Kyung H. Lee's research while affiliated with University of Texas at Dallas and other places

A gas sensor using a multi-walled carbon nanotube (MWCNT) sheet, which can detect oxygen (O2) gas, is presented and its output characteristics are evaluated in this study. A simple, cost effective and novel fabrication technique is described compared to dispersing CNTs into a liquid or polymer. The sheets are spun from a MWCNT forest grown on a silicon substrate; its electrical resistance decreases linearly with O2 exposure. The MWCNT sheet has a large surface area and many individual MWCNT contact points; this leads to a linear sensitivity, a fast response time, repeatability, and stability. It is well known that the surface distribution and areal density of MWCNTs have a significantly affect on their sensing characteristics. The sensors fabricated using dispersed CNTs on a substrate, either with separated CNTs of low density or with overlapping CNTs of low resistance, reveal much lower sensitivities. The large surface area and uniform distribution of the gas sensor, however, allow for the higher interaction of the MWCNTs with the O2 molecules, increasing the sensor's characteristics. Moreover, the MWCNT sheet does not need purification or a complex transfer process to be used as a sensor, making it suitable for practical applications.

This paper presents carbon nanotubes (CNTs) used as transparent heaters, which offer great advantages in miniaturization, high efficiency, low power consumption, and rapid response. Previously proposed transparent single-walled carbon nanotube (SWCNT) based heaters used to replace indium tin oxide (ITO) heaters were fabricated either by dielectrophoresis or the piece-wise alignment of read-out electronics around randomly dispersed CNTs. These methods require steps for purification, separation, and dispersion in a liquid or polymer in order to improve their electrical and optical properties. We studied a transparent film used for heating, fabricated by employing a multi-walled carbon nanotube (MWCNT) sheet. The sheet was made from a super-aligned MWCNT forest; the heater was fabricated by direct coating onto a glass substrate. The characteristics of the MWCNT sheet, i.e. a high transmittance of ∼90% and a sheet resistance of ∼756 Ω/sq, are comparable to previously reported SWCNT-based transparent films. These properties are directly applicable to applications such as window tinting and defrosters in production vehicles.

Understanding the critical factors that lead to the growth of spin-capable multi-walled carbon nanotubes (MWCNTs) will allow the development of a reproducible growth process. This paper reports that the spin capability of MWCNT forests depends strongly on the pre-annealing time of the as-deposited Fe film. In our results, a longer pre-annealing time of the Fe film not only increased the number of larger Fe nanoparticles, it also changed the shape of the Fe nanoparticles from non-faceted balls to faceted polyhedrons. The former was associated with a decrease in the density of the active, suitably-sized catalyst nanoparticles for the growth of MWCNTs. The latter reduced the rate of CNT nucleation. Both effects reduced the areal density of the resulting MWCNT forest, which in turn deteriorated the alignment of the MWCNTs in the forest and inhibited their spin capability. As a result, a longer pre-annealing of Fe catalyst films resulted in poorer MWCNT forest properties.

A multi-walled carbon nanotube (MWCNT) sheet-based strain sensor is presented in this paper, which possesses a novel fabrication technique that employs a simpler process than that of dispersing CNTs into a liquid or polymer. The sheets were spun from a MWCNT forest grown on a silicon substrate. The electrical resistance of the MWCNT sheets increased linearly with an increased tensile strain. The sheet did not require any chemical grafting or charging in order to work as a sensor, making it an ideal strain sensor. The proposed sensor exhibited excellent piezoresistive behavior under repetitive strain and relaxation, as well as a relatively high sensitivity compared to other methods, highlighting its potential application to high sensitivity pressure and force sensors.

One must control the size distribution of catalyst Fe nano-particles (NPs) very carefully if one is to have any chance of growing "super-aligned" carbon nanotube (CNT) forests which can be spun directly into yarns and pulled directly into long sheets. Control of the Fe Nps size is important during all phases, including: the catalyst deposition, annealing and forest growth. As a result, it is important to understand how NPs are affected by various experimental factors as well as how those catalyst NPs then cause the growth of the forests. This paper focuses on two key experimental factors: The as-deposited thickness of the Fe catalyst film and the use of hydrogen gas (H2) during anneal and growth. We found that the sheet resistance (Rs) of as-deposited Fe films is directly related to the average film thickness and can be used to estimate whether the films can catalyze the growth of super-aligned forests. The height of the CNT forests decrease with decreasing Rs, but only slowly. More importantly, CNTs grown on the largest and the smallest Rs films are less aligned. Instead, they are more curled and wavy due to the Fe NP dynamics. The use of Hydrogen (H2) affects the formation of Fe NPs from the as-deposited film as well as their composition during the forest growth. We find that the addition of H2 to a CNT forest growth process at 680 degrees C (C2H2/He [30/600 sccm]) increases the CNT alignment substantially. H2 can also reduce iron-oxides which otherwise would impede the formation of NPs. As a result, H2 has multiple roles: besides its chemical reactivity, H2 is important for catalyst reconstruction into NPs having a proper size distribution as well as surface density.

Spin-capable multiwalled carbon nanotube (MWCNT) forests that can form webs, sheets, and yarns provide a promising means for advancing various technologies. It is necessary to understand the critical factors to grow spin-capable carbon nanotubes (CNTs) in a repeatable fashion. Here we show how both the spinning capability and morphology of MWCNT forests are significantly changed by controlling the C2H2 concentration and ramp rate of temperature. The acetylene gas flow was varied in the range of 0.25–6.94% by volume. The MWCNTs grown at C2H2 concentrations between 1.47–3.37% are well-aligned and become spin-capable. The well-aligned forests have higher areal density and shorter distance between CNTs. The thermal ramp rate was also changed from 30 °C/min to 70 °C/min. A specific range of thermal ramp rate is also required to have the suitably sized nanoparticles with sufficient density resulting in higher CNT areal density for spinnable MWCNTs. A ramp rate of 50 °C/min forms suitable sized nanoparticles with sufficient density to produce CNT forests with a higher areal density and a shorter tube spacing.

Inspired by the high specific capacitances found using ultrathin films or nanoparticles of manganese oxides (MnO(x)), we have electrodeposited MnO(x) nanoparticles onto sheets of carbon nanotubes (CNT sheets). The resulting composites have high specific capacitances (C(sp) ≤ 1250 F/g), high charge/discharge rate capabilities, and excellent cyclic stability. Both the C(sp) and rate capabilities are controlled by the average size of the MnO(x) nanoparticles on the CNTs. They are independent of the number of layers of CNT sheets used to form an electrode. The high-performance composites result from a synergistic combination of large surface area and good electron-transport capabilities of the MnO(x) nanoparticles with the good conductivity of the CNT sheets. Such composites can be used as electrodes for lithium batteries and supercapacitors.

Pre-annealing in either H2 or He is shown to have a significant impact on the formation of either Fe nanoparticles or Fe contiguous films. This suggests various ways to form suitable-sized nanoparticles for the growth of well-aligned, spin-capable MWCNT forests. The reduction of the native iron oxide in as-deposited Fe films suppresses the formation of contiguous films, helping to form nanoparticles. In contrast, contiguous films are formed when the native iron oxide is crystallized into hematite, α-Fe2O3, by annealing the Fe film in He gas. The reduction of hematite in H2 gas breaks up the contiguous films into nanoparticles. This break-up of Fe contiguous films into nanoparticles is suppressed by other effects such as Ostwald ripening processes at the forest growth temperature of 780 °C. Understanding the catalyst dynamics for forming either Fe nanoparticles or contiguous films provides ways to control the MWCNT alignment.

Growing spin-capable multi-walled carbon nanotube (MWCNT) forests in a repeatable fashion will become possible through understanding the critical factors affecting the forest growth. Here we show that the spinning capability depends on the alignment of adjacent MWCNTs in the forest which in turn results from the synergistic combination of a high areal density of MWCNTs and short distance between the MWCNTs. This can be realized by starting with both the proper Fe nanoparticle size and density which strongly depend on the sheet resistance of the catalyst film. We prove that a simple measurement of the sheet resistance can allow one to reliably predict the growth of spin-capable forests. Further investigation into the properties of pulled MWCNTs sheets demonstrated the relationship between their electrical resistance and optical transmittance. Overlaying either 3, 5, or 10 sheets pulled out from a single forest produces much more repeatable characteristics.