Associate Professor Alfred Tok

Phone: (+65) 6790 4935
Office: ABN-B2c-17
Current Research Interests
Prof Tok leads the Nanomaterials Group that focuses on the synthesis, processing, consolidation and applications of nanomaterials (rare earth, carbon-based, functional ceramic nanoparticles). His current research is broadly based on 3 areas: carbon-based sensors, synthesis of nanostructured materials and hard & tough materials.

1) Carbon-based Field-Effect Transistor Sensors
The biosensors market, which is currently at USD 9.9 billion, is expected to reach USD 18.9 billion in 2019 (GIA Report, 2014) propelled by the growing population and health issues. Our group capitalizes on this emergent market and researches on disposable and low-cost sensor suitable for real-time sensing in field conditions. Our group focuses on sensors for biological and gas detection applications.

a) Biological Sensors

Our group is able to scale-up and improve on the synthesis of graphene oxide to obtain the largest graphene oxide average flake size (700 µm2). These flakes have improved electrical and optical properties (145kΩ/square at 98% transparency). Our group also demonstrates the possibility of the extended growth of graphene oxide using chemical vapor deposition, thus contributing to the fundamentals of carbon growth science. These novel materials are then optimized for use in biosensors (Figure 1) for the physiological-relevant detection of sodium ions in sweat and interleukin-6 proteins in circulation. Our group seeks to elucidate these ultra-sensitive sensing mechanisms and then spin-off to other applications. Our work has been published in Nanoscale, Journal of Materials Chemistry C, European journal of applied physiology, etc.

Figure 1: (a) Top and (b) side view illustration of fabricated biological sensor for interleukin-6 proteins.

b) Gas Sensors

Our group showed that Single-Walled Carbon Nanotube (SWCNT) based gas sensing does not only occur mainly via the Schottky modulation as reported by many. Instead, it is a combination of different effects and by increasing the types and peculiarity of the sensing pathways, the sensitivity and selectivity of the SWCNT based electronic gas sensor can be enhanced. Our studies yielded resistive gas sensors, based on Ag-SWCNTs, which are selective to nitrogen oxides even at ambient temperatures (Figure 2). 

Figure 2: Current response of gas sensor to toxic NO gas at room temperature.

2) Synthesis of Nanostructured Materials using Atomic Layer Deposition (ALD)
Atomic layer deposition (ALD) has evolved to be a unique tool for nanotechnology with atomic level control of the depositions, 3D conformity and homogeneity. Film depositions can be realized for complex non-planar topographies for a wide range of applications such as energy conversion and storage, nanoparticle catalysts, nanostructures for drug delivery, gas separations, sensing, and photonic applications. Our group focuses on ALD materials for solar cell, hydrogen generation and smart window applications.

a) Solar Cell Application

MoS2 is a transition metal dichalcogenide with a structure analogous to graphene and has been extensively researched and applied in various areas. Our group works with MoS2-based NaYF4: Yb, Er nanocomposite (Figure 3) using a simple one pot decomposition method which enables the upconversion process to extend the absorption spectrum to the NIR range for efficient solar harvesting. This nanocomposite exhibits the rare phenomenon of negative photoresponse. We are also exploring ways to obtain 3D-structured MoS2 using environmentally friendly and non-toxic ALD suitable for industries.

Figure 3: Transmission Electron Microscopy image of MoS2-UCNP nanocomposite

b) Hydrogen Generation from H2O

Photoelectrochemical (PEC) water splitting, which converts solar energy into chemical energy in the form of H2, is a promising approach to address energy security issues, and has attracted great attention in recent years. By controlling the particle size of polystyrene spheres, our group obtains 3-D inverse opal photonic crystals with various reflection peaks using a custom-made ALD machine (Figure 4). Our group has demonstrated 3D TiO2 inverse opal–coupled upconversion nanoparticles photoanode for enhanced near-infrared light harvesting and that upconversion is responsible for the photoresponse upon near-infrared exposure. Related works has been published in Small, Energy Environ. Sci., Advanced Materials etc.

Figure 4: Custom-made Atomic Layer Deposition machine 

c) Smart Windows

Building energy efficiency is a very important area of engineering, research and development in the world today, with 25% of the world’s energy being consumed just to maintain a comfortable interior environment. Our group works on electrochromic TiO2 photonic crystal-based smart glass (Figure 5) that can be used as building glass facade to modulate both the optical and thermal properties. Currently our cross-institute team has a gradient-deposition patent and several publications related to electrochromics, electrochemistry, nanoparticles and printing of electrodes.

Figure 5:  TiO2 crystal-based smart glass color change properties and its eventual usage scheme.

3) Hard & Tough Materials for Ballistic Protection Application

The next generation of military vehicular and soldier system requires light-weight materials with high strength-to-weight ratio. Our research focuses on the synthesis and densification of nanostructured materials & desired composite architecture to significantly raise the ballistic protection capability. The B-C-N-O group of compounds are potential candidates to form novel materials for ballistic protection application as they inherent the unique properties from both boron nitride and boron carbide which are known for their light weight, high hardness, low friction coefficient and high wear resistance. Prof Tok leads a team of collaborators in armour material research ranging from high temperature synthesis of novel superhard materials and consolidation by state-of-the-art Spark Plasma Sintering to advanced characterisation techniques such as depth of penetration test using Two-Stage Light-Gas Gun (Figure 6).

Figure 6: Synthesis and testing of hard & tough materials

4) Institute for Sports Research

Our group is involved in the Institute for Sports Research, working on the damping property of midsoles which is based on carbon nanotube (CNT). CNT’s high aspect ratios (length/diameter) is particularly desirable for mechanical reinforcement, and it is found that the vertical aligned (VA)CNTs perform well in damping, to dissipate the energy absorbed under compression (Figure 7).  Our present job is to tune the damping property of VACNT by adjusting the length, diameter and area density etc. parameters and try to reinforce the polymer with VACNT to fabricate midsole material (Figure 8) with better cushion property.

Figure 7: A schematic illustration shows a nanotube array compressed to folded springs and then regaining the free length upon the release of compressive load.

Figure 8: Midsole of the sports footwear

5) NRF-CREATE

In accordance with the objectives of the Energy Thrust Program of the NRF-CREATE Project, our group is focused on the design and synthesis of highly functional nanomaterials, which enables energy harvesting and conservation. Recently, novel graphene oxide synthesized nanoballs and nanoflowers were synthesized. These exhibit potentials for supercapacitors and energy applications. In general, these activities results in above 50 publications, 17 patent applications and projects discussions with companies regarding commercialization possibilities.

Figure 9: Synthesis of graphene nanoballs and nanoflowers for energy harvesting and conservation.

Biographical Information
Alfred Tok (PK; Ph.D, NTU; C.Eng, MIMMM; MBA, NTU) has been a faculty in the School of Materials Science and Engineering since 2003. He studied Mechanical Engineering at the Queensland University of Technology, Australia, and graduated with a first class honors in 1995. He was also conferred the Dean's Award for Excellence for being top graduate on the course. After graduation, he had worked as a mechanical engineer at ST Aerospace Engineering. In 1997, he was awarded 2 scholarships at Nanyang Technological University (NTU) to pursue his PhD in Mechanical Engineering. After graduation, he joined the Materials Engineering school as a Research Fellow in 2000 and took up the position of Assistant Professor in 2003. In 2009, he was bestowed the National Day Commendation Medal (PK) from the Singapore Prime Minister’s Office. In the same year, he obtained his Nanyang MBA in the Dean’s Honors List. He was then appointed Division Head of Materials Technology in MSE in 2009 and presently, he is the Deputy Director of the Institute for Sports Research. He also consults extensively for companies from various industries.

The research group collaborates actively with NIMS (Japan), Loughborough University (UK), The Hebrew University of Jerusalem (Israel), Ben-Gurion University of the Negev (Israel), Austrian Institute of Technology (Austria), Vestas Wind Systems (Denmark), University of New South Wales (Australia), ST Kinetics (SG), Defence Science and Technology Agency (SG), DSO National Laboratories (SG), Globalfoundries (SG) and Ministry of Defence (SG). 

Teaching

  • MS2012 Introduction to Manufacturing Processes
  • MS3015 Materials Aspects in Design
  • MS7001 Materials Laboratory Techniques
Selected Publications
1. L.T. Su, S.K. Karuturi, J. Luo, L. Liu, X. Liu, J. Guo, T.C. Sum, R. Deng, H.J. Fan, X Liu*, A.I.Y. Tok*, “Photon Upconversion in Heteronanostructured Photoanodes for Enhanced Near-Infrared Light Harvesting,” Advanced Materials 25 11 1603-1607 (2013). Back Cover Article [IF: 15.41]

2. R.I. Made, E.J.R. Phua, S.S. Pramana, C.C. Wong, Z. Chen, A.I.Y. Tok, C.L. Gan*, “Improved Mechanical and Thermomechanical Properties of Alumina Substrate via Iron Doping,” Scripta Materialia 68 869-872 (2013) [IF: 3.16]

3. J. Huang, M. Larisika, D.W.H. Fam, Q. He, M.A. Nimmo, C. Nowak*, A.I.Y. Tok*, “The extended growth of graphene oxide flakes using ethanol CVD,” Nanoscale, 5 7 2945-51 (2013)  [IF: 6.23]

4. Xie, S. S.; Su, L. T.; Guo, J.; Vasylkiv, O.; Borodianska, H.; Zhu, X.; Krishnan, G. M.; Su, H.; Tok, A. I. Y*. “Non-Catalytic Facile Synthesis of Superhard Phase of Boron Carbide (B13C2) Nanoflakes and Nanoparticles.” Journal of Nanoscience and Nanotechnology 12 596 (2012). [IF:1.44]

5. Karuturi, S. K.; Cheng, C.; Liu, L.; Su, L. T.; Fan, H. J.; Tok, A. I. Y*. “Inverse opals coupled with nanowires as photoelectrochemical anode.” Nano Energy 1 322-327 (2012) Journal Cover [IF: 10.21]

6. J. Luo, S.V. Karuturi, L. Liu, L.T. Su, A.I.Y. Tok, H.J. Fan*, “CdSe Sensitization of Complex Nanostructures using Atomic Layer Deposition for Solar to Hydrogen Conversion”, Nature Scientific Reports 2 451 (2012)  [IF: NA]

7. X. Teng, Y.H. Zhu, W. Wei, S.C. Wang, J.F. Huang, A.I.Y Tok, Y. Han, Q.C. Zhang, J.A. Capobianco, L. Huang*, “Lanthanide-Doped NaScFx Nanoparticles: Crystal Structure Evolution and Multicolor Tuning,” Journal of the American Chemical Society 134 8340-8343 (2012) [IF: 11.44]

8. S. K. Karuturi, J. S. Luo, C. W. Cheng, L. J. Liu, L. T. Su, H. J. Fan*, and A. I. Y Tok*, “Three-dimensional Ordered Hierarchical Nanobushes Photoanode for Highly Efficient Photoelectrochemical Cells” Advanced Materials 24 (30) (2012) [IF: 10.88]

9. C. Cheng, S.K. Karuturi, L. Liu, H. Li, L.T. Su, A.I.Y. Tok* & H.J. Fan*, “Quantum-Dot-Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation.” Small 8 37-42 (2012). Frontispiece Article. [IF: 7.33]