Kaili Jiang | Tsinghua University (original) (raw)
Papers by Kaili Jiang
We report the in situ transmission electron microscope (TEM) observation of the catalytic gasific... more We report the in situ transmission electron microscope (TEM) observation of the catalytic gasification and growth of carbon nanotubes (CNTs). It was found that iron catalysts can consume the CNTs when pumping out the precursor gas, acetylene, at the growth temperature, and reinitiate the growth when acetylene is re-introduced. The switching between gasification and growth of CNTs can be repeated many times with the same catalyst. To understand the phenomenon, thermogravimetric analysis (TGA) coupled with mass spectroscopy was used to study the mechanism involved. It was shown that the residual water molecules in the growth chamber of the TEM react with and remove carbon atoms of CNTs as carbon monoxide vapor under the action of the catalyst, when the precursor gas is pumped out. This result contributes to a better understanding of the water-assisted and oxygen-assisted synthesis of CNT arrays, and provides useful clues on how to extend the lifetime and improve the activity of the catalysts.
Surface-enhanced Raman scattering (SERS) has attracted wide attention because it can enhance norm... more Surface-enhanced Raman scattering (SERS) has attracted wide attention because it can enhance normally weak Raman signal by several orders of magnitude and facilitate the sensitive detection of molecules. Conventional SERS substrates are constructed by placing metal nanoparticles on a planar surface. Here we show that, if the planar surface was substituted by a unique nanoporous surface, the enhancement effect can be dramatically improved. The nanoporous surface can be easily fabricated in batches and at low costs by cross stacking superaligned carbon nanotube films. The as-prepared transparent and freestanding SERS substrate is capable of detecting ambient trinitrotoluene vapor, showing much higher Raman enhancement than ordinary planar substrates because of the extremely large surface area and the unique zero-dimensional at one-dimensional nanostructure. These results not only provide a new approach to ultrasensitive SERS substrates, but also are helpful for improving the fundamental understanding of SERS phenomena.
The cooling effect accompanying field electron emission has been considered for a single carbon n... more The cooling effect accompanying field electron emission has been considered for a single carbon nanotube (CNT) used as a field emission (FE) electron source. An improved model for the failure mechanism of field emitting CNTs has been proposed and validated. Our model predicts a maximum temperature (T-max) located at an interior point rather than the tip of the CNTs, and the failure of the CNT emitters tends to take place at the T-max point, inducing a segment by segment breakdown process. A combination of Joule heating and electrostatic force effect is proposed responsible for initiating the failure of the field emitting CNT and validated by in situ FE observation.
Polarized light emission from multiwalled carbon nanotube ͑MWNT͒ bundles due to current heating i... more Polarized light emission from multiwalled carbon nanotube ͑MWNT͒ bundles due to current heating is observed. The spectra of the emitted light fit well with the blackbody radiation distribution. And the emitted light is partially polarized with a degree of 0.33 along the axis of MWNT bundle, which is qualitatively explained in terms of one-dimensional structure of carbon nanotubes ͑CNTs͒. Negative temperature-dependent resistance is also observed, which is different from normal metal filaments. The MWNT bundles are very stable at high temperature in vacuum during light emitting, indicating that CNTs can be a good candidate as polarized incandescent light sources.
is higher than that observed for Co 3 O 4 with an average particle size of 20 nm (25 K). This is ... more is higher than that observed for Co 3 O 4 with an average particle size of 20 nm (25 K). This is in accordance with the smaller average particle size, which corresponds to a higher fraction of surface spins, and hence for which bulk behavior is expected to occur at higher temperatures. Relationships between block temperature and particle size have also been studied for Mn 3 O 4 and c-Fe 2 O 3 , and the peak temperature was also found to increase with decreasing particle size. In conclusion, mesostructured Co 3 O 4 with Ia3d symmetry has been nanocast from cubic Ia3d mesoporous vinylsilica. The vinyl groups play an important role in entrapping Co(NO 3 ) 2 inside the pores. This process seems to be extendable to other metal oxides. The cobalt oxide synthesized has interesting magnetic properties, and may be the basis of magnetic nanocomposites, if the pore system can be filled with another ferromagnetic material. It will be very interesting to see what kind of magnetic properties result from the interaction of two different, ordered, ferromagnetic structures on a nanometer size scale. Experimental 0.5 g of vinyl-functionalized silica (synthesized using 20 % vinyltriethoxysilane and aged at 100 C [4], hereafter called vinylsilica) was soaked in 5 mL of 0.8 M Co(NO 3 ) 2 ethanolic solution, dried at 200 C, re-impregnated, and then calcinated at 450 C for 6 h. The amount of cobalt oxide in the composite was then typically 50± 55 wt.-%. The silica was then dissolved in a 2 M NaOH aqueous solution, with the NaOH solution being replaced 3 times. The samples were characterized using standard techniques, such as XRD, N 2 sorption analysis, and TEM. The magnetic properties were measured using a superconducting quantum interference device (SQUID).
A straightforward roll-to-roll process for fabricating flexible and stretchable superaligned carb... more A straightforward roll-to-roll process for fabricating flexible and stretchable superaligned carbon nanotube films as transparent conducting films is demonstrated. Practical touch panels assembled by using these carbon nanotube conducting films are superior in flexibility and wearability-and comparable in linearity-to touch panels based on indium tin oxide (ITO) films. After suitable laser trimming and deposition of Ni and Au metal, the carbon nanotube film possesses excellent performance with two typical values of sheet resistances and transmittances (208 V & À1 , 90% and 24 V & À1 , 83.4%), which are comparable to ITO films and better than the present carbon nanotube conducting films in literature. The results provide a route to produce transparent conducting films more easily, effectively, and cheaply, an important step for realizing industrial-scale applications of carbon nanotubes for transparent conducting films.
A superaligned carbon nanotube (CNT) array is a special kind of vertically aligned CNT array with... more A superaligned carbon nanotube (CNT) array is a special kind of vertically aligned CNT array with the capability of being converted into continuous fi lms and yarns. The as-produced CNT fi lms are transparent and highly conductive, with aligned CNTs parallel to the direction of drawing. After passing through volatile solutions or being twisted, CNT fi lms can be further condensed into shrunk yarns. These shrunk yarns possess high tensile strengths and Young's moduli, and are good conductors. Many applications of CNT fi lms and shrunk yarns have been demonstrated, such as TEM grids, loudspeakers, touch screens, etc.
We found that very thin carbon nanotube films, once fed by sound frequency electric currents, cou... more We found that very thin carbon nanotube films, once fed by sound frequency electric currents, could emit loud sounds. This phenomenon could be attributed to a thermoacoustic effect. The ultra small heat capacity per unit area of carbon nanotube thin films leads to a wide frequency response range and a high sound pressure level. On the basis of this finding, we made practical carbon nanotube thin film loudspeakers, which possess the merits of nanometer thickness and are transparent, flexible, stretchable, and magnet-free. Such a singleelement thin film loudspeaker can be tailored into any shape and size, freestanding or on any insulating surfaces, which could open up new applications of and approaches to manufacturing loudspeakers and other acoustic devices.
We report the in situ transmission electron microscope (TEM) observation of the catalytic gasific... more We report the in situ transmission electron microscope (TEM) observation of the catalytic gasification and growth of carbon nanotubes (CNTs). It was found that iron catalysts can consume the CNTs when pumping out the precursor gas, acetylene, at the growth temperature, and reinitiate the growth when acetylene is re-introduced. The switching between gasification and growth of CNTs can be repeated many times with the same catalyst. To understand the phenomenon, thermogravimetric analysis (TGA) coupled with mass spectroscopy was used to study the mechanism involved. It was shown that the residual water molecules in the growth chamber of the TEM react with and remove carbon atoms of CNTs as carbon monoxide vapor under the action of the catalyst, when the precursor gas is pumped out. This result contributes to a better understanding of the water-assisted and oxygen-assisted synthesis of CNT arrays, and provides useful clues on how to extend the lifetime and improve the activity of the catalysts.
Surface-enhanced Raman scattering (SERS) has attracted wide attention because it can enhance norm... more Surface-enhanced Raman scattering (SERS) has attracted wide attention because it can enhance normally weak Raman signal by several orders of magnitude and facilitate the sensitive detection of molecules. Conventional SERS substrates are constructed by placing metal nanoparticles on a planar surface. Here we show that, if the planar surface was substituted by a unique nanoporous surface, the enhancement effect can be dramatically improved. The nanoporous surface can be easily fabricated in batches and at low costs by cross stacking superaligned carbon nanotube films. The as-prepared transparent and freestanding SERS substrate is capable of detecting ambient trinitrotoluene vapor, showing much higher Raman enhancement than ordinary planar substrates because of the extremely large surface area and the unique zero-dimensional at one-dimensional nanostructure. These results not only provide a new approach to ultrasensitive SERS substrates, but also are helpful for improving the fundamental understanding of SERS phenomena.
The cooling effect accompanying field electron emission has been considered for a single carbon n... more The cooling effect accompanying field electron emission has been considered for a single carbon nanotube (CNT) used as a field emission (FE) electron source. An improved model for the failure mechanism of field emitting CNTs has been proposed and validated. Our model predicts a maximum temperature (T-max) located at an interior point rather than the tip of the CNTs, and the failure of the CNT emitters tends to take place at the T-max point, inducing a segment by segment breakdown process. A combination of Joule heating and electrostatic force effect is proposed responsible for initiating the failure of the field emitting CNT and validated by in situ FE observation.
Polarized light emission from multiwalled carbon nanotube ͑MWNT͒ bundles due to current heating i... more Polarized light emission from multiwalled carbon nanotube ͑MWNT͒ bundles due to current heating is observed. The spectra of the emitted light fit well with the blackbody radiation distribution. And the emitted light is partially polarized with a degree of 0.33 along the axis of MWNT bundle, which is qualitatively explained in terms of one-dimensional structure of carbon nanotubes ͑CNTs͒. Negative temperature-dependent resistance is also observed, which is different from normal metal filaments. The MWNT bundles are very stable at high temperature in vacuum during light emitting, indicating that CNTs can be a good candidate as polarized incandescent light sources.
is higher than that observed for Co 3 O 4 with an average particle size of 20 nm (25 K). This is ... more is higher than that observed for Co 3 O 4 with an average particle size of 20 nm (25 K). This is in accordance with the smaller average particle size, which corresponds to a higher fraction of surface spins, and hence for which bulk behavior is expected to occur at higher temperatures. Relationships between block temperature and particle size have also been studied for Mn 3 O 4 and c-Fe 2 O 3 , and the peak temperature was also found to increase with decreasing particle size. In conclusion, mesostructured Co 3 O 4 with Ia3d symmetry has been nanocast from cubic Ia3d mesoporous vinylsilica. The vinyl groups play an important role in entrapping Co(NO 3 ) 2 inside the pores. This process seems to be extendable to other metal oxides. The cobalt oxide synthesized has interesting magnetic properties, and may be the basis of magnetic nanocomposites, if the pore system can be filled with another ferromagnetic material. It will be very interesting to see what kind of magnetic properties result from the interaction of two different, ordered, ferromagnetic structures on a nanometer size scale. Experimental 0.5 g of vinyl-functionalized silica (synthesized using 20 % vinyltriethoxysilane and aged at 100 C [4], hereafter called vinylsilica) was soaked in 5 mL of 0.8 M Co(NO 3 ) 2 ethanolic solution, dried at 200 C, re-impregnated, and then calcinated at 450 C for 6 h. The amount of cobalt oxide in the composite was then typically 50± 55 wt.-%. The silica was then dissolved in a 2 M NaOH aqueous solution, with the NaOH solution being replaced 3 times. The samples were characterized using standard techniques, such as XRD, N 2 sorption analysis, and TEM. The magnetic properties were measured using a superconducting quantum interference device (SQUID).
A straightforward roll-to-roll process for fabricating flexible and stretchable superaligned carb... more A straightforward roll-to-roll process for fabricating flexible and stretchable superaligned carbon nanotube films as transparent conducting films is demonstrated. Practical touch panels assembled by using these carbon nanotube conducting films are superior in flexibility and wearability-and comparable in linearity-to touch panels based on indium tin oxide (ITO) films. After suitable laser trimming and deposition of Ni and Au metal, the carbon nanotube film possesses excellent performance with two typical values of sheet resistances and transmittances (208 V & À1 , 90% and 24 V & À1 , 83.4%), which are comparable to ITO films and better than the present carbon nanotube conducting films in literature. The results provide a route to produce transparent conducting films more easily, effectively, and cheaply, an important step for realizing industrial-scale applications of carbon nanotubes for transparent conducting films.
A superaligned carbon nanotube (CNT) array is a special kind of vertically aligned CNT array with... more A superaligned carbon nanotube (CNT) array is a special kind of vertically aligned CNT array with the capability of being converted into continuous fi lms and yarns. The as-produced CNT fi lms are transparent and highly conductive, with aligned CNTs parallel to the direction of drawing. After passing through volatile solutions or being twisted, CNT fi lms can be further condensed into shrunk yarns. These shrunk yarns possess high tensile strengths and Young's moduli, and are good conductors. Many applications of CNT fi lms and shrunk yarns have been demonstrated, such as TEM grids, loudspeakers, touch screens, etc.
We found that very thin carbon nanotube films, once fed by sound frequency electric currents, cou... more We found that very thin carbon nanotube films, once fed by sound frequency electric currents, could emit loud sounds. This phenomenon could be attributed to a thermoacoustic effect. The ultra small heat capacity per unit area of carbon nanotube thin films leads to a wide frequency response range and a high sound pressure level. On the basis of this finding, we made practical carbon nanotube thin film loudspeakers, which possess the merits of nanometer thickness and are transparent, flexible, stretchable, and magnet-free. Such a singleelement thin film loudspeaker can be tailored into any shape and size, freestanding or on any insulating surfaces, which could open up new applications of and approaches to manufacturing loudspeakers and other acoustic devices.