Q&A?   Please contact:

Prof. Kai LIU

Rm. B815, Yifu Technology and Science Building

School of Materials Science and Engineering

Tsinghua University Beijing 100084,China   

E-mail : liuk@tsinghua.edu.cn 

Tel: +86-10-62799182

1. Interface interactions in 2-dimensional (2D) materials

Interface interactions are of fundamental importance for 2D materials as they are ultrathin and susceptible to the interfaces at 2D/metal electrodes, at 2D/substrates, or between 2D layers. These interfaces are crucial for determing the properties of 2D materials and their van der Waals heterostructures. Although various types of 2D materials have been intensively explored in recent years, there are still lots of open questions and challenges in the investigation of these interfaces. Here we are, by mechanical or optical approaches, probing interface interactions between 2D homo-/hetero-layers, between 2D/metal heterostructures, and between 2D/'active' substrates, aiming to understand the physics at interfaces and modulate the properties of 2D mateirals for cooperative devices with multiple functionalities .

Selected publications:
J. Mater. Res., 31, 832 (2016) (invited review) ;
Adv. Mater., 27, 6841 (2015);
Nano Lett.,14, 5097 (2014);
Nano Lett., 14, 5329 (2014).

2. Novel mechanical and electronic devices driven by metal-insulator transition

Vanadium dioxide (VO2) is a strong-correlated oxide that undergoes a thermally driven metal−insulator phase transition (MIT) accompanied by a structural transition slightly above room temperature (68 °C). However, whether electronic or structural correlation dominates the transition has been debated for decades. On one hand, we employ liquid electrolyte to electrostatically accumulate surface charges to trigger a surface MIT on single-crystal VO2 nanobeams, and free up ultrahigh density of electrons from the original valence band, resulting in a tunable and dense two-dimensional electron system on a substrate that has the same chemical composition as the electron layer. On the other hand, we utilize the dramatic structural change during the MIT, delivering a metal/VO2 bimorph structure with a large mechanical actuation of bending and torsional motions responding to diverse stimuli of heat, electric current, and light, under both ambient and aqueous conditions. These actuators offer potential applications in microelectromechanical systems, microfluidics, robotics, drug delivery, and artificial muscles.

Selected publications:
Nano Lett., 15, 8365 (2015);
Adv. Mater., 26, 1746 (2014);
J. Appl. Phys., 113, 043707 (2013);
Nano Lett., 12, 6302 (2012);
Nano Lett., 12, 6272 (2012).

3. Applications of super-aligned carbon nanotubes in flexible devices

Owing to the achievement of controlled growth, it becomes possible to realize practical macro-scale applications of CNTs. For example, transparent conducting films (TCFs) and yarns are two important applications of CNTs. However, the challenge lies in a facile and cost-effective fabrication of high-performance products. We achieve a roll-to-roll production of CNT films from spinnable super-aligned carbon nanotube (SACNT) arrays on 8-inch Si wafers, producing TCFs with excellent electrical conductivity, transparency, flexibility, and durability as conducting layers in touch panels of cell phones. These films can be further cross-stacked into stretchable conductors. We also twist high-strength CNT yarns and CNT/polymer composite yarns from spinnable SACNT arrays. These yarns are flexible, lightweight, scratch-resistant, very stable in the lab environment, resistant to extremely humid ambient, and as a result, can be woven into high-strength and heatable fabrics.

Selected publications:
Nanoscale, 4, 3389 (2012);
Adv. Func. Mater., 21, 2721 (2011);
ACS Nano, 4, 5827 (2010);
Adv. Func. Mater.,20, 885 (2010);
Nanotechnology,21,045708 (2010).

4. Controlled growth of vertically aligned carbon nanotubes

Controlled and large-scale synthesis of nanomaterials is always a big challenge and an obstacle to practical applications. One major reason for this is the unclarity of growth mechanism. We design a special apparatus to label the growth of CNTs, which can be readily read by an optical microscope post growth. The growth rates of CNTs are thus measured, and the growth mechanism can be explored. Based on the progress, we can further control shapes of CNT tips by altering the termination process of CNT growth, as well as diameters, lengths, and physical properties of spinnable CNTs. All of the work deepens the understanding of CNT growth mechanism and steps CNTs towards applications.

Selected publications:
Nano Lett., 8, 700 (2008);
Adv. Mater., 19, 975 (2007);
Carbon, 45, 2379 (2007);
Carbon, 43, 2850 (2005).

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