F.C. Cheong

Large-scale synthesis and field emission properties of vertically oriented CuO nanowire films

Using a simple method of direct heating of bulk copper plates in air, oriented CuO nanowire films were synthesized on a large scale. The length and density of nanowires could be controlled by growth temperature and growth time. Field emission (FE) measurements of CuO nanowire films show that they have a low turn-on field of 3.5?4.5?V??m?1 and a large current density of 0.45?mA?cm?2 under an applied field of about 7?V??m?1. By comparing the FE properties of two types of samples with different average lengths and densities (30??m, 108?cm?2 and 4??m, 4 ? 107?cm?2, respectively), we found that the large length?radius ratio of CuO nanowires effectively improved the local field, which was beneficial to field emission. Verified with finite element calculation, the work function of oriented CuO nanowire films was estimated to be 2.5?2.8?eV.

The manipulation and assembly of CuO nanorods with line optical tweezers

We present a simple technique for manipulating and assembling one-dimensional (1D) CuO nanorods. Our technique exploits the optical trapping ability of line optical tweezers to trap, manipulate and rotate nanorods without physical contact. With this simple and versatile method, nanorods can be readily arranged into interesting configurations. The optical lin et weezers could also be used to manipulate an individual nanorod across two conducting electrodes. This work demonstrates the potential of optical manipulation and assembly of 1D nanostructures into useful nanoelectronics devices. M This article features online multimedia enhancements (Some figures in this article are in colour only in the electronic version)

Large area patterned arrays of aligned carbon nanotubes via laser trimming

We develop a simple to implement, inherently parallel and high throughput technique for the fabrication of large areas of patterned aligned multi-wall carbon nanotube (CNT) arrays deposited on silicon or quartz substrate. This technique makes use of a parallel or converging laser beam from a high power pulsed laser for the destruction of aligned CNTs with a copper grid as lithography mask to define patterned aligned CNT arrays. The wavelength of the laser beam used is 248 nm and the average energy per pulse is 500 mJ. Using this technique, an extensive area of patterned CNT arrays as large as 3 × 5 mm2 can be fabricated without the use of any pre-patterned substrate. In addition, we were able to control the size of the features created by (1) using different copper grids and (2) using a converging beam. Exposing the sample to different numbers of laser pulses allows us to generate families of CNTs with different uniform lengths. Furthermore, using two overlapping grids as a lithography mask, we managed to create a regular array of features with sizes as small as 2.5 μm.

Preparation of cantilevered W tips for atomic force microscopy and apertureless near-field scanning optical microscopy

Tip characteristics play an important role in the resolution and sensitivity of scanning probe microscopy. Extensive efforts have been devoted to tip fabrication. Most of the research is focused on scanning tunneling microscopy applications, which require sharp and short tips. Long tips that can be bent into cantilevered tips have great potential in atomic force microscopy/apertureless near-field scanning optical microscopy applications. However, the fabrication of such tips has been rarely reported. The present work is carried out with the aim of optimizing the conditions suitable for fabricating long and sharp tungsten tips. Besides topography, optical, and spectroscopic information, electrical and magnetic measurements can also be carried out with such tips obtained with the recipe reported in this article. The long tips also make it possible to measure deep grooves/trenches.

Patterning and fusion of CuO nanorods with a focused laser beam

We report a simple technique that facilitates the micropatterning of an aligned array of CuO nanorods on a substrate as well as the fusion of the nanorods into fused junctions. The technique utilizes a focused laser beam from a He–Ne laser with moderate power to melt away the pointed end of as-grown CuO nanorods resulting in the formation of microballs at the tips of the truncated nanorods. The size of the microballs and the length of the truncated CuO nanorods were found to be dependent on the laser power used during the process. The nature of the microballs formed was investigated by high-resolution transmission electron microscopy and Raman spectroscopy. Such a focused beam provides an effective means to modify the morphology of the as-grown nanorod array and to pattern the aligned CuO nanorod array into interesting and potentially useful configurations. In addition, the focused laser beam was utilized to fuse and join nanorods, which could potentially be useful in the fabrication of nanorod circuits and network repair.

Multicolored Carbon Nanotubes: Decorating Patterned Carbon Nanotube Microstructures with Quantum Dots

In this work, techniques to create patterned array of multiwalled nanotube (MWNT) microstructures decorated with quantum dots (QDs) were presented. Using aligned array of intertwined MWNTs as the supporting template, a droplet of solution comprising QDs was deposited onto the MWNTs. When the solution evaporated away, QDs were left behind on the MWNT template. Coupled with the technique of laser pruning, a wide variety of QDs-decorated MWNT microstructures were created. In addition, the aligned array of MWNTs was found to be an effective nanosieve that could effectively sort out QDs with a size difference of approximately 0.5 nm. In this case, a droplet of solution comprising QDs of different sizes was placed on aligned array of MWNTs. As the solution spread across as well as trickled down the MWNTs, the smaller QDs were found to venture further and deeper into the MWNTs. Again coupled with laser pruning, fluorescence microscopy revealed multicolored MWNT microstructures due to preferential decoration of these QDs with difference sizes. As a result, multicolored/multicomponents hybrid functional materials were achieved.

Rapid, High-Throughput Tracking of Bacterial Motility in 3D via Phase-Contrast Holographic Video Microscopy

Tracking fast-swimming bacteria in three dimensions can be extremely challenging with current optical techniques and a microscopic approach that can rapidly acquire volumetric information is required. Here, we introduce phase-contrast holographic video microscopy as a solution for the simultaneous tracking of multiple fast moving cells in three dimensions. This technique uses interference patterns formed between the scattered and the incident field to infer the three-dimensional (3D) position and size of bacteria. Using this optical approach, motility dynamics of multiple bacteria in three dimensions, such as speed and turn angles, can be obtained within minutes. We demonstrated the feasibility of this method by effectively tracking multiple bacteria species, including Escherichia coli, Agrobacterium tumefaciens, and Pseudomonas aeruginosa. In addition, we combined our fast 3D imaging technique with a microfluidic device to present an example of a drug/chemical assay to study effects on bacterial motility.

Annealing effects on the elastic modulus of tungsten oxide nanowires

Three-point bend test coupled with transmission electron microscopy (TEM) analysis was carried out on individual tungsten oxide nanowires (NWs) before and after annealing. Three-point bend test monitors the change in the Young’s modulus of the NW after annealing, while TEM provides nanostructural detail changes on the same NW. In this way, insight into the correlation between the mechanical properties of a NW and its nanostructure details can be obtained. Annealing increased the diameter of the NWs by forming a uniform amorphous/polycrystalline outer coating. The coating results in a decrease in Young’s moduli for thicker NWs. On the other hand, annealing led to increased Young’s moduli of thinner NWs, which is attributed to the improved crystallinity in these NWs after annealing. This study points to a more refined strategy for more in-depth understanding of the relationship between the nanostructures and elastic mechanical properties of NWs.

Manipulation Assembly of CuxS Dendrites Using Optical Tweezers

We present the applications of optical tweezers to manipulate and assemble Cu x s dendrites in fluidic medium. Our techniques involve the use of an optical tweezers, a single focused laser beam as non-contact manipulators to assemble Cu x S at designated location in a sample chamber. Using this simple tool, we are able to readily arrange the dendrites into interesting configurations. Besides a simple focused laser beam, an asymmetrical line optical tweezers was utilized to manipulate and propel the Cu x S along the length of the line tweezers. Finally, an application of using dendrites as a possible micro-machine powered by optical force has been explored in this work.

A simple technique for dynamic optical tweezers using mirror on a vibrating membrane

We report a simple and economical technique to create a wide variety of laser pattern for optical tweezing. The main feature of this technique is a reflective gold mirror that is mounted on a stretched latex membrane which can be vibrated with sound wave at a frequency within 100~600Hz. Due to the vibrating gold mirror, laser beam that is reflected off the mirror exhibits a wide variety of controlled patterns. With the reflected laser pattern directed into an optical microscope for optical tweezing, we were able to form different dynamic configurations of structures made of colloidal microspheres. Different formations of colloidal microspheres that correspond to the reflected laser patterns created by the sound-vibrated mirror have been observed.