Nature Nanotechnology. doi:10.1038/nnano.2017.155 Authors: Giang D. Nguyen, Hsin-Zon Tsai, Arash A. Omrani, Tomas Marangoni, Meng Wu, Daniel J. Rizzo, Griffin F. Rodgers, Ryan R. Cloke, Rebecca A. Durr, Yuki Sakai, Franklin Liou, Andrew S. Aikawa, James R. Chelikowsky, Steven G. Louie, Felix R. Fischer & Michael F. Crommie
Nature Nanotechnology. doi:10.1038/nnano.2017.181 Authors: Yuhang Jiang, Jinhai Mao, Dean Moldovan, Massoud Ramezani Masir, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Francois M. Peeters & Eva Y. Andrei The photon-like propagation of the Dirac electrons in graphene, together with its record-high electronic mobility, can lead to applications based on ultrafast electronic response and low dissipation. However, the chiral nature of the charge carriers that is responsible for the high mobility also makes it difficult to control their motion and prevents electronic switching. Here, we show how to manipulate the charge carriers by using a circular p–n junction whose size can be continuously tuned from the nanometre to the micrometre scale. The junction size is controlled with a dual-gate device consisting of a planar back gate and a point-like top gate made by decorating a scanning tunnelling microscope tip with a gold nanowire. The nanometre-scale junction is defined by a deep potential well created by the tip-induced charge. It traps the Dirac electrons in quantum-confined states, which are the graphene equivalent of the atomic collapse states (ACSs) predicted to occur at supercritically charged nuclei. As the junction size increases, the transition to the optical regime is signalled by the emergence of whispering-gallery modes, similar to those observed at the perimeter of acoustic or optical resonators, and by the appearance of a Fabry–Pérot interference pattern for junctions close to a boundary.
Nature Nanotechnology. doi:10.1038/nnano.2017.161 Authors: Ji Ho Sung, Hoseok Heo, Saerom Si, Yong Hyeon Kim, Hyeong Rae Noh, Kyung Song, Juho Kim, Chang-Soo Lee, Seung-Young Seo, Dong-Hwi Kim, Hyoung Kug Kim, Han Woong Yeom, Tae-Hwan Kim, Si-Young Choi, Jun Sung Kim & Moon-Ho Jo
Nature Nanotechnology. doi:10.1038/nnano.2017.184 Author: Ivan V. Vlassiouk An improved industrial manufacturability has been achieved for a hybrid water-treatment membrane that exhibits high water permeance, prolonged high salt and dye rejection under cross-flow conditions and better resistance to chlorine treatment.
Nature Nanotechnology. doi:10.1038/nnano.2017.160 Authors: Aaron Morelos-Gomez, Rodolfo Cruz-Silva, Hiroyuki Muramatsu, Josue Ortiz-Medina, Takumi Araki, Tomoyuki Fukuyo, Syogo Tejima, Kenji Takeuchi, Takuya Hayashi, Mauricio Terrones & Morinobu Endo
Nature Nanotechnology. doi:10.1038/nnano.2017.149 Authors: Zuocheng Zhang, Xiao Feng, Jing Wang, Biao Lian, Jinsong Zhang, Cuizu Chang, Minghua Guo, Yunbo Ou, Yang Feng, Shou-Cheng Zhang, Ke He, Xucun Ma, Qi-Kun Xue & Yayu Wang The recent experimental observation of the quantum anomalous Hall effect has cast significant attention on magnetic topological insulators. In these magnetic counterparts of conventional topological insulators such as Bi2Te3, a long-range ferromagnetic state can be established by chemical doping with transition-metal elements. However, a much richer electronic phase diagram can emerge and, in the specific case of Cr-doped Bi2(SexTe1−x)3, a magnetic quantum phase transition tuned by the actual chemical composition has been reported. From an application-oriented perspective, the relevance of these results hinges on the possibility to manipulate magnetism and electronic band topology by external perturbations such as an electric field generated by gate electrodes—similar to what has been achieved in conventional diluted magnetic semiconductors. Here, we investigate the magneto-transport properties of Cr-doped Bi2(SexTe1−x)3 with different compositions under the effect of a gate voltage. The electric field has a negligible effect on magnetic order for all investigated compositions, with the remarkable exception of the sample close to the topological quantum critical point, where the gate voltage reversibly drives a ferromagnetic-to-paramagnetic phase transition. Theoretical calculations show that a perpendicular electric field causes a shift in the electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and, in turn, a magnetic phase transition.
Nature Nanotechnology. doi:10.1038/nnano.2017.68 Authors: Chuan Zhao, Tenzin Norden, Peiyao Zhang, Puqin Zhao, Yingchun Cheng, Fan Sun, James P. Parry, Payam Taheri, Jieqiong Wang, Yihang Yang, Thomas Scrace, Kaifei Kang, Sen Yang, Guo-xing Miao, Renat Sabirianov, George Kioseoglou, Wei Huang, Athos Petrou & Hao Zeng Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2 meV T–1). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12 T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.
Nature Nanotechnology. doi:10.1038/nnano.2017.9 Author: Manuel Melle-Franco Triangulene, an elusive open-shell magnetic molecule, is synthesized and characterized by electron microscopy.
Nature Nanotechnology. doi:10.1038/nnano.2017.100 Authors: Ang-Yu Lu, Hanyu Zhu, Jun Xiao, Chih-Piao Chuu, Yimo Han, Ming-Hui Chiu, Chia-Chin Cheng, Chih-Wen Yang, Kung-Hwa Wei, Yiming Yang, Yuan Wang, Dimosthenis Sokaras, Dennis Nordlund, Peidong Yang, David A. Muller, Mei-Yin Chou, Xiang Zhang & Lain-Jong Li Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.
Nature Nanotechnology. doi:10.1038/nnano.2017.108 Authors: Matthias Kühne, Federico Paolucci, Jelena Popovic, Pavel M. Ostrovsky, Joachim Maier & Jurgen H. Smet
Nature Nanotechnology. doi:10.1038/nnano.2017.129 Authors: Jie Zhao, Guangmin Zhou, Kai Yan, Jin Xie, Yuzhang Li, Lei Liao, Yang Jin, Kai Liu, Po-Chun Hsu, Jiangyan Wang, Hui-Ming Cheng & Yi Cui
Nature Nanotechnology. doi:10.1038/nnano.2017.130 Author: Kian Ping Loh Two independent methods using near-field coupling to surface plasmon polaritons and magnetic brightening allow the observation of dark excitons in WSe2.
Nature Nanotechnology. doi:10.1038/nnano.2017.105 Authors: Xiao-Xiao Zhang, Ting Cao, Zhengguang Lu, Yu-Chuan Lin, Fan Zhang, Ying Wang, Zhiqiang Li, James C. Hone, Joshua A. Robinson, Dmitry Smirnov, Steven G. Louie & Tony F. Heinz
Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons
Nature Nanotechnology. doi:10.1038/nnano.2017.106 Authors: You Zhou, Giovanni Scuri, Dominik S. Wild, Alexander A. High, Alan Dibos, Luis A. Jauregui, Chi Shu, Kristiaan De Greve, Kateryna Pistunova, Andrew Y. Joe, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Mikhail D. Lukin & Hongkun Park Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom. Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication.