Browsing by Author "Li, Shisheng"

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  • 1D Crystallographic Etching of Few-Layer WS2
    (2024-11-12) Li, Shisheng; Lin, Yung Chang; Chiew, Yiling; Dai, Yunyun; Ning, Zixuan; Zhang, Yaming; Nakajima, Hideaki; Lim, Hong En; Wu, Jing; Neitoh, Yasuhisa; Okazaki, Toshiya; Sun, Yang; Sun, Zhipei; Suenaga, Kazu; Sakuma, Yoshiki; Tsukagoshi, Kazuhito; Taniguchi, Takaaki
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Layer number-dependent band structures and symmetry are vital for the electrical and optical characteristics of 2D transition metal dichalcogenides (TMDCs). Harvesting 2D TMDCs with tunable thickness and properties can be achieved through top-down etching and bottom-up growth strategies. In this study, a pioneering technique that utilizes the migration of in situ generated Na-W-S-O droplets to etch out 1D nanotrenches in few-layer WS2 is reported. 1D WS2 nanotrenches are successfully fabricated on the optically inert bilayer WS2, showing pronounced photoluminescence and second harmonic generation signals. Additionally, the modulation of inkjet-printed Na2WO4-Na2SO4 particles to switch between the etching and growth modes by manipulating the sulfur supply is demonstrated. This versatile approach enables the creation of 1D nanochannels on 2D TMDCs. The research presents exciting prospects for the top-down and bottom-up fabrication of 1D-2D mixed-dimensional TMDC nanostructures, expanding their use for electronic and optoelectronic applications.
  • Broadband Plasmon-Enhanced Four-Wave Mixing in Monolayer MoS2
    (2021-07-28) Dai, Yunyun; Wang, Yadong; Das, Susobhan; Li, Shisheng; Xue, Hui; Mohsen, Ahmadi; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Two-dimensional transition-metal dichalcogenide monolayers have remarkably large optical nonlinearity. However, the nonlinear optical conversion efficiency in monolayer transition-metal dichalcogenides is typically low due to small light-matter interaction length at the atomic thickness, which significantly obstructs their applications. Here, for the first time, we report broadband (up to μ150 nm) enhancement of optical nonlinearity in monolayer MoS2 with plasmonic structures. Substantial enhancement of four-wave mixing is demonstrated with the enhancement factor up to three orders of magnitude for broadband frequency conversion, covering the major visible spectral region. The equivalent third-order nonlinearity of the hybrid MoS2-plasmonic structure is in the order of 10-17 m2/V2, far superior (μ10-100-times larger) to the widely used conventional bulk materials (e.g., LiNbO3, BBO) and nanomaterials (e.g., gold nanofilms). Such a considerable and broadband enhancement arises from the strongly confined electric field in the plasmonic structure, promising for numerous nonlinear photonic applications of two-dimensional materials.
  • Deterministic Modification of CVD Grown Monolayer MoS2 with Optical Pulses
    (2021-05-21) Turunen, Mikko T.; Hulkko, Eero; Mentel, Kamila K.; Bai, Xueyin; Akkanen, Suvi Tuuli; Amini, Mohammad; Li, Shisheng; Lipsanen, Harri; Pettersson, Mika; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Transition metal dichalcogenide monolayers have demonstrated a number of exquisite optical and electrical properties. Here, the authors report the optical modification of topographical and optical properties of monolayer MoS2 with femtosecond pulses under an inert atmosphere. A formation of three-dimensional structures on monolayer MoS2 with tunable height up to ≈20 nm is demonstrated. In contrast to unmodified monolayer MoS2, these optically modified structures show significantly different optical properties, such as lower photoluminescence intensity and longer fluorescence lifetime. The results suggest a novel way to modify transition metal dichalcogenide materials for mechanic, electronic and photonic applications.
  • Difference frequency generation in monolayer MoS2
    (2020-10-14) Wang, Yadong; Ghotbi, Masood; Das, Susobhan; Dai, Yunyun; Li, Shisheng; Hu, Xuerong; Gan, Xuetao; Zhao, Jianlin; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Difference frequency generation has long been employed for numerous applications, such as coherent light generation, sensing and imaging. Here, we demonstrate difference frequency generation down to atomic thickness in monolayer molybdenum disulfide. By mixing femtosecond optical pulses at wavelength of 406 nm with tunable pulses in the spectral range of 1300-1520 nm, we generate tunable pulses across the spectral range of 550-590 nm with frequency conversion efficiency up to ∼2 × 10-4. The second-order nonlinear optical susceptibility of monolayer molybdenum disulfide, χ(2)eff, is calculated as ∼1.8 × 10-8 m V-1, comparable to the previous results demonstrated with second harmonic generation. Such a highly efficient down-conversion nonlinear optical process in two-dimensional layered materials may open new ways to their nonlinear optical applications, such as coherent light generation and amplification.
  • Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism
    (2021-03-25) Lin, Yung Chang; Karthikeyan, Jeyakumar; Chang, Yao Pang; Li, Shisheng; Kretschmer, Silvan; Komsa, Hannu Pekka; Chiu, Po Wen; Krasheninnikov, Arkady V.; Suenaga, Kazu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4–15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2. Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.
  • Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons
    (2021-08-18) Wang, Yadong; Das, Susobhan; Iyikanat, Fadil; Dai, Yunyun; Li, Shisheng; Guo, Xiangdong; Yang, Xiaoxia; Cheng, Jinluo; Hu, Xuerong; Ghotbi, Masood; Ye, Fangwei; Lipsanen, Harri; Wu, Shiwei; Hasan, Tawfique; Gan, Xuetao; Liu, Kaihui; Sun, Dong; Dai, Qing; García De Abajo, F. Javier; Zhao, Jianlin; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    All-optical control of nonlinear photonic processes in nanomaterials is of significant interest from a fundamental viewpoint and with regard to applications ranging from ultrafast data processing to spectroscopy and quantum technology. However, these applications rely on a high degree of control over the nonlinear response, which still remains elusive. Here, we demonstrate giant and broadband all-optical ultrafast modulation of second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides mediated by the modified excitonic oscillation strength produced upon optical pumping. We reveal a dominant role of dark excitons to enhance SHG by up to a factor of ∼386 at room temperature, 2 orders of magnitude larger than the current state-of-the-art all-optical modulation results. The amplitude and sign of the observed SHG modulation can be adjusted over a broad spectral range spanning a few electronvolts with ultrafast response down to the sub-picosecond scale via different carrier dynamics. Our results not only introduce an efficient method to study intriguing exciton dynamics, but also reveal a new mechanism involving dark excitons to regulate all-optical nonlinear photonics.
  • Molybdenum Disulfide/Double-Wall Carbon Nanotube Mixed-Dimensional Heterostructures
    (2022-05-04) Bai, Xueyin; Xu, Zhenyu; Zhang, Qiang; Li, Shisheng; Dai, Yunyun; Cui, Xiaoqi; Yoon, Hoon Hahn; Yao, Lide; Jiang, Hua; Du, Mingde; Zhang, Yi; Kauppinen, Esko I.; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Mixed-dimensional heterostructures which combine materials with different dimensions have emerged to expand the scope and functionality of van der Waals heterostructures. Here, a direct synthesis method of molybdenum disulfide/double-wall carbon nanotube (MoS2/DWCNT) mixed-dimensional heterostructures by sulfurating a molten salt, Na2MoO4, on a substrate covered with a DWCNT film is reported. The synthesized heterostructures are comprehensively characterized and their stacking order is confirmed to be MoS2 under the DWCNTs, although the DWCNT film is transferred on the substrate first. Moreover, field-effect transistors based on the heterostructure are fabricated for photodetection, and an abnormal negative photoresponse is discovered due to the strong carrier transfer in the mixed-dimensional heterostructures under light incidence. The MoS2/DWCNT heterostructure results provide a new approach for the synthesis and applications of mixed-dimensional heterostructures.
  • Optical Control of High-Harmonic Generation at the Atomic Thickness
    (2022-11-09) Wang, Yadong; Iyikanat, Fadil; Bai, Xueyin; Hu, Xuerong; Das, Susobhan; Dai, Yunyun; Zhang, Yi; Du, Luojun; Li, Shisheng; Lipsanen, Harri; García De Abajo, F. Javier; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light–matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.
  • Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies
    (2022-01-20) Wang, Yadong; Iyikanat, Fadil; Rostami, Habib; Bai, Xueyin; Hu, Xuerong; Das, Susobhan; Dai, Yunyun; Du, Luojun; Zhang, Yi; Li, Shisheng; Lipsanen, Harri; García de Abajo, F. Javier; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2, such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to >= 200 via broadband (from approximate to 1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding approximate to 94% at approximate to 2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier-exciton and carrier-phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.
  • Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy
    (2017-06-05) Karvonen, Lasse; Säynätjoki, Antti; Huttunen, Mikko J.; Autere, Anton; Amirsolaimani, Babak; Li, Shisheng; Norwood, Robert A.; Peyghambarian, Nasser; Lipsanen, Harri; Eda, Goki; Kieu, Khanh; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.
  • Single-step chemical vapour deposition of anti-pyramid MoS2/WS2vertical heterostructures
    (2021-02-28) Bai, Xueyin; Li, Shisheng; Das, Susobhan; Du, Luojun; Dai, Yunyun; Yao, Lide; Raju, Ramesh; Du, Mingde; Lipsanen, Harri; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Van der Waals heterostructures are the fundamental building blocks of electronic and optoelectronic devices. Here we report that, through a single-step chemical vapour deposition (CVD) process, high-quality vertical bilayer MoS2/WS2 heterostructures with a grain size up to ∼60 μm can be synthesized from molten salt precursors, Na2MoO4 and Na2WO4. Instead of normal pyramid vertical heterostructures grown by CVD, this method synthesizes an anti-pyramid MoS2/WS2 structure, which is characterized by Raman, photoluminescence and second harmonic generation microscopy. Our facile CVD strategy for synthesizing anti-pyramid structures unveils a new synthesis route for the products of two-dimensional heterostructures and their devices for application.
  • Ultrafast transient sub-bandgap absorption of monolayer MoS2
    (2021-01-29) Das, Susobhan; Wang, Yadong; Dai, Yunyun; Li, Shisheng; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.