; Nordlund, Dennis; Sainio, Sami; Heinz, Tony F.; Leone, Stephen R.; Lindenberg, Aaron M.; Nakano, Aiichiro; Ajayan, Pulickel; Vashishta, Priya; Fritz, David; Lin, Ming-Fu; Bergmann, UweFemtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d5/2 core level (M5-edge, 572–577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400 ± 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.Item Characterizations of two-photon absorption process induced by defects in aluminum nitride using Z-scan method(IOP Publishing, 2024) Zhou, Jingan; Li, Tao; Zhao, Xuan; Zhang, Xiang; Doumani, Jacques; Xu, Mingfei; He, Ziyi; Luo, Shisong; Mei, Zhaobo; Chang, Cheng; Robinson, Jacob T.; Ajayan, Pulickel M.; Kono, Junichiro; Zhao, Yuji; Smalley-Curl InstituteIn this work, we reported two-photon absorption (TPA) measurements for aluminum vacancies in Aluminum nitride single crystals. We measured the linear transmission and identified the defect levels. Using the Z-scan method, we measured the TPA coefficients of the transitions between defect levels from 380 nm to 735 nm. The transition occurs between the aluminum vacancies defect levels. Furthermore, the power dependence shows good linear fitting, confirming the TPA mechanism. These results will be helpful for the design and fabrication of ultra-low loss waveguides and integrated photonics in the ultraviolet spectral range.Item Collagen-rich airway smooth muscle cells are a metastatic niche for tumor colonization in the lung(Springer Nature, 2019) Lee, Yu-Cheng; Kurtova, Antonina V.; Xiao, Jing; Nikolos, Fotis; Hayashi, Kazukuni; Tramel, Zoe; Jain, Antrix; Chen, Fengju; Chokshi, Mithil; Lee, Ciaran; Bao, Gang; Zhang, Xiang; Shen, Jianjun; Mo, Qianxing; Jung, Sung Yun; Rowley, David; Chan, Keith SysonMetastases account for the majority of cancer deaths. While certain steps of the metastatic cascade are well characterized, identification of targets to block this process remains a challenge. Host factors determining metastatic colonization to secondary organs are particularly important for exploration, as those might be shared among different cancer types. Here, we showed that bladder tumor cells expressing the collagen receptor, CD167a, responded to collagen I stimulation at the primary tumor to promote local invasion and utilized the same receptor to preferentially colonize at airway smooth muscle cells (ASMCs)—a rich source of collagen III in lung. Morphologically, COL3-CD167a-driven metastatic foci are uniquely distinct from typical lung alveolar metastatic lesions and exhibited activation of the CD167a-HSP90-Stat3 axis. Importantly, metastatic lung colonization could be abrogated using an investigational drug that attenuates Stat3 activity, implicating this seed-and-soil interaction as a therapeutic target for eliminating lung metastasis.Item Collagen-rich airway smooth muscle cells are a metastatic niche for tumor colonization in the lung(Springer Nature, 2019) Lee, Yu-Cheng; Kurtova, Antonina V.; Xiao, Jing; Nikolos, Fotis; Hayashi, Kazukuni; Tramel, Zoe; Jain, Antrix; Chen, Fengju; Chokshi, Mithil; Lee, Ciaran; Bao, Gang; Zhang, Xiang; Shen, Jianjun; Mo, Qianxing; Jung, Sung Yun; Rowley, David; Chan, Keith SysonMetastases account for the majority of cancer deaths. While certain steps of the metastatic cascade are well characterized, identification of targets to block this process remains a challenge. Host factors determining metastatic colonization to secondary organs are particularly important for exploration, as those might be shared among different cancer types. Here, we showed that bladder tumor cells expressing the collagen receptor, CD167a, responded to collagen I stimulation at the primary tumor to promote local invasion and utilized the same receptor to preferentially colonize at airway smooth muscle cells (ASMCs)—a rich source of collagen III in lung. Morphologically, COL3-CD167a-driven metastatic foci are uniquely distinct from typical lung alveolar metastatic lesions and exhibited activation of the CD167a-HSP90-Stat3 axis. Importantly, metastatic lung colonization could be abrogated using an investigational drug that attenuates Stat3 activity, implicating this seed-and-soil interaction as a therapeutic target for eliminating lung metastasis.Item Cryo-mediated exfoliation and fracturing of layered materials into 2D quantum dots(AAAS, 2017) Wang, Yan; Liu, Yang; Zhang, Jianfang; Wu, Jingjie; Xu, Hui; Wen, Xiewen; Zhang, Xiang; Tiwary, Chandra Sekhar; Yang, Wei; Vajtai, Robert; Zhang, Yong; Chopra, Nitin; Odeh, Ihab Nizar; Wu, Yucheng; Ajayan, Pulickel M.Atomically thin quantum dots from layered materials promise new science and applications, but their scalable synthesis and separation have been challenging. We demonstrate a universal approach for the preparation of quantum dots from a series of materials, such as graphite, MoS2, WS2, h-BN, TiS2, NbS2, Bi2Se3, MoTe2, Sb2Te3, etc., using a cryo-mediated liquid-phase exfoliation and fracturing process. The method relies on liquid nitrogen pretreatment of bulk layered materials before exfoliation and breakdown into atomically thin two-dimensional quantum dots of few-nanometer lateral dimensions, exhibiting size-confined optical properties. This process is efficient for a variety of common solvents with a wide range of surface tension parameters and eliminates the use of surfactants, resulting in pristine quantum dots without surfactant covering or chemical modification.Item K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2(Springer Nature, 2022) Bae, Soungmin; Matsumoto, Kana; Raebiger, Hannes; Shudo, Ken-ichi; Kim, Yong-Hoon; Handegård, Ørjan Sele; Nagao, Tadaaki; Kitajima, Masahiro; Sakai, Yuji; Zhang, Xiang; Vajtai, Robert; Ajayan, Pulickel; Kono, Junichiro; Takeda, Jun; Katayama, IkufumiIn transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.Item Micro-Extinction Spectroscopy (MExS): a versatile optical characterization technique(Springer, 2018) Kumar, Anjli; Villarreal, Eduardo; Zhang, Xiang; Ringe, EmilieMicro-Extinction Spectroscopy (MExS), a flexible, optical, and spatial-scanning hyperspectral technique, has been developed and is described with examples. Software and hardware capabilities are described in detail, including transmission, reflectance, and scattering measurements. Each capability is demonstrated through a case study of nanomaterial characterization, i.e., transmission of transition metal dichalcogenides revealing transition energy and efficiency, reflectance of transition metal dichalcogenides grown on nontransparent substrates identifying the presence of monolayer following electrochemical ablation, and scattering to study single plasmonic nanoparticles and obtain values for the refractive index sensitivity and sensing figure of merit of over a hundred single particles with various shapes and sizes. With the growing integration of nanotechnology in many areas, MExS can be a powerful tool to both characterize and test nanomaterials.Item Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides(The Optical Society, 2016) Razanoelina, Manjakavahoaka; Bagsican, Filchito Renee; Kawayama, Iwao; Zhang, Xiang; Ma, Lulu; Murakami, Hironaru; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Tonouchi, MasayoshiAs novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 1011 cm−2, which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.Item Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys(Springer Nature, 2018) Taghinejad, Hossein; Eftekhar, Ali A.; Campbell, Philip M.; Beatty, Brian; Taghinejad, Mohammad; Zhou, Yao; Perini, Christopher J.; Moradinejad, Hesam; Henderson, Walter E.; Woods, Eric V.; Zhang, Xiang; Ajayan, Pulickel; Reed, Evan J.; Vogel, Eric M.; Adibi, AliComposition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.Item Super-elasticity at 4 K of covalently crosslinked polyimide aerogels with negative Poisson’s ratio(Springer Nature, 2021) Cheng, Yang; Zhang, Xiang; Qin, Yixiu; Dong, Pei; Yao, Wei; Matz, John; Ajayan, Pulickel M.; Shen, Jianfeng; Ye, MingxinThe deep cryogenic temperatures encountered in aerospace present significant challenges for the performance of elastic materials in spacecraft and related apparatus. Reported elastic carbon or ceramic aerogels overcome the low-temperature brittleness in conventional elastic polymers. However, complicated fabrication process and high costs greatly limited their applications. In this work, super-elasticity at a deep cryogenic temperature of covalently crosslinked polyimide (PI) aerogels is achieved based on scalable and low-cost directional dimethyl sulfoxide crystals assisted freeze gelation and freeze-drying strategy. The covalently crosslinked chemical structure, cellular architecture, negative Poisson’s ratio (−0.2), low volume shrinkage (3.1%), and ultralow density (6.1 mg/cm3) endow the PI aerogels with an elastic compressive strain up to 99% even in liquid helium (4 K), almost zero loss of resilience after dramatic thermal shocks (∆T = 569 K), and fatigue resistance over 5000 times compressive cycles. This work provides a new pathway for constructing polymer-based materials with super-elasticity at deep cryogenic temperature, demonstrating much promise for extensive applications in ongoing and near-future aerospace exploration.Item Synthesis and Characterization of 2H and 1T’ Molybdenum Ditelluride (MoTe2)(2017-04-21) Zhang, Xiang; Ajayan, Pulickel MadhavapanickerMolybdenum ditelluride (MoTe2) is a member of the two-dimensional transition metal dichalcogenides (TMDs) family, which owns unique structures and interesting properties compared to other group VI TMDs. MoTe2 has two different stable structures at room temperature, the semiconducting 2H phase and metallic 1T’ phase. 2H MoTe2 has a small band gap of ~ 1 eV, which is quite similar to that of Si. Besides, due to the energy difference between the two phases is very small, theoretically, it is possible to reversibly switch from 2H phase to 1T’ phase in some conditions. In this thesis, controllable synthesis of 2H and 1T’ MoTe2 is demonstrated. As-grown samples are characterized by optical microscopy, Raman spectroscopy, SEM, AFM, XPS, XRD, UV-vis, etc. We also fabricated MoTe2-based electronic devices and measured their electrical properties.Item Two-dimensional molybdenum ditelluride (MoTe2): synthesis, characterization, and application(2018-04-19) Zhang, Xiang; Ajayan, Pulickel MRecent research efforts in two-dimensional (2D) materials have shown an increasing focus on molybdenum ditelluride (MoTe2). Unlike other TMDs, MoTe2 is distinguished by the existence of two stable phases, hexagonal 2H phase and monoclinic 1T’ phase, both of which can be synthesized directly. 2H MoTe2 is a semiconductor with a band gap of ~ 1 eV, while 1T’ MoTe2 is metallic which can be transformed to a type-II Weyl semimetal at low temperature. The semiconductor-metal junction between 2H and 1T’ MoTe2 shows the potential to resolve the issue of the existence of high Schottky barrier in traditional devices. MoTe2 does not only possesses a myriad of physical properties to further unravel but also shows great potential towards various industrial applications such as analog circuits and spintronics. Here in this thesis, chapter 1 gave a brief introduction to 2D materials. Several common synthesis methods, characterization methods, and applications were discussed. Chapter 2 focused on the phase-controlled synthesis of large-area MoTe2 films and 2H/1T’ MoTe2 heterostructures by chemical vapor deposition. A series of techniques have been used to systematically characterize the synthesized MoTe2 films. In chapter 3, several types of MoTe2-based devices were fabricated and measured. We demonstrated an electrical device across the one-step-synthesized 2H/1T’ MoTe2 in-plane heterostructure, where 1T’ phase serves as the contact electrodes for the 2H phase channel. An improved current density was observed compared with deposited metal electrodes on top. In chapter 4, we studied the Raman enhancement on MoTe2 films. MoTe2-based 2D heterostructures exhibit the potential as novel platforms for surface-enhanced Raman scattering (SERS) application. Our recent efforts in the spatial phase-targeted synthesis of 2H and 1T’ MoTe2 were presented in chapter 5. This strategy was suitable not only for large-scale patterns but also for small features. In chapter 6, we used the ultrafast electron diffraction (UED) to investigate the nonradiative process in 2H MoTe2 at SLAC National Accelerator Laboratory. This thesis doesn’t only study the fundamental properties of MoTe2, but also paves the way towards the large-scale application of MoTe2 in electronic and optoelectronic devices.Item Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2ﾠheterostructures(Springer Nature, 2016) Chen, Hailong; Wen, Xiewen; Zhang, Jing; Wu, Tianmin; Gong, Yongji; Zhang, Xiang; Yuan, Jiangtan; Yi, Chongyue; Lou, Jun; Ajayan, Pulickel M.; Zhuang, Wei; Zhang, Guangyu; Zheng, JunrongVan der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2ﾠheterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.Item Ultrafast non-radiative dynamics of atomically thin MoSe2(Springer Nature, 2017) Lin, Ming-Fu; Kochat, Vidya; Krishnamoorthy, Aravind; Bassman, Lindsay; Weninger, Clemens; Zheng, Qiang; Zhang, Xiang; Apte, Amey; Tiwary, Chandra Sekhar; Shen, Xiaozhe; Li, Renkai; Kalia, Rajiv; Ajayan, Pulickel; Nakano, Aiichiro; Vashishta, Priya; Shimojo, Fuyuki; Wang, Xijie; Fritz, David M.; Bergmann, UwePhoto-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.Item Ultrafast probes of electron–hole transitions between two atomic layers(Springer Nature, 2018) Wen, Xiewen; Chen, Hailong; Wu, Tianmin; Yu, Zhihao; Yang, Qirong; Deng, Jingwen; Liu, Zhengtang; Guo, Xin; Guan, Jianxin; Zhang, Xiang; Gong, Yongji; Yuan, Jiangtan; Zhang, Zhuhua; Yi, Chongyue; Guo, Xuefeng; Ajayan, Pulickel M.; Zhuang, Wei; Liu, Zhirong; Lou, Jun; Zheng, JunrongPhase transitions of electron-hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. To investigate interfacial dynamical transitions of charged quasiparticles, however, remains a grand challenge. By employing ultrafast mid-infrared microspectroscopic probes to detect excitonic internal quantum transitions and two-dimensional atomic device fabrications, we are able to directly monitor the interplay between free carriers and insulating interlayer excitons between two atomic layers. Our observations reveal unexpected ultrafast formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2. The result suggests carriers in the doped graphene are no longer massless, and an effective mass as small as one percent of free electron mass is sufficient to confine carriers within a 2D hetero space with energy 10 times larger than the room-temperature thermal energy. The interlayer excitons arise within 1 ps. Their formation effectively blocks charge recombination and improves charge separation efficiency for more than one order of magnitude.Item Ultrahigh resistance of hexagonal boron nitride to mineral scale formation(Springer Nature, 2022) Zuo, Kuichang; Zhang, Xiang; Huang, Xiaochuan; Oliveira, Eliezer F.; Guo, Hua; Zhai, Tianshu; Wang, Weipeng; Alvarez, Pedro J.J.; Elimelech, Menachem; Ajayan, Pulickel M.; Lou, Jun; Li, Qilin; NSF Nanosystems Engineering Research Center Nanotechnology-Enabled Water TreatmentFormation of mineral scale on a material surface has profound impact on a wide range of natural processes as well as industrial applications. However, how specific material surface characteristics affect the mineral-surface interactions and subsequent mineral scale formation is not well understood. Here we report the superior resistance of hexagonal boron nitride (hBN) to mineral scale formation compared to not only common metal and polymer surfaces but also the highly scaling-resistant graphene, making hBN possibly the most scaling resistant material reported to date. Experimental and simulation results reveal that this ultrahigh scaling-resistance is attributed to the combination of hBN’s atomically-smooth surface, in-plane atomic energy corrugation due to the polar boron-nitrogen bond, and the close match between its interatomic spacing and the size of water molecules. The latter two properties lead to strong polar interactions with water and hence the formation of a dense hydration layer, which strongly hinders the approach of mineral ions and crystals, decreasing both surface heterogeneous nucleation and crystal attachment.