Group Update

(updated Nov 20, 2017)

Ms Wang Hongli will have her Ph.D thesis oral defense on 10:00 am, Nov 21, 2017, at E1-06-07. Her thesis title is:"Nanoscale Characterization of the Piezoelectric and Ferroelectric Properties of PZN-PT Single Crystals by Scanning Probe Microscopy Techniques". The abstract of her thesis is show below:

Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3 - x%PbTiO3 (PZN-PT) have been attracting considerable attention for their widespread applications in non-volatile random access memories (NVRAMs), microelectromechanical systems (MEMS), and high-performance transducers, sensors and actuators due to their superior piezoelectric and ferroelectric properties. Compared to state-of-the-art lead zirconate titanate (PZT) ceramics, PZN-PT single crystals exhibit excellent piezoelectric and electromechanical properties (i.e., with d33>2000pc/N, and k33 >90%). This has stimulated remarkable motivation to investigate the piezoelectric and ferroelectric properties, especially the nanoscale domain structure and evolutions. As Scanning Probe Microscopy (SPM) based techniques provide useful and effective techniques for local structure and property characterization, this thesis is focused on using various SPM techniques to characterize the nanoscale domain and their evolution under mechanical, electrical and thermal fields. In the first part of this project, the piezoelectric properties and domain structure of PZN-PT single crystals are studied by using piezoresponse force microscopy (PFM). The local PFM images along x, y and z axis (3D) have been measured to identify the possible polarization orientations. In the second part, the ferroelectric properties are studied by using Piezoresponse Force Spectroscopy (PFS) and Switching Spectroscopy PFM (SS-PFM). The results show that the imprint is polarization direction dependent for both as-polished and poled samples. Since imprint is an indication of internal field, therefore, internal field is dependent of the polarization states of PZN-PT single crystals. Domain growth kinetics has also been studied, and it found that the domain size scales linearly and approximately logarithmically with pulse duration and pulse duration, respectively. In the third part, surface potential and relaxation of PZN-PT single crystals are studied. The results confirm the existence of the screening layer on the surface of the material. Comparing the relaxation behavior in ambient air with that in argon gas, we find that the relaxation time in argon is much shorter than that in air, and the relaxation in air is dominated by the space charge transport phenomena. In the fourth part, the domain structure and piezoelectric response of PZN-PT as functions of temperature, flexural stress or different humidity are investigated. By measuring the 3D domain structure at different temperatures, it is found that certain polarization rotation routes are preferred for phase transition of PZN-PT single crystals with different compositions.  The results also show that the piezoresponse decreases under flexural stress.  Besides, the piezoresponse under different humidity reveals that the piezoresponse increases with increasing humidity. The hysteresis loops under different humidity demonstrates that the coercive bias decrease with higher humidity. These results can be attributed to the formation of water layer in the top end of the cantilever tip. The finding of this project are of considerable importance for a detail understanding of PZN-PT single crystals behaviors, which is crucial for the enhancement of reliability and advancement of piezoelectric and ferroelectric device applications. Finally, based on the findings of the present work, possible reasons for large scatters in the reported properties of these relaxor single crystals are discussed. The property matrices generated for differently poled PZN-PT single crystals in the present work can be directly used for the device design.
(updated Nov 9)

Mr. Wang Zhongting has passed his oral qualify exam on Nov 9, 2017, his proposed thesis title is: "SPM techniques and MD simulation on functional properties of lithium ion battery materials".

(updated Nov 8, 2017)

Mr. Yang Peifa, Ph.D student from Beihang University, China, is attached with group from Oct to Jan for three months.

(Updated September 4, 2017)

Dr. Du Chunbao, attached in the group since Sept 2016, has finished his attachment and returned to China for new career, we wish him success in his new career in the future. 

(Updated August 31, 2017)


(Updated August 18, 2017)

We have numbers of new students joint group this year, Welcome all new members of the research group.

Mr. Lu Xin, 2015-16 FYP student with the group, currently Research Engineer at ECE department, joint the group as M.Eng. student from August 1, he will be joint supervised by A/Zeng Kaiyang (ME) and A/Prof. Ho Ghim Wei (ECE) on the project related to pyroelectric properties of PVDF materials.

Mr. Oh Jin An Sam, A*STAR scholarship holder, registered as Ph.D student with NUS Graduate School of Integrated Science and Engineering (NGS) joint the group from August 1, his research project will be All-solid state batteries. The project is a collaboration project with SIMTECH.

Ms. Sun Qiaomei, B.Eng and M.Eng from Nanjing University of Aeronautics and Astronautics (NUAA), China, joint the group as Ph.D student from August 2017. She will work on the project of All-solid state batteries.

Ms. Liu Yue, B.Eng from Wuhan University, M.Eng from Dalian University of Technology, China, joint the groupas Ph.D student from August 2017, She is sponsored by NUS Ring-fenced scholarship, working on the collaboration project with Rolls-Royce Singapore Pte Ltd and SIMTECH.

Mr. Zeng Qibin, B.Eng from Chinese University of Mining, M.Eng from Chinese Academy of Science, joint the group as Ph.D student from August 2017. He is sponsored by the scholarship from MoE Tier2 project.

Ms. Zhang Nan, a Ph.D student from Northwest Polytechnic University, Xi'an, China, sponsored by Chinese Scholarship Council (CSC) joint the group as exchange student in August 2017. Her term with the group is from August 2017 to Dec 2018.

(Updated August 10)

Ms. Yu Bingxue has completed her first month industrial attachment with Oxford Instruments in Shanghai, China in July 2017.

(Updated June 15, 2017)

Ms. Yu Bingxue has passed the written qualify examination for her Ph.D candidature in May 2017. She will go to Oxford Instruments in Shanghai for Industrial Attachment for one month in July 2017. 

(Updated May 12, 2017)

Congratulation to Ms. Lu Wanheng who passed the Ph.D oral examination on April 28, 2017. She is 12 Ph.D student graduated from our group since 2005.

(updated April 12, 2017)

Ms. Lu Wanheng is going to have her Ph.D oral examination on April 28, 2017, 2:00 to 4:00 pm at Blk EA-06-04. Her thesis title is: Characterizing Resistive Switching Phenomena of Binary Transition Metal Oxides by Scanning Probe Microscopy Techniques

The Thesis Abstract is:

As the conventional charge-based non-volatile memories (NVMs), such as the dynamic random access memory (DRAM), have approached to their practical and physical limits, to overcome this problem, many candidates have been developed as the non-charge-based NVMs. One of the promising candidates is the resistance random access memory (ReRAM) which is based on the resistive switching phenomena, i.e., the conversion of the materials’ resistance states stimulated by an external bias. Among the materials exhibiting a resistive switching phenomenon, transition metal oxides have been extensively explored, and their structures and properties significantly vary as the variety of chemical elements and compositions. On the other hand, as continuously miniaturizing and pursing excellent performance of the memories, the design and research of materials are scaling down from macro to micro, and even to the nanoscale. This transition to the nanoscale can shorten the ionic transport length and increase the chemical and electrostatic gradients, consequently blurring the boundary between the physical and electrochemical phenomena. This indicates that at the nanoscale the ionic and electrochemical phenomena under the electrical field are possible and able to change the physical properties of materials dramatically. An excellent example of the nanoionics-based memory is that based on the resistance switch induced by the ionic motions and electrochemical reactions.

In this work, three representative types of binary transitional metal oxides with simple structures and unique properties are selected, including NiO (p-type semiconductor), TiO2 (n-type semiconductor), and VO2 (the conductivity variations can be trigged by multiple external stimulus). By employing the advanced scanning probe microscopy (SPM) techniques, the resistive switching properties of these three kinds of binary transition metal oxides at the nanoscale are studied in detail. Specifically, this work are focused on the following aspects: 1) experimentally evidencing the ionic and electrochemical phenomena during the resistive switching processes in the transition metal oxides; 2) identifying the role of these ionic and electrochemical phenomena in the resistive switching of the transition metal oxides; 3) probing the effects of the defect concentration and the film thickness on the resistive switching of the transition metal oxides; and 4) studying the conductivity variation induced by multiple external stimulus as well as the coupling between these stimulus.

The SPM techniques are ideal characterization tools to study materials’ properties at the nanoscale due to the capability to probe and manipulate the structural, electrical, magnetic, and mechanical functionalities on the nanometer. A series of SPM techniques is applied in this work, including the atomic force microscopy (AFM), conductive atomic force microcopy (C-AFM), Kelvin probe force microscopy (KPFM) and electrochemical strain microscopy (ESM). AFM can provide information on the surface topography; C-AFM is used to measure the resistance or conductivity of various materials; KPFM can show the surface potential changes in the samples by the external electrical field; and ESM is mainly adopted to examine the local ionic/electrochemical phenomena.

The results of resistive switching on NiO approve the migration of the oxygen ions (or oxygen vacancies) as well as a moisture-involved electrochemical reaction during the resistive switching process.  This moisture-involved electrochemical reaction plays a determinative role in the resistive switching behavior of NiO: no resistive switching behavior can be observed in the environments without moisture (such as synthetic air or Argon). NiO deposited at lower oxygen partial pressure (with a lower concentration of the oxygen vacancies or defects) shows a higher conductivity when it is switched into the low resistance state. NiO does not show any resistive switching behavior when the film thickness was less than 10 nm. While the film thickness is more than 10 nm, NiO exhibits the resistive switching behavior.

For TiO2 samples, either in a vertical or a planar configuration, the migration of oxygen vacancy and the moisture-related electrochemical reaction are found to be associated with the resistive switching behaviors. A higher concentration of the oxygen vacancies during the TiO2 deposition (with lower oxygen partial pressure) show a bipolar resistive switching. It is found that the film thickness has a limited impact on the resistive switching of TiO2.

For VO2 samples, multiple external stimulus, including mechanical stress, thermal field and electrical field can cause the changes of conductivity: 1) increasing the mechanical stress improve the conductivity of VO2; 2) when the temperature is above 70 , the conductivity of VO2 sharply increases; 3) magnifying the voltage firstly increases the conductivity but changes VO2 into the high resistance state when the voltage higher than 5 V. Besides, the nucleation and growth processes of the phase transition that are contributed to conductivity variations are tracked in this work.

Results in this work have illustrated the resistive switching properties of transition metal oxides not only at the nanoscale but also from the point of view of the ionic and electrochemical processes, which will contribute to a full understanding of the resistive switching of transition metal oxides and constitute one part of the researches on the transition metal oxides in our research group. Although recently more attention and concerns are being attracted on the SPM research of the ionic and electrochemical phenomena, especially in the area of energy storage and conversion materials, the SPM studies are rare in terms of resistive switching of transition metal oxides. Therefore, combining various SPM techniques to reveal the presence and the role of the ionic and electrochemical phenomena in the resistive switching processes of transition metal oxides is one of the features of this thesis. In addition, the work in this thesis evidences the advantages of the SPM techniques in characterizing the resistive switching behavior driven by multi stimulus.

(updated March 16, 2017)

Ms Chen Jie and Ms Tian Tian have completed their attachments at the group and went back to their home universities, respectively, We wish them successful in their future research.

(Updated Jan 19, 2017)


(updated Dec 19)

Mr. Wang Zhongting has passed Ph.D written QE on Dec 9. 

(updated Nov. 16)

Congratulation to Ms. Yang Shan who passed the Ph.D oral examination today. She is 11st Ph.D student graduated from our group since 2005.

(updated Nov. 14)

Ms. Yang Shan is going to have her Ph.D oral examination on Nov 16, 2016, 11:00 am to 1:00 pm at Blk EA 02-14. Her thesis title is: NANOSCALE CHARACTERIZATION OF THIN FILM Li1.2Mn0.54Ni0.13Co0.13O2 CATHODE IN LITHIUM ION BATTERIES BY USING SCANNING PROBE MICROSCOPY TECHNIQUES.

The thesis abstract is: 

The widely-used lithium-ion batteries (LIBs) have many advantages, such as non-memory effect, long cycle life, good portability, and high energy conversion efficiency. These features make them the most attractive power sources for portable electronic products, electric vehicles and energy storage systems. Up to now, most of studies are focused on the understanding of the functionalities of the LIBs at device level and atomic level. The knowledge at mesoscopic level has yet to be understood thoroughly. Scanning probe microscopy (SPM) based techniques are therefore powerful tools to investigate the detailed phenomenon associated with lithium ion diffusion in LIBs materials at mesoscopic level. Hence, the aim of the present study is to extend the applications of SPM techniques in the study of LIB materials. This thesis has covered explorative studies on nanoscale characterization of Li1.2Co0.13Ni0.13Mn0.54O2 thin film cathode, using a combination of various SPM techniques, i.e. Atomic Force Microscopy (AFM), Electrochemical Strain Microscopy (ESM) with dual AC resonance tracking (DART) and Band Excitation (BE), conductive-AFM (c-AFM), Biased-AFM, and AM-FM (Amplitude Modulation-Frequency Modulation). This study has taken the first step in characterizing Li1.2Co0.13Ni0.13Mn0.54O2 cathode by using the SPM-based techniques. The findings of this study are of considerable importance since they provide a new perspective and novel method to understand this cathode material at the mesoscopic length scale, bridging the gap between atomic and device-level studies.

(updated Oct 27)

Ms. Tian Tian and Mr. Du Chunbao, both are Ph.D students from Northwest Polytechnic University, Xi'an, China, are attached with the group for six months, from Oct 2016 to March 2017. Welcome Tian Tian and Chunbao.

(updated August 24).

Ms. Yang Shan has submitted her Ph.D thesis in August 2016. The title of the thesis is: Nanoscale Characterization of Cathode Materials in Lithium Ion Battery by Scanning Probe Microscopy Techniques.

(updated July 22, 2016).

Ms. Yu Bingxue, B.Eng from Huazhong University of Science and Technology, China, joint the group as Ph.D student from July 2016. Bingxue will work on the characterization of transitional metal oxide materials. Welcome Bingxue!

Mr. Qiao Yiming, student from Shanghai Jiaotong University, China, attached to the group from July to Sept, 2016 for summer internship.

Ms. Sun Yao has passed her oral qualify examination (OQE) on July 11, 2016.

(updated March 10, 2016)

Ms. Chen Jie, B.Eng from Inner Mongolia University of Technology, M.Eng from Chongqing University, currently Ph.D student from Chongqing University, also Research and Development Engineer in Chongqing Materials Research Institute Co., Ltd., join the group from March 1, 2016 for one year. Welcome Chen Jie!!

(updated Feb. 23, 2016)

From Jan. 2016, Ms. Sun Yao and Mr. Wang Zhongting are attached with Institute of High Performance Computation (IHPC), A*Star, Singapore, under the supervision of Dr. Cheng Yuan to conduct the molecular dynamic simulation work.  

(updated Jan 4, 2016)

Mr. Wang Zhongting, B.Eng and M.Eng from Harbin Institute of Technology, China, M.Sc from Department of Materials Science and Engineering, NUS, previously attached with group from May - Dec. 2015, joint the group as Ph.D student from Jan 2016. Zhongting will work on computer simulation of various SPM techniques as well as comparison with the experimental work. Welcome Zhongting!!

After oral examination and submission of revised thesis, Dr. Xiao Juanxiu joint a research group at Department of Materials Science and Engineering, NUS, as a Research Fellow, we wish her every success in her future career. 

(updated Dec 29 2015)

Congratulation to Ms. Xiao Juanxiu who passed the Ph.D oral examination today. She is 10th student graduated from our group since 2005.

(updated Dec 28 2015)

Ms Sun Yao has passed Ph,D written qualify examination on Dec 22, 2015.

(updated Dec 16 2015)

Ms.Xiao Juanxiu will have her Ph.D oral examination on Dec. 29, 2015, 11:00 am to 1:00 pm at EA-02-14. Her thesis title is: Characterizing the Multifunctional Properties of ZnO-based Thin Films by Advanced Scanning Probe Microscopy Techniques. 

The abstract of the thesis is below:

ZnO based materials have recently attracted considerable attention due to their multifunctional properties and their potential applications in many fields. The resistive switching (RS) and polarization switching behaviors have been extensively studied. However, the underlying mechanisms remain unclear. In addition, the coexistence of resistive switching and polarization switching processes complicates the analysis and understanding of the multifunctional properties. Hence this study has been focused on nano- to micro-scales characterization and understanding of the correlations among the multifunctional properties in the ZnO-based materials. Three types of ZnO-based materials were fabricated and investigated in this work. The undoped ZnO thin films and copper doped ZnO (ZnO:Cu) thin films were deposited by a pulse laser deposition (PLD) technique, and the undoped ZnO nanorods were deposited by a hydrothermal method.

The characterizations were mainly based on various advanced Scanning Probe Microscopy (SPM) techniques. The domain structures were characterized by Dual AC Resonance Tracking Piezoresponse Force Microscopy (DART-PFM) and Band Excitation Piezoresponse Fore Microscopy (BE-PFM) techniques. The polarization switching and priezoresponse hysteresis loops were studied by Piezoresponse Force Microscopy (PFM), Piezoresponse Force Spetroscopy (PFS) and Switching Spectroscopy Piezoresponse Force Microscopy (SS-PFM) techniques. The resistive switching behavior and the surface potential were investigated by conductive Atomic Force Microscopy (c-AFM) and Kelvin Probe Force Microscopy (KPFM) techniques respectively. 

       The main results are presented into four sections in this dissertation. The characterization and correlations between the resistive switching and polarization switching behaviors for undoped ZnO are provided in the first section. Persuasive mechanisms for resistive switching behavior are also proposed. In addition, the polarization switching behavior in ZnO:Cu thin films and the related effects are studied in more detail in the second section. The correlations between the resistive switching and polarization switching behaviors for ZnO:Cu thin films are further studied in the third section. Furthermore, to complete the study of ZnO-based system, the characteristics of the size effects on multifunctional properties of the ZnO nanorods are studied in the fourth section. Finally, general conclusions and recommendations for future research work are proposed at the end of this dissertation. 

Ms. Sun Yao will have her Ph.D written QE on Dec 22, 2015.
Mr.Wu Jiaxiong will go back to Beihang University on Dec. 20, 2015 after three month attachment at the group.

(updated Sept 2015)
Ms. Wang Hongli has passed her Ph.D oral QE in May 2015.
Mr. Wang Zhongting, has joint the group as part-time research from August 2015. He is currently M.Sc student at Department of Materials Science and Engineering at NUS. He got B.Eng and M.Eng degrees from Harbin Institute of Technology. Welcome Zhongting!!

Mr. Wu Jiaxiong, from Department of Applied Physics, Beihang University, has joint the group for three month until Dec. 2015, he will conduct the related research on Lithium ion battery materials. Welcome JIaxiong!!
(updated March 2015)
Ms. Sun Yao, from Peking University, China, has joint the group as Ph.D studet from Jan. 2015. Ms. Sun is going to work on the project related to "Bioferroelectricity and Biopiezoelectricity". Welcome Sun Yao!
Ms. Wang Hongli is going to have her Ph.D oral QE in May 13, 2015.
Ms. Lu Wanheng has passed her Ph.D oral QE in Sept. 2014.
(updated May 30, 2014)
Dr. Li Tao, who has been worked in this group for last 6 years (since 2008 as FYP student, then Ph.D student and research fellow), will leave the group on May 31, 2014 for new career. Dr. Li has contributed significantly to the group's research activities, including (i) Biopiezoelectricity and Bioforroelectricity research during her Ph.D period; and (ii) In Situ electrochemical characterization of battery materials". Dr. Li has also helped others in many aspects on their research. We will miss Dr. Li and wish a great future and more success on her future research career.
Ms. Wang Hongli has passed her Ph.D written QE in March. 2014.
Ms. Yang Shan has passed her Ph.D oral QE in Feb. 2014.

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