Semiconductor is the unsung hero behind most electronic devices that have become ubiquitous in the past decades from transistors to integrated circuits. A smartphone with modern computing power might have never fitted in your pocket if not for semiconductor. Its widespread use in electronics partly originates from its electrical conductivity between a conductor and an insulator. However, this property also means electric current produced in semiconductor cannot be conducted away quickly, leading to a weak current or low effectiveness for the device. Prof. Wallace Leung, Chair Professor of Innovative Products and Technologies, Department of Mechanical Engineering, led a research team to develop semiconductor nanofibres, with diameter between 60 and 80 nanometres (about 1/1000 of the thickness of a human hair, while one nanometre equals one billionth of a metre). These nanometres encase highly conductive nano-cores, such as a few layers of graphene or carbon nanotubes (CNT), at the centre of the nanofibres. The new nanostructure combines the best of both worlds – it retains the useful properties of a semiconductor, while being at least 100 times more efficient in conducting electric charges, bringing about higher sensitivity and effectiveness.
The mechanism: electrons and holes
When a semiconductor is subject to charging or illumination by light, the excited charges (e.g. electrons) jump out of its energy level leaving behind positively-charged holes. The negatively-charged electrons tend to recombine with the positive-charged “holes” thereby reducing the flow of electrons or charges undermining the effectiveness of the device. For the semiconductor nanofibres with embedded graphene developed by Prof. Leung and his team, the free electrons are immediately transported away, greatly reducing the chance of recombining with the holes. Prof. Leung elaborated, “Graphene sheets produced from shearing of graphite are purified and added to the precursor solution that will be electrospun to produce nanofibres. Upon electrospinning, nanofibres are produced with graphene sheet rolled up inside the fibres. These nanofibres are heated to remove the water and organic matters to produce finally the semiconductor nanofibres. The diameter of these nanofibres is only 60 to 80 nanometres. The highly conductive graphene rolled-up core provides a dedicated superhighway for electrons while the outer shell retains the behaviour of normal semiconductor. In our experiments with solar cells and air purifiers, semiconductor nanofibres embedded with graphene/CNT significantly enhance the performance efficiency of these devices. Also, given the graphene sheets are rolled up, there are no edges as with conventional graphene sheets for which electrons will “drop-off” or recombine with the holes at these edges. In our configuration, electrons will only travel uni-directionally along the axis of the nanofibres, which is most desirable.”
Enhanced solar cell efficiency
Dye sensitized solar cell is a very promising source of renewable energy and it is the most environmental friendly. Despite the advancement in technology, the efficiency of these solar cells is quite low. Prof. Leung and his team have embedded CNT in the semiconductor nanofibres for the dye sensitized solar cell. “First, CNT which is carbon based absorbs light itself. This harvests light that has not been absorbed by the small molecule dyes attached to the semiconductor nanofibres. Second, the CNT core conducts electrons away quickly reducing the chance of the electrons in recombining with the holes, so that more electrical current is being generated,” commented Prof. Leung. In their experiment on dye sensitized solar cells made with CNT-embedded semiconductor nanofibres, the efficiency is increased by 66% as compared to the device without CNT. This is indeed very encouraging.
Improved performance in air purifiers
The semiconductor titanium dioxide is also used as a photocatalyst in air purification. However, it is not very effective because it only harvests ultraviolet light which only makes up 5% of the light spectrum. To improve air-purifying efficiency, Prof. Leung and his team embedded graphene/CNT in composite nanofibres made of titanium dioxide, zinc and bismuth (TZB composite) that can harvest up to 50% of the light spectrum. “After the graphene is embedded in the composite TZB nanofibres to produce TZB-Gr nanofibres, the surface area for absorbing light and trapping harmful gaseous molecules is greatly increased by 80%. In our experiment, an air purifier made with graphene-embedded composite nanofibres is at least 7 times more effective than commercial air purifiers with titanium dioxide nanoparticles. This is due to the increase in surface area, increased light harvesting, and the fast transport of electrons once they are generated thereby reducing electron-hole recombination.”
According to Prof. Leung, the team will work on other applications of graphene/CNT embedded semiconductor nanofibres, such as ultra-sensitive and fast-response biological-chemical sensors, hydrogen production by water splitting and high-capacity lithium batteries with low impedance.
A United States patent on the technology has been granted to Prof. Leung in 2015. At the 45th International Exhibition of Inventions of Geneva, Switzerland in March 2017, a Gold Medal with the Congratulations of Jury was awarded to Prof. Leung and his team for the technology.
