Research Interests:

Synthesis and characterization of novel nanomaterials including semiconducting nanowires, nanotubes, metal oxide nanowires and nanodots;

Study on electronic transport, optical and mechanical properties of low dimensional nanomaterials and nanocomposites;

Applications of nanomaterials in structural and functional composites and in energy storage;

Application of nanoscience to biology and medicine;

Fabrication and test of nanoelectronic devices.

 

Research highlights:

 

·         Dye Sensitized Solar Cells

The research is to develop high efficiency dye sensitized solar cell through innovating its photoanode and counter electrodes. In this project, exploiting the unique anisotropic photocatalysis of rutile crystal, we expect to develop a multiple step growth approach via the assistance of self-assembled monolayer (SAM) to synthesize long rutile nanorod arrays with controllable structure for high efficiency DSSCs. Besides titanium dioxide nanorods, carbon nanotube thin film and its composite with TiO₂ will be used to fabricate novel photoanode and counter electrodes for high efficiency solar cells at low cost. Through this research, we aim to understand the electron transport mechanism in the CNT integrated photoanode and counter electrode and to improve the solar cell efficiency by optimizing the solar cell structure at the nanoscale.

 

 

Figure caption. Schematics of TiO₂ nanorod dye sensitized solar Cell. (a) J-V curve of the DSSCs made from the 1S, 2S and 3S NR sample.  (b) The fill factor (FF), short circuit current (J_sc) and open circuit voltage (V_oc)) as a function of the nanorod height.

 

·         Nanostructured thermoelectric materials for power generation

Thermoelectric (TE) materials can be used to develop solid-state power generator based on the Seebeck effect and solid-state refrigerator based on the Peltier effect. The TE material based energy conversion devices will help to solve the energy crisis because they can harvest waste thermal energy and then convert it to electrical energy (see Figure below). The objectives of this research are to (1) develop adequate and efficient hydrothermal and solvothermal techniques to synthesize nanostructured TE materials such as bismuth selenide (Bi₂Se₃), skutterudite (CoSb₃), lead telluride (PbTe), and indium doped lead telluride (In_xPb_1-xTe), and (2) understand how the quantum-confinement effect and the interface scattering to phonons affect the thermoelectric properties of the materials.

 

    

Figure. Left: Schematic of a thermoelectric generator. Right panel: SEM and TEM images of the as–prepared Bi₂Se₃ samples synthesized in DMF at 200ºC for 24 h.  a) SEM image, b) TEM image, c) HRTEM image, d) Figure of merit (ZT) vs. temperature. For details, please refer to Kadel et al., “Synthesis and Thermoelectric Properties of Bi2Se3 Nanostructures” Nanoscale Research Letters 6:57, 2011.

 

·         Electron filed emission properties of carbon nanotube arrays synthesized by plasma enhanced chemical vapor deposition

Carbon nanotubes (CNTs) have been considered as the next generation field emission material due to their large aspect ratio, high mechanical stiffness, good electrical conductivity, and excellent thermal and chemical stability. Vertically aligned multi-walled carbon nanotubes (MWCNTs) were grown on Cu substrates by PECVD and were then treated by NH₃ plasma to modify their microstructure. A great improvement on the field emission properties of the MWCNTs was obtained after the plasma treatment. Specifically, the MWCNTs after 1.0 min plasma treatment demonstrated the best field emission performance with the field enhancement factor increasing from 319 to 546, the turn-on electric field decreasing from 11.93 V/mm to 8.84 V/mm, and the highest emission current reaching 0.85 mA, as compared to the untreated sample. The improvement was attributed to the morphology change: the tapered structure of the MWCNTs generated by the plasma etching and the increased inter-tip distance due to the cluster structure of the thin tips. The open-ended tips and the increased structural defects introduced by NH₃ plasma also played a role in the enhancement of field emission properties of the MWCNTs.

 

Figure. Field emission characteristics of the MWCNTs before and after NH₃ plasma treatment. (a) I-V curves. The inset shows the corresponding F-N plots and the upper right inset shows the schematic of the field emission measurement setup. (b) Field enhancement factor (b) and turn-on electric field (E_turn-on) of MWCNTs according to NH₃ plasma treatment time. For details, please refer to Chen et al, “Electron field emission properties of vertically aligned carbon nanotube point emitters” Diamond and Related Materials 25, 134 (2012); Neupane et al., “Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper”, Carbon 50, 2641 (2012),

 

·         Study of Radial Modulus of Single-walled Carbon Nanotubes by Atomic Force Microscopy

The understanding of the mechanical properties, especially the radial modulus (radial elasticity), of single-walled carbon nanotubes (SWCNT) is very important in developing SWCNT-based nanodevices. We have measured the radial modulus by combining tapping mode and contact mode atomic force microscopies on as-grown horizontally aligned SWCNTs. It is found that the radius of the SWCNTs has a significant influence on the radial modulus. As shown in the Figure, the measured radial modulus (E_radial) decreases from 57 to 9 Gpa as the radius of the SWCNTs increases from 0.92 to 1.91 nm.  We have analyzed our experimental data using Hertzian theory with the consideration of the deformation of the AFM tip-SWCNT-substrate system. The experimental data agrees well with the reported simulation result using a modified molecular structure mechanics (MSM) model.

 

 

Figure caption. (a) AFM image of horizontally aligned SWCNTs grown on quartz substrate; (b) Measured radial modulus (E_radial) vs. SWCNT radius (black squares) compares with data from nanoindentation experiment (blue dots) and modified MSM calculation data (red triangles, data courtesy of Mr. Y.L. Liu). Inset: The measured radial deformation (Dh) vs. applied normal force (Fz) as well as the fitting to the experimental data by using the power function relation Δh∝Fz^2/3 for SWCNTs with radius from 1.91 to 0.42 nm. For details, please see Yang et al., APL 98, 041901, 2011.

 

·         Growth Mechanism and optical properties of Ruthenium Dioxide and Nickel Oxide Nanostructures

(1) RuO₂: Low-dimensional RuO₂ nanostructures hold a promising future in nanoelectronics application due to their low resistivity and excellent thermal and chemical stabilities over a wide temperature range. The understanding of the growth mechanism of the RuO₂ nanostructures is essential for controlling their growth and developing novel devices. We have studied the growth mechanism by combing experiment and electronic structure calculation. The experiment reveals that the O₂ flow rate, concentration and pressure play significant roles in the formation of RuO₂ nanostructures, as show in the Figure. Our calculation confirms that RuO₂ has higher energy compared to RuO₃ and RuO₄. The less stable RuO₃ (as compared to RuO₄) is more suitable for nanorod growth and a rich O₂ environment at the RuO₂ source is a preferable condition to grow RuO₂ nanorods.

 

   

Figure caption. RuO₂ nanostructures synthesized at various conditions. (a) Polygonal prisms with uniform width formed at low O₂ flow rate, (b) top-view of nanorods with square-shaped hollow tips at high O₂ flow rate, (c) hierarchical pine tree-liked structure formed at high pressure, and (d) flower-liked nanostructures formed at low O₂ concentration. For details, please see Neupane et al. Journal of Materials Sciences 46, 4803 (2011).

 

(2) NiO: NiO nanowalls have demonstrated their potential use as field emitter, electrodes for high performance Lithium ion battery and electrochemical supercapacitor.Vertically aligned and well interconnected NiO nanowalls with thin nanodiscs of cubic NiO as the in situ formed building blocks were fabricated via a template-free approach. Surface morphology analysis confirmed the formation of 15 nm thick and 1-1.5 μm wide nanowalls which have the crystal structure of cubic NiO with their growth plane along the [111] direction. An optical band gap of about 3.8 eV for the NiO nanowalls was obtained from the optical absorption measurement. NiO nanowalls exhibited a broad UV emission band centered at around 390 nm.

 

 

Figure caption. (A) SEM image of a large area of the NiO nanowalls synthesized at 100 ^oC for 24 h (sample 1); (B) SEM image of the NiO nanowalls synthesized at 200 ^oC for 24 h (sample 2); (c) UV-vis absorption spectra for NiO nanowalls for sample 1 and sample 2, respectively. Insets show the plot of (ahg)² vs. hg  for sample 1 and sample 2, respectively; and (d) Photoluminescence emission spectra for NiO nanowalls for sample 1 and sample 2, respectively.  For details, please refer to Kumari et al., “Vertically Aligned and Interconnected Nickel Oxide Nanowalls Fabricated by Hydrothermal Route”, Crystal Research and Technology 44, 495 (2009).

 

·         Structure of Flattened Carbon Nanotubes

Carbon nanotubes (CNTs) behavior either as semiconductor or metal depending on the arrangement of carbon atoms in their cylindrical graphitic layers. However, CNTs are not always in cylindrical shape, they could be partially or fully collapsed to form flattened CNTs (F-CNTs). The F-CNTs have substantially different electrical properties from their cylindrical counterparts. Therefore, a detailed study on the structure of the F-CNTs is essential to the understanding of the electrical property and the application of CNTs. In this research, the structure of the F-CNTs has been investigated by transmission electron microscopy (TEM). Two types of flattenings of CNTs in our experiment have been observed. The first type of flattening is parallel to and symmetric about the long axis of the CNT. The second type of flattening is a spiraling of fully flattened CNT around the long axis of the CNT. The flattening and the spiraling are ascribed to axial compression and torsion on the CNTs.

 

Figure caption. TEM images of a F-CNT capped with Co catalyst particle before (a) and after (b) rotation of 35°. A CNT with parallel flattening (c) and its model (d). A long spiraling of a fully flattened CNT (e), its model (f), its close-up (g), the high magnification of a node C (h), and the model of the node C (i).

For details, please see Li et al., “Structure of flattened carbon nanotubes”, Carbon 45, 2938 (2007).

 

·         Electrical properties of carbon nanotube junctions

The electrical properties of the carbon nanotube junctions were investigated using 4-probe scanning tunnelling microscope (STM). The transport property measurement was carried out on three terminal junctions such as T-junction (Figure a) and Y-junction (Figure c). In contrast to the reported three terminal nanotube junctions where a rectification-like asymmetric I –V behaviour has been reported, the T– or Y– junctions here display symmetric I –V curves. For the T–junction, the I –V curves measured with two probes across the junction are nonlinear at a high source current range (Figure b); however, they are linear at a low source current range, similar to that of the Y-junction (Figure d). Both types of junctions do not show a gate effect when a bias voltage is applied to one of the branches while the transport is measured between the other two branches. When the silicon substrate was used as the back gate, the gate effect of the junction nanotubes was not observed either. The lack of rectifying behaviour and gating effect is consistent with the metallic conduction state of the junctions. For details, please refer to Kim et al, Nanotechnology 19, 485201 (2008).

 

 

·         Growth mechanism of carbon nanotube junctions

The research emphasis is on studying, identifying and understanding fundamental mechanisms of synthesis of new functional materials - Y-shaped carbon nanotube junctions (Y-junctions). In this project, thiophene (C₄H₄S) has been used as carbon source to grow nanotubes on Co catalyst nanoparticles, and the influence of the concentration of C₄H₄S vapor on the structure of the nanotubes has been investigated. It is found that nanotube junctions will grow when the C₄H₄S concentration is in the range of 0.76-1.5%. Otherwise, carbon fibers will form at high C₄H₄S concentration and Co₉S₈ nanowire-filled nanotube will form at low concentration.

 

Figure caption. Transmission electron microscopy images of (a) carbon fibers, (b) branched nanotube, (c) Co₉S₈ nanowire-filled nanotube, and (d) hollow nanotubes grown at various thiophene (C₄H₄S) vapor concentrations 3.0, 1.5, 0.51, and 0.37 vol.%, respectively. For details, please refer to Li et al, J. Phys. Chem. B 110(47), 23694 (2006); Du et al.,  J. Phys. Chem. C, 2007, 111, (39), pp 14293–14298; Du et al,  J. Phys. Chem. C, 2008, 112, (6), pp 1890–1895

 

 

 

 

Home