Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J51: Optoelectronic Properties of 2D Materials |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Shaowei Li, University of California, Berkeley Room: Mile High Ballroom 1D |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J51.00001: van der Waals photothermoelectric effect in atomic layer heterojunctions Yunqiu (Kelly) Luo, Tong Zhou, Mahesh R Neupane, Alex Matos Abiague, Ryan Bailey-Crandell, Michael J Newburger, Igor Lyalin, Igor Zutic, Roland Kawakami Two-dimensional (2D) van der Waals (vdW) heterostructures provide exceptional opportunities for new physics and devices due to their unprecedented ability to tune the electronic, optical, magnetic and spintronic properties by atomic layer stacking and electrostatic gating. Harnessing this versatility requires a fundamental understanding of light-matter interactions and establishing new functionalities for photon-charge and photon-spin conversions. Here, we report the first observation of a highly-tunable vdW photothermoelectric effect in dual-gated MoS2/graphene junctions with a striking multiple-polarity switching of photocurrent as a function of junction bias and carrier density. In stark contrast to photovoltaic effects arising from excitonic absorption in MoS2, the vdW photothermoelectric effect originates from photoexcitation of hot electrons in graphene and thermoelectric transport across the vdW junction. Systematic studies of photoconductance as a function of photon energy and intensity reveal vdW photothermoelectric effect as the dominant mechanism for photocurrent generation at room temperature, as opposed to excitonic absorption. These findings provide an important step for understanding and control of vdW-interface devices. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J51.00002: Realizing precise charge neutrality in van der Waals heterostructure p-n junctions through data intensive photoresponse imaging Trevor Arp, Fatemeh Barati, Shanshan Su, Roger Lake, Nathaniel Gabor In atomically thin materials, the narrow phase space for exciton–phonon interactions could be used to engineer unusual devices that mimic molecular optical transitions. A van der Waals heterostructure tuned to a charge neutral state allows for the formation of excitons where the electron and hole are localized in different materials. Such interlayer excitons have a well-defined dipole moment making them sensitive to the interlayer electric field and out-of-plane vibrations. To isolate the behavior of interlayer excitons and explore their dynamics, we employ Multi-Parameter Dynamic Photoresponse Microscopy on encapsulated heterostructures composed of monolayer MoSe2 and bilayer WSe2. Gathering a large set of spatial photocurrent images as a function of source-drain and gate voltages, we unambiguously identify the charge neutrality condition. We observe that the system can be tuned to exhibit clear rectification with highly uniform spatial dependence in the heterojunction consistent with the formation of a two-dimensional distributed p-n junction at charge neutrality. Under these precise charge neutrality conditions, we observe striking spectroscopic features that reveal highly unusual exciton-phonon interactions. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J51.00003: Enhanced intrinsic photovoltaic effect in tungsten disulphide nanotubes Yijin Zhang, Toshiya Ideue, Masaru Onga, Feng Qin, Ryuji Suzuki, Alla Zak, Reshef Tenne, Jurgen Hubert Smet, Yoshihiro Iwasa Transition metal dichalcogenides (TMDs) are representative 2D materials. Group VI-B TMDs in 2H-type crystal structure have a semiconducting band structure. One of their unique features is the controllability of the crystal symmetry. The inversion symmetry inherent in bulk TMDs can be broken by isolating a monolayer. Various opto-electronic functionalities have been demonstrated by breaking this inversion symmetry. A further reduction of crystal symmetry can be achieved by rolling up 2D sheets to form a tubular structure. In such TMD nanotubes, not only the inversion symmetry but also mirror and rotation symmetries can be broken simultaneously, leading to a polar structure. We have investigated the photovoltaic effect in intrinsic WS2 with different crystal symmetries and observed a sizable photovoltaic effect from TMD nanotubes without p-n junction. Such a phenomenon is called bulk photovoltaic effect (BPVE). In our study BPVE was observed in WS2 nanotubes only, indicating a importance of the polar structure to enhance BPVE. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J51.00004: Carrier transport dynamics in 2D semiconductors optoelectronic devices Pan P Adhikari, Peijian Wang, Kanishka Kobbekaduwa, Exian Liu, Hao Zeng, Jianbo Gao
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Tuesday, March 3, 2020 3:18PM - 3:30PM |
J51.00005: Nanoscale Mapping of Photoconductivity Enhanced by Localized Charge Traps in the Grain Structures of MoS2 Monolayer Hyesong Jeon, Myungjae Yang, Taeyoung Kim, Takhee Lee, Seunghun Hong We present a method for the nanoscale mapping of photoconductivity and charge trap density in the grain structures of a monolayer molybdenum disulfide (MoS2). In this method, the lateral current and noise maps were measured by scanning the MoS2 surface with a nanoscale conducting contact probe. The measured data were analyzed to obtain the sheet resistance and charge trap density maps in the MoS2 grain structures. Interestingly, the sheet resistance was found inversely proportional to the charge trap density in the grains. It was explained by the sulfur vacancies working as both charge hopping sites and traps in the MoS2 monolayer. When illuminated with a light, the regions with a relatively high trap density showed a high photoconductivity. These results indicate that high photoconductivity was enhanced by charge traps. This method can be a powerful tool for the basic research about noises and the device applications based on two-dimensional materials. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J51.00006: Effect of light-matter interaction on the ballistic conductance of an irradiated dice material Dipendra Dahal, Godfrey Gumbs We derived closed-form analytic expressions for the transmission coefficient of quasiparticles impinging on a square electrostatic potential barrier and step formed by split gates on a dice material which is irradiated by circularly polarized light. A consequence of modifying the states by the dressing field is to suppress Klein tunneling for head-on collision. Our results for transmission are employed in the Landauer-Buttiker formalism to examine the ballistic transport properties for varying barrier widths, incident quasiparticle energy, and light intensity. A comparison is made between our results for dressed and undressed states. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J51.00007: Spatially resolved photoconductivity mapping of MoS2 / WS2 lateral heterostructures Samuel Berweger, Hanyu Zhang, Prasana K Sahoo, Benjamin M Kupp, Jeffrey L Blackburn, Elisa M Miller, Dmitri Voronine, Thomas M Wallis, Pavel Kabos, Sanjini U Nanayakkara The presence of free carriers directly affects the optical properties of transition metal dichalcogenides. For instance, free carriers directly underpin the competition between neutral excitons and carrier-bound trions and they strongly affect radiative and nonradiative decay pathways. However, despite their fundamental importance for TMDs, spatially resolved studies of the carrier distribution and the associated photoresponse remain challenging. Here we use scanning microwave microscopy (SMM, also called microwave impedance microscopy, MIM) to study the spatial carrier distribution in MoS2/WS2 lateral heterostructures under dark conditions as well as under illumination with photon energy-resolved narrowband illumination. We find strong spatial variations in the photoconductive response throughout the flakes studied. We further find significant long-term dynamics in the optically generated free carriers that manifest themselves in persistent charging and discharging and are strongly correlated with observed spatial variations in the steady-state photoconductive response. A comparison with spatially resolved photoluminescence mapping reveals excellent agreement between the local conductivity determined by SMM and the interplay between trion and exciton emission characteristics. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J51.00008: Controllable beam slicing through fractional phase shifts accumulated at an atomically-thin interface Myungjae Lee, Fauzia Mujid, Andrew Ye, Jiwoong Park The interface between different media has been one of the crucial sources of light–matter interactions where properties of light, such as intensity, phase, or polarization, may change. One previously unexplored regime is when light propagates parallel to and bisected by the interface. Here, we realized an optically suspended and atomically-thin interface based on wafer-scale monolayer MoS2 on fused silica that is immersed inside index-matching liquid. With this system we observed that a beam propagating along the interface is sliced into two sub-beams leaving a node at the interface. We quantified the upper and lower bounds of absorption and concluded that the atomically-thin interface behaves like a partial mirror that reflects light with an additional phase shift, which results in self-interference with the incident beam. In addition, we found that the degree of interference can be tuned by using interfaces with different number of MoS2 layers. Our results indicate that the value of phase shift by a monolayer is a fraction of π, not an integer of π, which is unexpected from classical interfaces. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J51.00009: Engineering the exciton spontaneous emission time in MoSe2 monolayer Bo HAN, Cédric Robert, Honghua FANG, Marina Semina, Delphine Lagarde, Emmanuel Courtade, Takashi Taniguchi, Kenji Watanabe, Thierry Amand, Bernhard Urbaszek, Mikhail Glazov, Xavier Marie Encapsulation of monolayers such as MoSe2 in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. In time-resolved photoluminescence measurements we demonstrate that the exciton radiative lifetime in the MoSe2 monolayer can be tuned by a factor 10 (typically from 1 to 10 ps) as a function of the hBN thickness. This variation is a consequence of the Purcell effect and it is in very good agreement with the calculated dependence using transfer matrix techniques [1]. We show that the exciton linewidth measured in cw photoluminescence can be controlled as well. The variation of the charged exciton lifetime due to cavity-like effects will also be discussed. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J51.00010: Optical Switching Based on a Single Layer of WSe2 Zheng Sun, Ke Xu, Jonathan Beaumariage, Jierui Liang, Susan Fullerton-Shirey, David Wayne Snoke Two-dimensional materials are an emerging class of new materials with a wide range of electrical and optical properties and potential practical applications. Single layers materials such as semiconducting transition metal dichalcogenides with MX2stoichiometry, where M is a transition metal element from group VI (M=Mo, W) and X is a chalcogen (X=S, Se, Te) are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Here, we report a type of optical switching based on a single-layer WSe2 transistor. A side gate controls the polarization of the ions in PEO:CsClO4, after which we can turn on and off the photoluminescence using a back gate. An on-off ratio of 90 is observed under constant pump light intensity. We believe the “off” and “on” states originate from the net electric field produced by the polarized ions and the back-gate, which aligns the dipole within the WSe2 either in the same way or in the opposite way. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J51.00011: Excitonic Complexes in a Charge Tunable WSe2 Monolayer Device Cedric ROBERT, Emmanuel Courtade, Delphine Lagarde, Takashi Taniguchi, Kenji Watanabe, Bernhard Urbaszek, Thierry Amand, Xavier Marie Transition Metal Dichalcogenides (TMD) monolayers have emerged as exciting 2D materials for optoelectronics and spintronics. Due to the large exciton binding energies and strong spin-orbit coupling, these materials can host various and robust excitonic complexes. The encapsulation of TMD monolayers into hBN combined with the constant improvement of the material quality now enable to reproducibly observe and control these excitonic complexes in the optical spectra. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J51.00012: Widely Tunable Mid-Infrared Light Emission in Thin-Film Black Phosphorus Chen Chen, Xiaobo Lu, Bingchen Deng, Xiaolong Chen, Qiushi Guo, Cheng Li, Chao Ma, Shaofan Yuan, Eric Sung, Kenji Watanabe, Takashi Taniguchi, Li Yang, Fengnian Xia In this work, we report the widely tunable mid-infrared light emission from dual-gate hexagonal boron nitride (hBN)/Black Phosphorus (BP)/hBN heterostructure devices. The photoluminescence (PL) from a ~20-layer BP flake can be continuously tuned from 3.7 to 7.7 μm with a moderate displacement field of 0.48 V/nm, spanning 4 μm in mid-infrared. The PL emission remains perfectly linear-polarized regardless of the bias field due to the preservation of the optical transition rule. We further performed first-principles calculation to investigate the gate dependence of the band structure and optical conductivity in BP. Moreover, together with theoretical analysis we show that the radiative decay probably dominates over other nonradiative decay channels in the PL experiments. Our results reveal the great potential of thin-film BP in widely tunable, mid-infrared light emitting and lasing applications. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J51.00013: Designing and probing electronic structures of a scalable transition metal dichalcogenide heterostructure: MoS2 monolayers on MoSe2 multilayers. Woojoo Lee, Li-Syuan Lu, Wen-Hao Chang, Chih-Kang Shih The emergence of van der Waals (vdW) 2D electronic materials and their heterostructures, especially the semiconducting family of transition metal dichalcogenides (TMD), has opened a new frontier of science and technology. Although interesting novel devices have been created using these 2D electronic materials and heterostructure, they are created only as discrete single devices. Namely, they are not scalable. A scalable material platform must satisfy the following conditions: (a) the ability to create single continuous layer of a continuous area of the same orientation, (b) the ability to stack dissimilar semiconducting layers to form wafer size heterostructures that are laterally homogeneous. Here we report successful development of a wafer size monolayer, quasi continuous and single orientation (but containing twins) platform of TMD that can be exfoliated and transferred to create wafer size TMD semiconducting heterostructures (in this case, monolayer MoS2 on multilayer MoSe2) with atomically clean interface. We use angle resolved photoemission to probe the electronic structure of such heterostructure and their respective constituents, revealing evidence of strong hybridization near the zone center. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J51.00014: Emergent optical properties in crystalline, semiconducting 2D covalent organic framework / TMD heterostructures Halleh Balch, Austin Evans, Raghunath Dasari, Ruofan Li, Simil Thomas, Hong Li, Danqing Wang, Jean-Luc E Bredas, Seth R. Marder, Daniel C. Ralph, William Dichtel, Feng Wang Two dimensional covalent organic frameworks are a new class of Van der Waals materials formed of periodic, covalently-bound lattices of planar organic molecules. The symmetry, lattice constant, optical, and electronic properties of 2D COFs can be controlled via choice of molecular constituents, which give rise to lattice properties not present in the component parts. To date, 2D COFs have been limited by large bandgaps, low coupling, and poor control of material morphology. Here, we present results on a new semiconducting 2D covalent organic framework and the unusual optical properties that emerge in COF / TMD heterostructures and when exfoliated to few-layer sheets. We characterize the structure by X-ray scattering, TEM, AFM, transport, and optical spectroscopy, and demonstrate facile manipulation onto arbitrary experimental platforms. Our data demonstrates that the formation of a highly crystalline and semiconducting 2D COF permits thickness-dependent optical properties not previously observed in 2D covalent organic frameworks. We further demonstrate thickness-dependent energy transfer dynamics in semiconducting COF / TMD heterostructures and outline directions of future research. |
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J51.00015: Sub-picosecond hot electron transfer in WS2/hBN/p-Si hybrid structure revealed by energy- and time-resolved photoemission electron microscopy Yaolong Li, Yunan Gao, Xiaoyong Hu, Qihuang Gong Carrier transfer plays a central role in all optoelectronic applications. It can occur much faster in low dimensional materials than in conventional bulk materials, which can be exploited in developing ultrafast and high-efficient optoelectronic devices. Here, we report an ultrafast hot electron transfer study in a hybrid structure of monolayer WS2 on p-type silicon substrate separated by thin layer of hexagonal boron nitride (hBN). We studied the ultrafast electron transfer dynamics by an energy- and time-resolved photoemission electron microscopy, and determined that photoexcited electrons transfer to p-type silicon via two paths of a direct hot-electron transfer on a sub-picosecond timescale and another one of intra-band carrier cooling and subsequent electron transfer on a timescale of a few picoseconds. The transfer rate and the relative weight of the two paths can be quantitatively determined, which depends on excitation wavelength and the thickness of the hBN layer. |
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