Changxin Chen

Ultrasonic nanowelding of carbon nanotubes to metal electrodes

As imple ultrasonic nanowelding technique has been developed to reliably bond single-wall carbon nanotubes (SWCNTs) onto metal electrodes, by pressing SWCNTs against electrodes under a vibrating force at ultrasonic frequency. The bonds formed have been demonstrated to be mechanically robust. Using this technique, a stable low-Ohmic contact between SWCNTs and metal electrodes was achieved, with resistances in the range of 8–24 k� for a 1 µ ml ong metallic SWCNT at room temperature. The performance of carbon nanotube field-effect transistors (FETs) fabricated using this ultrasonic nanowelding method has also been greatly improved. Transconductance as high as 3.6 µ Sa mong the solid-state back-gate individual nanotube FETs has been achieved. (Some figures in this article are in colour only in the electronic version)

Nanowelded Carbon‐Nanotube‐Based Solar Microcells

Photovoltaic (PV) cells are of immense interest due to their vast application potential in the fields of energy and communication. The adoption of ideal photoactive material and the design of optimum device structure are critical to achieving low-cost, high-efficiency PV cells. The semiconducting single-walled carbon nanotubes (SWNTs) are potentially an attractive material for PV applications due to their many unique structural and electrical properties. They are almost defect free to greatly decrease carrier recombination, bear a wide range of direct bandgaps matching the solar spectrum, and show strong photoabsorption and photoresponse from ultraviolet to infrared, and exhibit high carrier mobility [16] and reduced carrier transport scattering. Indeed, previous studies had attempted to fabricate SWNT films into photoelectrochemical solar cells. However, due to the inefficient separation and collection of photoexcited carriers and large intertube interaction, the maximum monochromatic incident photo-to-current conversion efficiency (IPCE) obtained for the cell is only 0.15%. Here, we report a novel approach that enables fabricating SWNT PV solar microcells with high power-conversion efficiency. In this cell, a directed array of monolayer SWNTs was nanowelded onto two asymmetrical metal electrodes with high and low work functions, respectively, resulting in a strong built-in electric field in SWNTs for efficient separation of photogenerated electron–hole pairs. Under solar illumination,

Manipulation of single-wall carbon nanotubes into dispersively aligned arrays between metal electrodes

A dispersively aligned carbon nanotube array has been achieved between metal electrodes by electric-field assisted manipulation of surface modified single-wall carbon nanotubes (SWCNTs). Surface modification of SWCNTs with octadecylamine (ODA) molecules creates well-dispersed SWCNT solutions and effectively obviates the entanglement between deposited SWCNTs. The alignment effect of SWCNTs is improved by aligning them with an ac electric field of high frequency in a high-volatility solvent. The good electrical contact between the aligned SWCNTs and the metal electrodes is obtained by adopting a UV-light radiation method for removing the grafted ODA on the aligned SWCNTs.

Poly (acrylic acid sodium) grafted carboxymethyl cellulose as a high performance polymer binder for silicon anode in lithium ion batteries

The design of novel binder systems is required for the high capacity silicon (Si) anodes which usually undergo huge volume change during the charge/discharge cycling. Here, we introduce a poly (acrylic acid sodium)-grafted-carboxymethyl cellulose (NaPAA-g-CMC) copolymer as an excellent binder for Si anode in lithium ion batteries (LIBs). The NaPAA-g-CMC copolymer was prepared via a free radical graft polymerization method by using CMC and acrylic acid as precursors. Unlike the linear, one-dimensional binders, the NaPAA-g-CMC copolymer binder is expected to present multi-point interaction with Si surface, resulting in enhanced binding ability with Si particles as well as with the copper (Cu) current collectors, and building a stable solid electrolyte interface (SEI) layer on the Si surface. The NaPAA-g-CMC based Si anode shows much better cycle stability and higher coulombic efficiency than those made with the well-known linear polymeric binders such as CMC and NaPPA.

Surface charges and optical characteristic of colloidal cubic SiC nanocrystals

Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

Carbon nanotube photovoltaic device with asymmetrical contacts

A photovoltaic (PV) device based on “high-work-function metal/single-walled carbon nanotube/low-work-function metal” hybrid junction has been studied theoretically by the self-consistent nonequilibrium Green’s function approach. The PV effect and power conversion efficiency (η) of the device under light illumination are simulated, with a monochromatic η of higher than 40% for incident photon energies near the nanotube band-gap energy predicted. It is shown that the gate voltage and gate oxide thickness have an important influence on the device η.

Hole doping and surface functionalization of single-walled carbon nanotube chemiresistive sensors for ultrasensitive and highly selective organophosphor vapor detection

We developed a chemiresistive sensor based on doped and functionalized semiconducting single-walled carbon nanotube (SWNT) networks for ultrasensitive and rapid detection of dimethyl methylphosphonate (DMMP) (simulant of nerve agent sarin) vapor. The semiconducting SWNT network was deposited between interdigitated electrodes and modified by solid organic acid tetrafluorohydroquinone (TFQ). The TFQ molecules could not only selectively bind DMMP onto the sidewalls of SWNTs via the strong hydrogen bonding interaction, but also tailor the electronic properties of SWNTs via heavy hole doping. This synergetic effect significantly improved the sensitivity of the devices, and enabled the sensors to easily detect DMMP at 20 parts-per-trillion (ppt) concentration with a response time of less than 2 min, without the need for pre-concentration of the analytes. This sensitivity is about five orders of magnitude higher than that of the unmodified SWNT chemiresistor, and also significantly higher than that of the functionalized SWNT chemiresistors previously reported. Moreover, the SWNT-TFQ sensors could be recovered when DMMP is replaced with referencing gas. The SWNT-TFQ sensors also show excellent selectivity toward DMMP over some interfering organic vapors. The response mechanism, i.e. charge transfer and dedoping was investigated.

Theoretical Simulation on the Assembly of Carbon Nanotubes Between Electrodes by AC Dielectrophoresis

The assembly of single-walled carbon nanotubes (SWCNTs) using the AC dielectrophoresis technique is studied theoretically. It is found that the comb electrode bears better position control of SWCNTs compared to the parallel electrode. In the assembly, when some SWCNTs bridge the electrode first, they can greatly alter the local electrical field so as to “screen off” later coming SWCNTs, which contributes to the formation of dispersed SWCNT array. The screening distance scales with the gap width of electrodes and the length of SWCNTs, which provides a way to estimate the assembled density of SWCNTs. The influence of thermal noise on SWCNTs alignment is also analyzed in the simulation. It is shown that the status of the array distribution for SWCNTs is decided by the competition between the thermal noise and the AC electric-field strength. This influence of the thermal noise can be suppressed by using higher AC voltage to assemble the SWCNTs.

Assessment of Optical Absorption in Carbon Nanotube Photovoltaic Device by Electromagnetic Theory

An electromagnetic (EM) scattering model is built for a kind of single-walled carbon nanotube (SWCNT) photovoltaic device excited by light. In this model, the exciting light is treated as classical EM wave with a very high frequency, and the SWCNTs in the device were treated as a lossy dielectric cylinder with frequency-dependent complex permittivity. Based on the EM scattering model, the Foldy-Lax multiple-scattering equation for the SWCNT cylinders can be derived, and then, the absorbed power of SWCNTs can be estimated. We also use EM simulation software - high frequency structure simulator (HFSS) - to extract the optical absorption of SWCNTs, and then the property of optical absorption of the device is studied more carefully; and the EM scattering model is also validated through HFSS simulation. From the results, some advices are given for the design of such kind of device.

Functionalized self-assembled monolayers on mesoporous silica nanoparticles with high surface coverage

Mesoporous silica nanoparticles (MSNs) containing vinyl-, propyl-, isobutyl- and phenyl functionalized monolayers were reported. These functionalized MSNs were prepared via molecular self-assembly of organosilanes on the mesoporous supports. The relative surface coverage of the organic monolayers can reach up to 100% (about 5.06 silanes/nm2). These monolayer functionalize MSNs were analyzed by a number of techniques including transmission electron microscope, fourier transform infrared spectroscopy, X-ray diffraction pattern, cross-polarized Si29 MAS NMR spectroscopy, and nitrogen sorption measurement. The main elements (i.e., the number of absorbed water, the reactivity of organosilanes, and the stereochemistry of organosilane) that greatly affected the surface coverage and the quality of the organic functionalized monolayers on MSNs were fully discussed. The results show that the proper amount of physically absorbed water, the use of high active trichlorosilanes, and the functional groups with less steric hindrance are essential to generate MSNs with high surface coverage of monolayers.