Yihong Wu Group



Synthesis of carbon nanowall (CNW)





The discovery of graphene in 2004 made it possible for the first time to synthesize low-dimensional carbon allotropes in all dimensionalities ranging from zero-dimensional (0D) fullerene to one-dimensional (1D) nanotubes, and finally 2D graphene. But a few years before graphene was discovered, we had reported on the successful synthesis of vertically aligned 2D carbon nanosheets, dubbed as carbon nanowalls (CNWs).


We first reported the findings in the MRS Spring Meeting 2001 under the title “Carbon nano-flakes grown by microwave CVD”. We subsequently renamed the structure as CNW to emphasize its novel form of morphology in the first journal paper on this topic – “Carbon Nanowalls Grown by Microwave Plasma Enhanced Chemical Vapor Deposition”, Adv. Mater. 14, 64-67(2002). As we pointed out in the introduction of this paper, despite the intensive efforts on fullerene and nanotubes up to that time, “it is a surprising fact that so far there has been no report on the growth of well-separated graphene sheets (or two-dimensional carbon) on other substances”.





Y. H. Wu et al, Adv. Mater. 14, 64-67(2002).



Growth mechanism of CNW using microwave enhanced CVD





We revealed the important role of electric field in the growth of CNW by microwave enhanced chemical vapour deposition (Nano letters 2, 355-359 (2002). The role of electric field was confirmed later by other researchers (e.g., Carbon 45, 2229-2234 (2007), Nanoscale 5, 5180–5204 (2013), 2D Mater. 5, 044002 (2018)).





See Gallery for more SEM images



Synthesis of a class of nanomaterials using CNW as template





We succeeded in coating the CNW with a dozen of nanostructured materials ranging from metal to magnetic materials, oxide and semiconductors which are important for practical applications (Adv. Funct. Mater. 12, 489-494 (2002)).





See Gallery for more SEM images



Electrical transport of CNW





We revealed the peculiar transport properties of CNW which suggests that the self-assembled 2D carbon network can be effectively used to form large scale Josephson junction arrays (JJAs) in CNW (J. Mater. Chem. 14, 469-477 (2004) Jpn. J. Appl. Phys. 50, 01AF02 (2011)). This is a significant finding because the same approach can be used to form JJAs in other types of materials.




J. Mater. Chem. 14, 469-477 (2004) Jpn. J. Appl. Phys. 50, 01AF02 (2011).


Edge contact to 2D carbon





We were the first to show experimentally that edge contact is superior than surface contact to 2D carbon due to its unique E-K dispersion (AIP Advances 2, 012132 (2012)). At the early stage of graphene studies, many groups employed the scanning tunnelling spectroscopy (STM) technique to verify the linear E-k dispersion — a hallmark for graphene. However, we noticed that almost all the E-k dispersion curves obtained by STM exhibit a E∝k^(3/2) relation instead of the linear relation claimed by the authors. We argued that this is because the measurement was performed from the side surface rather than from the edge, which is extremely difficult to accomplish using the STM. In this context, we carried out well-thought-out point-contact studies on 2D carbon from the edge and side surface, respectively, and revealed that only the edge contact is able to obtain the true linear E-k relation. The results suggest that the edge-contact is superior to side-contact which is important for device applications.









See AIP Advances 2, 012132 (2012) for more results.



Field emission study of 2D carbon





One of the most promising applications of 2D carbon is in field emission as pointed out in our earliest paper (J. Mater. Chem. 14, 469-477 (2004)). The advantage of field emitters based on low-dimensional materials is that the large longitudinal to lateral aspect ratio significantly enhances the local electrical field and thus reduces the emission threshold field. Therefore, one often uses the so-called field enhancement factor as an indicator to claim how superior their emitters are when compared with others reported in literature. We pointed out that this is somewhat misleading as the simple physics tells that the enhancement factor is strongly dependent on the cathode-anode distance. When the cathode-anode distance is comparable to the lateral size of the emitter, the enhancement factor should be unity. However, it is a formidable challenge to perform field emission at an anode-cathode distance in the nanometre regime. To address this challenge, we leveraged on the nano-probe system that he has developed and had performed the very first systematic studies of field emission from single piece of 2D carbon and verified experimentally that the enhancement factor indeed approached unity when cathode-anode distance approaches the thickness of 2D carbon. We developed an analytical model which successfully accounts for the experimental results (Appl. Phys. Lett. 103, 033115 (2013)). Based on the results, we developed a method to dynamically control the field emission from 2D carbon (J. Vac. Sci. & Technol. B 32, 051803 (2014)) using either a time-varying magnetic or electric field.









(Appl. Phys. Lett. 103, 033115 (2013)) J. Vac. Sci. & Technol. B 32, 051803 (2014))


Spin-injection in graphene





The work of 2D carbon was also extended to the spin injection in graphene. We developed a method to enhance the spin injection efficiency in graphene by introducing a Cu barrier, which significantly enhances the spin injection efficiency without increasing the junction barrier (Appl. Phys. Lett. 101, 022406 (2012) Appl. Phys. A111, 339–345(2013). Based on these results, a prototype spin-valve device with only one ferromagnetic electrode was fabricated and characterized.






Appl. Phys. Lett. 101, 022406 (2012) Appl. Phys. A111, 339–345(2013)




Department of Electrical and Computer Engineering

National University of Singapore

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Email: elewuyh@nus.edu.sg