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Combustion Synthesis of Aluminum Nitride Particles and Whiskers (p 2293-2300)
Steven M. Bradshaw, John L. Spicer
Published Online: Dec 21 2004 12:00AM
DOI: 10.1111/j.1151-2916.1999.tb02082.x


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Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species
Yi Cui, Qingqiao Wei, Hongkun Park, and Charles M. Lieber
Science 17 August 2001: 1289-1292.
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Heterostructures of Single-Walled Carbon Nanotubes and Carbide Nanorods
Y. Zhang, T. Ichihashi, E. Landree, F. Nihey, and S. Iijima
Science 10 September 1999: 1719-1722.
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Nanobelts of Semiconducting Oxides
Zheng Wei Pan, Zu Rong Dai, and Zhong Lin Wang
Science 9 March 2001: 1947-1949.
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Science 9 March 2001:
Vol. 291. no. 5510, pp. 1947 - 1949
DOI: 10.1126/science.1058120
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Nanobelts of Semiconducting Oxides
Zheng Wei Pan,1 Zu Rong Dai,1 Zhong Lin Wang1,2*

Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

1 School of Materials Science and Engineering,
2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA.
*  To whom correspondence should be addressed. E-mail: zhong.wang@mse.gatech.edu




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Binary semiconducting oxides, such as ZnO, SnO2, In2O3, and CdO, have distinctive properties and are now widely used as transparent conducting oxide materials (1) and gas sensors (2). For example, fluorine-doped SnO2 film is widely used in architectural glass applications because of its low emissivity for thermal infrared heat (1). SnO2 nanoparticles are regarded as one of the most important sensor materials for detecting leakage of several inflammable gases owing to their high sensitivity to low gas concentrations (2). Tin-doped indium oxide (In2O3:Sn, ITO) film is an ideal material for flat panel displays because of its high electrical conductivity and high optical transparency (1), and ZnO is regarded as an ideal alternative material for ITO because of its lower cost and easier etchability (1). The current studies of semiconducting oxides have been focused on two-dimensional films and zero-dimensional nanoparticles, which can be readily synthesized with various well-established techniques such as sputtering (for films) and sol-gel (for particles). In contrast, investigations of wirelike semiconducting oxide nanostructures are cumbersome because of the unavailability of nanowire structures.
As stimulated by the novel properties of carbon nanotubes, wirelike nanostructures have attracted extensive interest over the past decade because of their great potential for addressing some basic issues about dimensionality and space-confined transport phenomena as well as applications (3). Besides nanotubules (4, 5), many other wirelike nanomaterials, such as carbides [SiC (6-8) and TiC (6)], nitrides [GaN (9, 10) and Si3N4 (11)], compound semiconductors (12, 13), element semiconductors [Si (14-16) and Ge (14)], and oxide [Ga2O3 (17) and MgO (18)] nanowires, have been successfully fabricated. In geometrical structures, these nanostructures can be classified into two main groups: hollow nanotubes and solid nanowires, which have a common characteristic of cylindrical symmetric cross section. Here, we report another group of distinctly different semiconducting oxide nanostructures that have a rectangular cross section, in correspondence to a beltlike (or ribbonlike) morphology. The oxides with the nanobelt morphology cover cations with different valence states and materials with different crystallographic structures, and it seems to be a common structural characteristic for the family of semiconducting oxides.

Our synthesis is based on thermal evaporation of oxide powders under controlled conditions without the presence of catalyst (19). The desired oxide powders were placed at the center of an alumina tube that was inserted in a horizontal tube furnace, where the temperature, pressure, and evaporation time were controlled. In our experiments, except for the evaporation temperature, which was determined on the basis of the melting point of the oxides used, we kept the following parameters constant: evaporation time, 2 hours; chamber pressure, 300 torr; and Ar flowing rate, 50 standard cubic centimeters per minute. During evaporation, the products were deposited onto an alumina plate placed at the downstream end of the alumina tube. The as-deposited products were characterized and analyzed by x-ray diffraction (XRD) (Philips PW 1800 with Cu K radiation), scanning electron microscopy (SEM) (Hitachi S800 FEG), transmission electron microscopy (TEM) [Hitachi HF-2000 FEG at 200 kV and JEOL 4000EX high-resolution TEM (HRTEM) at 400 kV], and energy-dispersive x-ray spectroscopy (EDS).

Thermal evaporation of ZnO powders (purity: 99.99%; melting point: 1975°C) at 1400°C for 2 hours resulted in white woollike products that formed in high yield on the surface of the alumina plate. SEM observations reveal that the products consist of a large quantity of wirelike nanostructures with typical lengths in the range of several tens to several hundreds of micrometers; some of them even have lengths on the order of millimeters (Fig. 1A). EDS microanalysis and powder XRD measurement (Fig. 1B) show that the sample is wurtzite (hexagonal) structured ZnO with lattice constants of a = 3.249 Å and c = 5.206 Å, consistent with the standard values for bulk ZnO (20).

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Science 17 August 2001:
Vol. 293. no. 5533, pp. 1289 - 1292
DOI: 10.1126/science.1062711
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Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species
Yi Cui,1* Qingqiao Wei,1* Hongkun Park,1 Charles M. Lieber1, 2

Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, real-time electrically based sensors for biological and chemical species. Amine- and oxide-functionalized SiNWs exhibit pH-dependent conductance that was linear over a large dynamic range and could be understood in terms of the change in surface charge during protonation and deprotonation. Biotin-modified SiNWs were used to detect streptavidin down to at least a picomolar concentration range. In addition, antigen-functionalized SiNWs show reversible antibody binding and concentration-dependent detection in real time. Lastly, detection of the reversible binding of the metabolic indicator Ca2+ was demonstrated. The small size and capability of these semiconductor nanowires for sensitive, label-free, real-time detection of a wide range of chemical and biological species could be exploited in array-based screening and in vivo diagnostics.

1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA.
*  These authors contributed equally to the work.

  To whom correspondence should be addressed. E-mail: cml@cmliris.harvard.edu




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Planar semiconductors can serve as the basis for chemical and biological sensors in which detection can be monitored electrically and/or optically (1-4). For example, a planar field effect transistor (FET) can be configured as a sensor by modifying the gate oxide (without gate electrode) with molecular receptors or a selective membrane for the analyte of interest; binding of a charged species then results in depletion or accumulation of carriers within the transistor structure (1, 2). An attractive feature of such chemically sensitive FETs is that binding can be monitored by a direct change in conductance or related electrical property, although the sensitivity and potential for integration are limited.
The physical properties limiting sensor devices fabricated in planar semiconductors can be readily overcome by exploiting nanoscale FETs (5-9). First, binding to the surface of a nanowire (NW) or nanotube (NT) can lead to depletion or accumulation of carriers in the "bulk" of the nanometer diameter structure (versus only the surface region of a planar device) and increase sensitivity to the point that single-molecule detection is possible. Second, the small size of NW and NT building blocks and recent advances in assembly (9, 10) suggest that dense arrays of sensors could be prepared. Indeed, NT FETs were shown recently by Dai and co-workers to function as gas sensors (11). Calculations suggested that direct binding of electron-withdrawing NO2 or electron-donating NH3 gas molecules to the NT surface chemically gated these devices. However, several properties of NTs could also limit their development as nanosensors, including the following: (i) existing synthetic methods produce mixtures of metallic and semiconducting NTs, which make systematic studies difficult because metallic "devices" will not function as expected, and (ii) flexible methods for the modification of NT surfaces, which are required to prepare interfaces selective for binding a wide range of analytes, are not well established.

Nanowires of semiconductors such as Si do not have these limitations, as they are always semiconducting, and the dopant type and concentration can be controlled (7-9), which enables the sensistivity to be tuned in the absence of an external gate. In addition, it should be possible to exploit the massive knowledge that exists for the chemical modification of oxide surfaces, for example, from studies of silica (12) and planar chemical and biological sensors (4, 13), to create semiconductor NWs modified with receptors for many applications. Here we demonstrate the potential of NW nanosensors with direct, highly sensitive real-time detection of chemical and biological species in aqueous solution.

The underlying concept of our experiments is illustrated first for the case of a pH nanosensor (Fig. 1A). Here a silicon NW (SiNW) solid state FET, whose conductance is modulated by an applied gate, is transformed into a nanosensor by modifying the silicon oxide surface with 3-aminopropyltriethoxysilane (APTES) to provide a surface that can undergo protonation and deprotonation, where changes in the surface charge can chemically gate the SiNW. The single-crystal boron-doped (p-type) SiNWs used in these studies were prepared by a nanocluster-mediated vapor-liquid-solid growth method described previously (7, 8, 14). Devices were fashioned by flow aligning (10) SiNWs on oxidized silicon substrates and then making electrical contacts to the NW ends with electron-beam lithrography (7, 8, 15). Linear current (I) versus voltage (V) behavior was observed for all of the devices studied, which shows that the SiNW-metal contacts are ohmic, and applied gate voltages produced reproducible and predictable (7, 8) changes in the I-V. In the solid state FET (insets, Fig. 1B), the conductance (dI/dV) measured in air at V = 0 as a function of time (15) was stable for a given gate voltage and showed a stepwise increase with discrete changes of the gate voltage from 10 to -10 V; plots of conductance versus gate voltage were nearly linear.

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Science 10 September 1999:
Vol. 285. no. 5434, pp. 1719 - 1722
DOI: 10.1126/science.285.5434.1719
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Heterostructures of Single-Walled Carbon Nanotubes and Carbide Nanorods
Y. Zhang, 1* T. Ichihashi, 1 E. Landree, 2 F. Nihey, 1 S. Iijima 13

A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.

1 Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan.
2 Department of Materials Science and Engineering, Northwestern University, 2225 North Campus Drive, Evanston, IL 60208, USA.
3 Japan Science and Technology Corporation, c/o Department of Physics, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan.
*  To whom correspondence should be addressed. E-mail: zhang@frl.cl.nec.co.jp




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Heterostructures with well-defined crystalline interfaces are essential in electronic devices. The prospect of heterostructures made from single-walled carbon nanotubes (SWCNTs) (1) are of particular interest for nanodevices because of their useful size scale and their unique electronic properties. SWNTs can be either semiconducting or metallic, depending on their diameter and chirality (2). Carbide materials also play an important role in the electronics industry. For example, silicon carbide (SiC) is a useful wide-gap semiconductor for high-temperature, high-frequency, or high-power applications (3). Transition metal carbides, such as titanium carbide (TiC) and niobium carbide (NbC), are good metallic conductors with high melting points, high corrosion resistance, and low diffusion coefficients and are therefore suitable for interconnects in ultra-large-scale integrated circuits (4). Combining these materials with SWCNTs may open the possibility for new device applications. However, until now there have been no techniques available for fabricating SWCNT/carbide heterojunctions. Earlier investigations have reported a method for fabricating carbide nanorods from multiwalled carbon nanotubes (MWCNTs) through a vapor-solid reaction (5). This method has not been used for fabricating nanojunctions because of the difficulty of controlling the reaction region along the carbon nanotube (6). Recently, Hu et al. have reported a catalytic vapor growth method for fabricating heterojunctions between MWCNTs and Si nanowires (7). The electrical characterization has shown the typical rectifying behaviors for the metal-semiconductor junctions produced by this method. However, the need for specific catalysts and vapor reactants has limited the generality of the method and caused several demerits. The first demerit is the presence of catalyst clusters at or near the junctions produced by growing MWCNTs on Si nanowires. The second is the contamination of amorphous Si and SiOx layers on the surfaces of both nanotubes and nanowires while growing Si nanowires on the MWCNTs. The third is the unexpected doping of catalyst atoms in Si nanowires. These demerits have made it difficult to obtain a sharply defined interface and may influence the structure and properties of a junction with a smaller size, such as an individual SWCNT junction.
In this report, we describe a simple and clean method for fabricating SWCNT/carbide heterostructures with well-defined nanometer-scale interfaces. The method is based on a direct solid-solid reaction: C (nanotubes) + M (solid)  MC (solid), where M is either Si or a transition metal. The reaction is spatially restricted by partial contact between the surface of the solid reactant (M) and carbon nanotubes and by performance of the reaction in ultra-high vacuum or an inert atmosphere to avoid any volatile reactant. The carbide initially forms at the C/M interface once a sufficient temperature (T) is reached for the reaction to occur (roughly T > 800°C) (Fig. 1). The continuous transformation of the SWCNTs to carbide is controlled by the diffusion of M to the C/MC interface (8). However, the self-diffusion rate through bulk SiC or transition metal carbide is extremely slow in the temperature range of interest (800°C < T < 1000°C). A continuous supply of Si or metal atoms is therefore transported primarily via surface diffusion. This is consistent with the known formation of SiC films on the surface of Si (8). The diffusion path length, along with the annealing temperature and time, provide a means of controlling the formation of the SWCNT/MC heterostructure near the site of the contact between SWCNTs and the M substrate.

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Synthesis of Aluminum Nitride Nanowires from Carbon Nanotubes
Yingjiu Zhang, Jun Liu, Rongrui He, Qi Zhang, Xiaozhong Zhang, and Jing Zhu
pp 3899–3905
Publication Date (Web): September 19, 2001 (Article)
DOI: 10.1021/cm001422a