|Yihong Wu, PhD Kyoto|
Department of Electrical and Computer Engineering
National University of Singapore
Tel: (65) 6516-2139; Fax: (65) 6779-1103; Room: E4-05-19
E-mail : firstname.lastname@example.org
EE4433 Nanometer Scale Information Storage (Semester I)
EE5508 Semiconductor Fundamentals (Semester I)
EE6438 Magnetic Materials and Devices (Semester II)
EE1001 Emerging Technologies in EE (Semester I)
EE1001E Emerging Technologies in EE (Semester I)
TE2002 Engineering Mathematics II (Semester I)
EE5209 Physics and Modeling of Spin Electronic Devices (changed to Nanoelectronics in 2005)
EE5202 Information Storage Materials and Devices (changed to Nanometer Scale Information Storage in 2005)
EE6504 Nanoscale Engineering
EE4409 Principle and Practice of Optical Data Storage
EE2461 Engineering Mathematics II
EG1108 Electrical Engineering
Current Group Members (Gallery)
Yang Yumeng, Chen Zhixian, Qi Long, Xu Yanjunm, Luo Ziyan
Research Fellow and Research Engineer:
Wang Ying, Wang Xiaowei, Yang Yumeng
Awards received by past and current group members
Liu Tie received the Best Poster Award in ICMAT 2005 and a Poster Award (3rd prize) in MRS-S Conference on Advanced Materials 2006..
Awards received by collaborators
G. C. Han, E. L. Tan, B. Y. Zong, K. B. Li, B. Liu and Y. H. Wu, “Temperature dependence of thermally activated ferromagnetic resonance in tunneling magnetoresistive heads”, Asia-Pacific Data Storage Conference, 28-30 August 2006, Hsin Chu, Taiwan (APDSC’06 Outstanding Poster Award).
"Two-Dimensional Carbon - Fundamental
Properties, Synthesis, Characterization, and Applications", Pan Stanford
series on carbon-based nanomaterials, 2014
Y. M. Yang, Y.J. Xu, X.S. Zhang, Y. Wang, S.F. Zhang, R.-W. Li, M. S. Mirshekarloo, K. Yao, and Y.H. Wu, "Current-driven spin canting in Pt/FeMn bilayers”, Phys. Rev. B 93, 094402 (2016).
L. Qi, Y. Wang, L. Shen, and Y. H. Wu, "Chemisorption-induced n-doping of MoS2 by oxygen", Appl. Phys. Lett. 108, 063103 (2016).
Y. Wang, J. Chai, S. Wang, L. Qi, Y. Yang, Y. Xu, H. Tanaka, and Y. Wu, “Electrical oscillation in Pt/VO2 bilayer strips,” J. Appl. Phys.117, 064502 (2015).
Ying Wang, Yumeng Yang, and Yihong Wu, "Dynamic control of local field emission current from carbon nanowalls", J. Vac. Sci. Technol. B 32, 051803 (2014).
Ying Wang, Yumeng Yang, Zizheng Zhao, Chi Zhang, and Yihong Wu, "Local electron field emission study of two-dimensional carbon", Appl. Phys. Lett. 103, 033115 (2013).
Wu, Y.H., Wang, Y., Wang, J.Y., Zhou, M., Zhang, A.H., Zhang, C., Yang, Y.J., Hua, Y.N. and Xu, B.X. (2012). Electrical transport across metal/two-dimensional carbon junctions: Edge versus side contacts, AIP Advances, 2. pp.012132
Zhang, C., Wang, Y., Wu, B. and Wu, Y., "Enhancement of spin injection from ferromagnet to graphene with a Cu interfacial layer", Appl Phys Lett, 101, pp. 022406, 2012
Wu, B.L., Wu, Y.H. and Qiu, J.J. (2012), "Instability of exchange bias induced by an overlaid superconductor tab in antiferromagnet bilayers", Appl Phys Lett, 100.pp.242602
Wu, Y.H., Wang, H.M. and Choong, C. (2011). Growth of two-dimensional carbon nanostructures and their electrical transport properties at low tempertaure, Japanese Journal of Applied Physics, 50, pp. 01AF02.
Bakaul, S.R., Wu, B.L., Han, G.C. and Wu, Y.H. (2011). Effect of frozen magnetic flux on the electrical transport characteristics of superconductor-ferromagnet junction, Journal of Superconductivity and Novel Magnetism, 24, pp. 951-955.
Wu, Y.H., Yu, T. and Shen, Z.X. (2010). Two-dimensional carbon nanostructures: Fundamental properties, synthesis, characterization, and potential applications, J Appl Phys, 108..071301
H. M. Wang, Y. H. Wu, Z. H. Ni, and Z. X. Shen, “Electronic transport and layer engineering in multilayer graphene structures”, Appl. Phys. Lett. 92, 053504 (2008).
Y. Y. Wang, Z. H. Ni, Z. X. Shen, H. M. Wang, and Y. H. Wu, “Interference enhancement of Raman signal of graphene”, Appl. Phys. Lett. 92, 043121 (2008).
Haomin Wang, Catherine Choonga, Jun Zhang, Kie Leong Teo and Yihong Wu, "Differential conductance fluctuation of curved nanographite sheets in the mesoscopic regime", Solid State Comm. 145, 341-345 (2008).
T. Dietl, T. Andrearczyk, A. Lipińska, M. Kiecana, Maureen Tay, and Yihong Wu, "Origin of ferromagnetism in Zn1−xCoxO from magnetization and spin-dependent magnetoresistance measurements ", Phys. Rev. B 76, 155312 (2007).
Z. H. Ni, H. M. Wang, J. Kasim, H. M. Fan, T. Yu, Y. H. Wu, Y. P. Feng and Z. X. Shen, "Graphene Thickness Determination Using Reflection and Contrast Spectroscopy", Nano Lett. 7, 2758-2763 (2007).
Kebin Li, Yihong Wu, Zaibing Guo, Yuankai Zheng, Guchang Han, Jinjun Qiu, Ping Luo, Lihua An, and Tiejun Zhou, "Exchange Coupling and Its Applications in Magnetic Data Storage", J. Nanosci. Nanotechnol. 7, 13–45 (2007) (Review).
Yuankai Zheng, Yihong Wu, Kebin Li, Jinjun Qiu, Guchang Han, Zaibing Guo, Ping Luo, Lihua An, Zhiyong Liu, Li Wang, Seng Ghee Tan, Baoyu Zong, and Bo Liu, "Magnetic Random Access Memory (MRAM)", J. Nanosci. Nanotechnol. 7, 117–137 (2007) (Review)
M. Tay, Y. H. Wu, G. C. Han, T. C. Chong, Y. K. Zheng, S. J. Wang, Y. B. Chen and X. Q. Pan, "Ferromagnetism in inhomogeneous Zn1–xCoxO thin films", J. Appl. Phys. 100, 063910 (2006).
T. Liu, Y. H. Wu, Z. L. Zhao, Y. K. Zheng and A. O. Adeyeye, "Transport properties and micromagnetic modeling of magnetic nanowires with multiple constrictions", Thin Solid Films, 505 (1-2): 35-40 MAY 18 2006.
KB Li, Wu YH, Han GC, Qiu JJ, Zheng YK, Guo ZB, An LH, Luo P, "Electrical and magnetic properties of nano-oxide added spin valves", Thin Solid Films, 505 (1-2): 22-28 MAY 18 2006.
G.C. Han, Li KB, Zheng YK, Qiu JJ, Luo P, An LH, Guo ZB, Liu ZY, Wu YH, "Fabrication of sub-50 nm current-perpendicular-to-plane spin valve sensors", Thin Solid Films, 505 (1-2): 41-44 MAY 18 2006.
P. Esquinazi, D. Spemann, K. Schindler, R. Höhne, M. Ziese, A. Setzer, K.-H. Han, S. Petriconi, M. Diaconu, H. Schmidt, T. Butz and Y.H. Wu, “Proton irradiation effects and magnetic order in carbon structures”, Solid Films 505, 85-89 (2006).
Li HL, Wu YH, Guo ZB, Wang SJ, Teo KL, Veres T, “Effect of antiphase boundaries on electrical transport properties of Fe3O4 nanostructures”, Appl. Phys. Lett. 86, 252507 (2005).
Li KB, Qiu JJ, Han GC, Guo ZB, Zheng YK, Wu YH, Li JS, “Effect of capping layer on interlayer coupling in synthetic spin valves”, Phys. Rev. 71, 014436 (2005).
Wang L, Qiu JJ, McMahon WJ, Li KB and Wu YH, “Nano-oxide-layer insertion and specular effects in spin valves: Experiment and theory”, Phys. Rev. B 69, 214402 (2004).
Wang L, McMahon WJ, Liu B, Wu YH, Chong CT, “Interface or bulk scattering in the semiclassical theory for spin valves”, Phys. Rev. 69 (21): Art. No. 214403 JUN 2004.
Wu YH, Yang BJ, Zong BY, Sun H and Feng YP, “Carbon nanowalls and related materials”, Journal of Materials Chemistry 14, 469-477 (2004) (Feature article).
Han GC, Wu YH, Tay M, Li KB, and Chong TC, “Epitaxial growth of ferromagnetic Co:TiO2 thin films by co-sputtering”, J. Magn. Magn. Mat. 268, 159-164 (2004).
Wu YH, “Nano Spintronics for Data Storage”, Encyclopedia for Nanoscience and Nanotechnology, vol.7, 493-544, American Scientific Publishers, 2003.
Wu YH, Shen YT, Liu ZY, Li KB, and Qiu JJ, “Point dipole response from a magnetic force microscopy tip with a synthetic antiferromagnetic coating”, Appl. Phys. Lett. 82, 1748-1750 (2003).
Han GC, Zong BY, Luo P, and Wu YH, “Angular dependence of the coercivity and remanence of electrodeposited nano-wire”, J. Appl. Phys. 93, 9202-9207 (2003).
Zheng YK, Wu YH, Qiu JJ, Li KB, Guo ZB, Han, GC, An LH, Luo P, You D, Liu ZY, and Shen YT, “A low-switching-current flux-closed magneto-resistive random access memory”, J. Appl. Phys. 93, 7307-7309 (2003).
Guo ZB, Zheng YK, Li KB, Liu ZY, Shen YT, and Wu YH, “Observation of a flux closure state in NiFe/IrMn exchange biased rings”, J. Appl. Phys. 93, 7435-7437 (2003).
Wu YH, Qiao PW, Chong TC, and Shen ZX, “Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition”, Adv. Mater. 14, 64-67 (2002).
Wu YH, Li KB, Qiu JJ, Guo ZB, and Han GC, “Antiferromagnetically coupled hard/Ru/soft layers and their applications in spin valves”, Appl. Phys. Lett. 80, 4413-4415 (2002).
Liu ZY, Dan Y, Qiu JJ, and Wu YH, “Magnetic force microscopy using focused ion beam sharpened tip with deposited antiferro-ferromagnetic multiple layers”, J. Appl. Phys. 91, 8843-8845 (2002).
Shen YT, Wu YH, Xie H, Li KB, Qiu JJ, and Guo ZB, “Exchange bias of patterned NiFe/IrMn film”, J. Appl. Phys. 91, 8001-8003 (2002).
Wu YH, Yang BJ, Han GC, Zong BY, Ni HQ, Luo P, Chong TC, Low TS, and Shen ZX, “Fabrication of a class of nanostructured materials using carbon nanowalls as the templates”, Adv. Funct. Mater.12, 489-494 (2002).
Wu YH, Qiao PW, Qiu JJ, Chong TC, and Low TS, “Magnetic nanostructures grown on vertically aligned carbon nanotube templates”, Nano Letters 2, 161-164 (2002).
Shen YT, Wu YH, Chong TC, Xie H, Guo ZB, Li KB, and Qiu JJ, “Asymmetry diffraction magneto-optical phenomenon of NiFe grating”, Appl. Phys. Lett. 79, 2034-2036 (2001).
Li KB, Wu YH, Qiu JJ, Han GC, Guo ZB, Xie H, and Chong TC , “Suppression of interlayer coupling and enhancement of magnetoresistance in spin valves with oxide layer”, Appl. Phys. Lett. 79, 3663-3665 (2001).
Zheng YK, Wang XY, You D, and Wu YH, “Switch-free read operation design and measurement of magnetic tunnel junction magnetic random access memory arrays”, Appl. Phys. Lett. 79, 2788-2790 (2001).
Wu YH, Arai K, and Yao T, “Temperature dependence of the photoluminescence of ZnSe/ZnS quantum-dot structures”, Phys. Rev. B 53, 10485-10488 (1996).
Wu YH, “Structure-dependent threshold current-density for CdZnSe-Based II-VI semiconductor-lasers”, IEEE J. Quan. Elect. 30, 1562-1573 (1994).
Wu YH, Khoo H, and Kogure T, “Read-only optical disk with superresolution”, Appl. Phys. Lett. 64, 3225-3227 (1994).
Kawakami Y, Yamaguchi S, Wu YH, Ichino K, Fujita S, Fujita S, “Optically pumped blue-green laser operation above room-temperature in Zn0.80Cd0.20Se-ZnS0.08Se0.92 multiple quantum-well structures grown by metalorganic molecular-beam epitaxy”, Jpn. J. Appl. Part 2 - Lett.30 (4A): L605-L607 APR 1 1991.
Wu YH, Kawakami Y, Fujita S, Fujita S, “Growth and optical-properties of novel wide-band-gap strained-layer single quantum-wells - Zn1-yCdySe/ZnSxSe1-x”, Jpn. J. Appl. Part 2 - Lett.30 (4A): L555-L557 APR 1 1991.
My past research activities centered on nanostructured materials / devices and their applications in data storage (see Special Issue on Nanotechnology for Information Storage: J. Nanosci. Nanotechnol. 7 (1), (2007)). From 1986 to 1999, I worked on semiconductor nanostructures, optoelectronics and optical data storage. Since 1998, I have been mainly working on nanomagnetism, spintronics, magnetic read sensors and 2D nanomaterials. My current research activities are focused on nanomagnetism / spintronics, 2D carbon and related 2D materials. Below is a brief introduction of selected research topics.
Current-induced spin-orbit torque in antiferromagnet (AFM)
Existing magnetic storage devices such as hard disk drive (HDD) and
magnetic random access memory (MRAM) store information in the
magnetization direction of ferromagnetic (FM) materials. This is a
natural choice because (i) the FM possesses a stable and externally
controllable magnetization direction and (ii) the direction (e.g., in
MRAM) or stray field (e.g., in HDD) of the magnetization can be readily
detected by advanced magnetic sensors. However, these properties of FM
also bring it disadvantages when the devices are becoming smaller and
smaller, such as limitation in packing density due to magnetic crosstalk
arising from the stray field, thermal instability of the FM elements,
and speed limitation due to the magnetization dynamics.
Magnetic interaction at ferromagnetic (FM)/ superconductor (SC) and antiferromagnet (AFM) / SC interfaces
Heterointerfaces between two different types of materials are the basic building blocks for many devices that are crucial for building up the modern information society such as transistors, laser diodes and spin-valve sensors. Apart from the spin-valves, most of these interfaces are formed between materials with ordered atomic lattices but non-ordered charges or spins. For next generation electronic devices, however, materials and interfaces involving collective behaviour or ordered phase of charges / spins will become increasingly important in order to create devices which can offer “more than Moore”. In this context, intensive researches have been carried out on various types of hetero-interfaces between materials with ordered electronic or spintronic phases such as ferromagnet (FM), antiferromagnet (AFM), ferroelectric (FE), superconductor (SC), topological insulator (TI), etc. Among them the AFM/FM and FM/S interfaces have received special attention in the last few decades. The former has already been widely used in magnetic sensors, memory, and recording media, whilst the latter has been investigated intensively as potential building block for superconductor based spintronics. Our researches in this area are focused FM/SC junctions with inhomogeneous magnetization distribution in the FM region such as magnetic disks and AFM/SC heterojunctions.
Spin-injection in graphene
Graphene has attracted special attention in spintronics applications due to its long spin-diffusion length. Like the case of other types of NM materials, spin-injection from FM to graphene is sensitive to the contact resistance between FM and graphene. In directly contacted FM/graphene junctions with a low interface resistance, the spin-injection efficiency is around 1%, which translates into a non-local magnetoresistance of 1 mΩ to 100 mΩ. A commonly adopted approach to mitigating the resistivity mismatch problem is to introduce a tunnelling barrier between FM and graphene. Spin injection efficiency between 2% to 35% and nonlocal MR ranging from 10 Ω to 130 Ω have been obtained. Despite being much more spin efficient, the resistance of tunnelling contact is significantly higher than that of transparent contact due to the non-conducting oxide layer; this may severely limits their on-current and high-frequency applications. Aiming at reducing the contact resistance and at the same time without sacrificing much the spin injection efficiency, recently we have proposed and studied FM/Cu bilayer as a possible spin injector for graphene-based spintronic devices. This was motivated by two factors: (1) the FM/Cu interface has been well established as an excellent spin-active interface in spin-valves 25 and (2) the bonding between Cu and graphene is of physisorption nature which helps preserve the intrinsic properties of graphene. Currently, we are working on dynamic spin injection using the FM/Cu contact.
Development of a combined nanofabrication / characterization tool for research on nanometer scale spintronics
Electron as a negatively charged elementary particle was discovered in 1897. About two decades later, photon (1922) and spin (1925) were discovered. Each of them has played a central role in the development of microelectronics, optoelectronics and magnetics industry. Although charge, photon and spin are deeply connected with one another, they have been used more or less “independently” by human being to create the infocom industry of the 20th century. Looking forward, a deep understanding on how these three interact with one another on the nanoscale is of great importance for creating devices with new functionalities. Although both single charge and single spin detection techniques have become available recently, they are still being probed “independently” in both the time and spatial domains, except for the case of vary small systems such as quantum dots. A true simultaneous detection technique for probing the spin-charge interactions in “real time” is a necessity for increasing our understanding of spin-charge interactions. To this end, we are currently developing a combined nano-fabrication and characterization tool consisting of (i) a scanning electron microscope with spin-polarization analysis (SEMPA) (ii) a scanning tunnel microscope (STM) or spin-dependent STM (SPSTM) (iii) four nano-probes (including the STM probe) (iv) a focused ion beam (FIB) (v) a sample preparation and fabrication chamber with variable temperature and magnetic field features.
- The SEMPA in this system will be used to study surface magnetism of various types of magnetic, half-metallic, and magnetic semiconductor thin films and nanostructures at nanometer scale.
- The STM and SPSTM will be used for atomic scale electronic and spin state detection applications.
- The nano-probes will be used to study the transport properties of various types of nanostructures including nanowires, nanotubes, nano-rings of magnetic, semiconductor and biological materials and the associated devices.
- The combination of SEM with STM ensures that the above-mentioned experiments can be performed at controlled positions with a nanometer scale spatial accuracy.
- The combination of SEMPA with the four nano-probes makes some of the measurements which so far are impossible an attainable task, such as in-situ study of spin-injection and spin transfer in magnetic nanostructures.
- The inclusion of an FIB makes it possible to perform in-situ modification and fabrication of nanostructures while performing magnetic and electrical measurements.
Development of new MFM tips
Magnetic force microscopy (MFM) has been and continues to be one of the primary imaging tools for studying magnetic nanostructures. There are two major issues with the MFM which have been addressed frequently during the last decade: (i) tip-sample interaction and (ii) moderate resolution. Although many techniques have been proposed and developed to resolve these two issues, the success still remains moderate. Most of these techniques are based on the modification of the MFM tips one at a time which suffer from a very low yield and poor reproducibility. To address this issue, in the last 2-3 years, we have proposed and verified experimentally a novel type of synthetic magnetic force microscopy tip which does not only allow for batch fabrication but also exhibits a resolution which approaches the theoretical limit. The key to achieving a superior performance over commercial tips is the introduction of a new tip coating structure which allows the tip to function as a point dipole in the as-fabricated form. The usefulness of the new tip has been tested based on a small number of prototypes. Currently, we are exploring the possibility of commercializing the tips and at the mean time using the new tips to study magnetic nanostructures. (info about different types of MFM tips).
Although the recent work on nanomaterials has been focused on the 0D and 1D systems, it was in the 2D system where the top-down approach of nanotechnology has been developed. This has led to the discovery of quantum Hall effect and the creation of new devices such as high electron mobility transistors, inter-sub-band infrared detectors and quantum cascade lasers in semiconductor systems and the discovery of giant magnetoresistance and invention of spin-valves in metallic systems. The work on the 2D systems had also become the foundation for the subsequent work on 1D and 0D systems. The above-mentioned 2D systems are obtained by the so-called top-down approach. Most of these 2D systems were realized in laminar structures of semiconductors, insulators and metals. In addition to these artificially structured 2D systems, there are also many naturally formed 2D systems such as graphite, MgB2, transition metal dichalcogenides, intercalation compounds of graphite, high-Tc superconductors, and many others. The common feature of these materials is that the electrical conduction is highly anisotropic, with a low resistance in the layer plane but a high resistance or being insulating perpendicular to the layers. It is worth noting that most of the layered compounds are also good superconductors. Regardless of whether it is an artificial laminar structure or a naturally formed layered compound, the 2D system formed in such a fashion is not a perfect 2D system in the sense that each 2D layer still has to interact with the adjacent layers either chemically or electronically. An ideal 2D system would be such that it consists of only a single nano-sheet without any electronic or chemical interactions with other types of materials or layers.
We questioned why there is no report on graphene synthesis and reported on the growth of 2D carbon by MPECDV in Adv. Mater. 14, 64-67 (2002). We also pointed out in Journal of Materials Chemistry 14, 469-477 (2004) that graphene can be obtained by "peeling off" carbon layer-by-layer from graphite.
The 2D carbon nano-sheets are very useful not only for fundamental physics studies but also for practical applications due to their large surface-to-volume ratio. In contrast to the closed boundary structure of 0D and 1D materials, the 2D nanosheets are characterized by open boundaries. Theoretical studies have shown that this may bring about nano-sheets unique transport and magnetic properties. We are currently focusing on superconducting instability, point-contact, local-field emission and spintronic properties of graphene and other 2D materials.
Point-contact study of 2D carbon
Local field emission study
Science as Art
Professor Wu Yihong grows nanostructures to study energy. See a slide show of his work here.
Venture shows high-efficiency backlight made with CVD carbon nanowalls (Source: Solid State Technology June, 2005)
Japanese venture Dailight uses carbon nano platelets as efficient emitters for a potential display backlight source. (Source: Dailight)
Monograph on Carbon Nanowalls by Profs Mineo Hiramatsu and Masaru Hori: