Research News and Highlights


Division of Physics and Applied Physics

The Home Of Extraordinary Research Breakthroughs
In the short span of a decade, the Division of Physics and Applied Physics has notched up an impressive number of world firsts in research achievements. These range from realising optical cooling in semiconductors to the creation of an invisibility cloak. Here are a few of the Division’s most notable research triumphs.
First To Attain Laser Cooling Of A Semiconductor (Xiong Qihua)
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Lasers can be used to cool specially designed materials in a phenomenon called optical refrigeration. This is also known as laser cooling of solids.
Optical refrigeration offers several important advantages over conventional methods of cooling. Since no coolant or moving part is involved in optical refrigeration, optical refrigeration is a vibration-free operation requiring little space, while delivering highly stable and reliable results.
With these key advantages, optical refrigeration can potentially be applied to systems such as aerospace detectors and sensors located in remote places.
At NTU, a research group led by Dr. Xiong Qihua scored a breakthrough by achieving net laser cooling of 40 Kelvin on cadmium sulphide nanobelts. It was the first time that optical refrigeration had been successfully employed to cool semiconductors.
With this discovery, all solid-state semiconductor cryocoolers can now be fabricated and project considerable promises of integration into electronic devices.
This triumph opened the door to the creation of materials with strong electron-longitudinal optical phonon coupling specifically to cater for the laser cooling of semiconductors. The next step forward is to study push laser cooling of solids down to liquid helium temperature regime and make macroscopic devices of optical cooler.
This discovery was featured on the cover of the international journal Nature in 2013.
Zhang, J., Li, D.H., Chen, R.J., Xiong, Q.H. 2013. Laser cooling of a semiconductor by 40 Kelvin. Nature. 493(7433), 504-508.
The Breakthrough That Made Possible Perovskite Solar Cells and Lasing (Sum Tze Chien)
When a molecular-scale composite is created from a hybrid of organic and inorganic constituents, it may possess valuable features of both organic and inorganic elements.
One such organic-inorganic hybrid composite is CH3NH3PbI3, a semiconductor with an optical bandgap of 1.55 eV.
Since its first discovery to the present time, solar cells have rapidly improved in performance. Today, it boosts 15% efficiency. This is the highest ever efficiency recorded for a low temperature solution processed solar cell.
The underlying reason for this rapid increase in the efficiency, however, remained a mystery for several years.
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In 2013, an interdisciplinary team led by Dr. Sum succeeded in uncovering the mechanics behind this phenomenon. The team’s work was subsequently published in
one of the world's top scientific journals, Science, in October 2013.
At Energy Research Institute @NTU (ERI@N), this new knowledge is currently being applied towards the development of a commercial prototype of the perovskite solar cell. This venture is undertaken in collaboration with Australian clean-tech firm Dyesol Limited.
Already, the team’s fundamental findings on technologically relevant materials have led to several spin-off discoveries of novel phenomena such as lasing in CH3NH3PbI3 halide perovskites. The findings have resulted in the filing of a patent and publication of another groundbreaking paper in the journal Nature Materials.
G.C. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum. 2013. Long-range balanced electron and hole-transport lengths in organic-inorganic CH3NH3PbI3. Science. 342, 344-347.
G. C. Xing, N. Mathews, S.S. Lim, N. Yantara, X. Liu, S. Dharani, M. Grätzel, S. Mhaisalkar, T.C. Sum. 2014. Low-temperature solution-processed wavelength tunable Perovskites for lasing. Nature Materials 13, 476-480.
The ‘Invisibility Cloak’ (Zhang Baile)
An invisibility cloak may sound like something out a Harry Potter movie but Dr. Zhang Baile succeeded in being the first to make a ‘cloak’ that renders objects invisible!
Dr Zhang and his team created the ‘cloak’ by attaching together two pieces of calcite together. This is a commonly available carbonate mineral that can bend light. Since the material bends light, any object beneath it becomes invisible to the human eye.
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Dr. Zhang and his collaborators have since successfully demonstrated the cloaking of living creatures, including a cat and a fish. The publication of the team’s work in open access journal
Nature Communications was billed as ‘breaking news’.
That is not all that the cloak can do. Dr. Zhang and his colleagues applied similar principles to make the material immune to heat conduction. In an experiment, the team successfully ‘cloaked’ – which is to say, insulated without a trace - a three-dimensional air bubble in a metal bulk from heat conductive flux.
This discovery is exciting because this new technology has the potential to block heat transfer by air gaps in electronics as well as to protect key electronic components from heat.
The team’s discovery was published in by the world’s premier physics letter journal Physical Review Letters, and was highlighted in the world’s leading physics’ magazine Physics World.
H. Chen, B. Zheng, L. Shen, H. Wang, X. Zhang, N. I. Zheludev, B. Zhang. 2013. Ray-optics cloaking devices for large objects in incoherent natural light. Nature Communications, 4, 2652.
H. Xu, F. Gao, X. Shi, H. Sun, B. Zhang. 2014. Ultrathin three-dimensional thermal cloak. Physical Review Letters. 112, 054301.

Proving That Nanobubbles Exist (Claus-Dieter Ohl)
Submerging a surface under water can lead to the formation of many nanometer-sized bubbles on the surface.
If the formation of such bubbles can be understood and controlled, it would have a huge bearing in the field of fluid mechanics; it would mean being able to better direct many facets of fluid mechanics from drag reduction to fluid-surface interaction in chemical and biological processes.
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Intriguingly, these nanobubbles have a much larger contact angle and longer lifetime than projected by classical theory. Because of this, many scientists theorised that these nanobubbles do not exist, but are, in fact, the result of contamination.
In a key breakthrough, the team led by Dr. Ohl developed a technique that could differentiate between contaminants and nanobubbles. Using that technique, the team proved optically that nanobubbles are indeed real gaseous objects.
This work was published in Physical Review Letters and highlighted in Physical Review Focus and Physics Today.
C. U. Chan, L. Chen, M. Arora, C. D. Ohl. 2015. Collapse of surface nanobubbles. Physical Review Letters. vol. 114.
Discovery of the Left-handed DNA G-quadruplex (Phan Anh Tuan)
DNA is universally known for its double helix structure. Less well known is the fact that DNA can be in the alternative form of a four-stranded structure known as G-quadruplex (or G4).
G-quadruplex is highly polymorphic, which is to say, it can exist in many different forms. However, only right-handed helical forms had been observed so far.
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G-quadruplex structure has been implicated in cellular processes and is particularly promising for application in the fields of therapeutics and nanotechnology.
Using nuclear magnetic resonance and X-ray crystallography, Dr. Phan and his colleagues became the first to observe a left-handed DNA G4 that contains structural features that can be exploited as unique recognition elements.
Their work was published in the journal Proceedings of the National Academy of Sciences of the United States of America in February 2015.
Chung W.J., Heddi B., Schmitt E., Lim K.W., Mechulam Y., Phan A.T. 2015. Structure of a left-handed DNA G-quadruplex. Proc. Natl. Acad. Sci. U.S.A. 112, 2729-2733.