Scholarly article on topic 'Molybdenum oxide nanowires: synthesis & properties'

Molybdenum oxide nanowires: synthesis & properties Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Liqiang Mai, Fan Yang, Yunlong Zhao, Xu Xu, Lin Xu, et al.

Molybdenum oxide nanowires have been found to show promise in a diverse range of applications, ranging from electronics to energy storage and micromechanics. This review focuses on recent research on molybdenum oxide nanowires: from synthesis and device assembly to fundamental properties. The synthesis of molybdenum oxide nanowires will be reviewed, followed by a discussion of recent progress on molybdenum oxide nanowire based devices and an examination of their properties. Finally, we conclude by considering future developments.

Academic research paper on topic "Molybdenum oxide nanowires: synthesis & properties"

Molybdenum oxide nanowires:

synthesis & properties

Molybdenum oxide nanowires have been found to show promise in a diverse range of applications, ranging from electronics to energy storage and micromechanics. This review focuses on recent research on molybdenum oxide nanowires: from synthesis and device assembly to fundamental properties. The synthesis of molybdenum oxide nanowires will be reviewed, followed by a discussion of recent progress on molybdenum oxide nanowire based devices and an examination of their properties. Finally, we conclude by considering future developments.

Liqiang Maiab'*, Fan Yanga, Yunlong Zhaoa, Xu Xua, Lin Xuab, Bin Hua, Yanzhu Luoa, and Hangyu Liua

aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, WUT-Harvard Joint Nano Key Lab, Wuhan University of Technology, Wuhan 430070, China

bDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA *E-mail: mlq@cmliris.harvard.edu

The field of transition metal oxides represents an exciting and rapidly expanding research area that spans the border between the physical and engineering sciences1-4. Molybdenum oxides (MoOx) are one of the most attractive metal oxides due to their special structural characteristics. MoOx comprises two simple binary oxides, namely, MoO3 and MoO2. MoO3 has several polymorphs, such as the thermodynamically stable a-MoO3 (space group Pnma), metastable P-MoO3 (P21/c), e-MoO3 (P21/m), and hexagonal metastable h-MoO3 (P63/m)5. MoO2, with its distorted rutile structure, is an unusual but interesting transition metal oxide because of its low metallic electrical resistivity (8.8 x 10-5 Qcm at 300 K in bulk samples), high melting point, and high chemical stability6. MoO2 has been used as a catalyst for alkane isomerization7-11, oxidation reactions12 and as a gas sensor13. It is also a promising anode material for Li-ion batteries14-18. As nanotechnology has developed, nanostuctrues have received significant attention. Interesting physical phenomena appear as the

scale of the building blocks approaches the nanoscale, such as the size effect, quantum conductance, and coulomb blockades. Nanowires are one of these building blocks that possess several distinct, practical properties, such as well-controlled dimensional composition, electronic radial transport, and crystallinity; this helps organize the nanoscale building blocks into assemblies and, ultimately, useful systems. Although research on molybdenum oxide (MoOx) nanowires started relatively late, MoOx nanowires are now showing potential for both fundamental research and applications in industry. MoOx nanowires represent attractive building blocks for active nanodevices. By controlling the growth and organization, they can be used to produce a number of novel, highly-efficient, robust, integrated nanoscale devices, including field emission devices (FED) and photodetectors.

Nanowires and nanorods are typically cylindrical, hexagonal, square, or triangular in cross-section. Nanobelts are typically rectangular in cross-section, with highly anisotropic dimensions. In this article, nanobelts and nanorods are regarded as special kinds of nanowire.

346 materialstoday july-august 2011 | volume 14 | number 7-8

ISSN:1369 7021 © Elsevier Ltd 2011

Fig. 1 "Bottom-up" fabrication procedure for MoOx nanowire devices.

Nowadays, the bottom-up technique is usually used for the assembly of MoOx nanowires and devices. A bottom-up design necessitates building with precisely controlled nanomaterial parameters (including chemical composition and structure), which determine the final performance of the device19,20. Fig. 1 shows the bottom-up procedure for MoOx nanowire device fabrication. To date, there has been no review that systematically summarizes the recent advances in MoOx nanowires from synthesis to devices and their properties. In this article, we focus on recent advances on MoOx nanowires based on our group's work.

Synthesis

Before 2000, nanomaterials based on molybdenum compounds, such as MoS221,22, MoSi223, MoSe224, etc., had been investigated extensively. It is only during the last few years that MoOx nanowires have attracted any attention in the literature. In 2000, Zach et al.25 used the electrode position method to prepare MoOx nanowires ranging from 20 nm to 1.3 pm in diameter for the first time. In the same year, carbon nanotubes were used as a template to synthesize a-MoO3 nanowires with a diameter of 5 - 15 nm26 After that, various strategies were developed to design and rationally synthesize MoOx nanowires with predictable control over the key structural, chemical, and physical properties. In 2001, Nesper et al.27 synthesized molybdenum oxide nanowires with a high aspect ratio using a template-directed hydrothermal process. In 2003, Wang et al.28 prepared nanowires of single-crystal a-MoO3 without using a catalyst or template. In 2007, Mai et al.29,30 synthesized a-MoO3 nanowires, and for the first time lithiated a-MoO3 nanowires to improve the electrochemical properties. A secondary reaction with a LiCl solution allowed the nanowires to retain their crystal structure and surface morphology, simultaneously. The crystallographic information and morphology can be seen in Fig. 2. In 2011, Cai et al.31 synthesized flower-like a-MoO3 nanowire arrays on diverse substrates using an atmospheric, catalyst-free, rapid flame synthesis technique. The growth rate, morphology, and surface coverage density of the a-MoO3 nanowires were all controlled.

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Fig. 2 (a) X-ray diffraction (XRD) patterns of an MoO3 nanowire before and after lithiation. The inset is the corresponding (020) diffraction peak. (b),(c) SEM, TEM, and HRTEM characterization of the nanowire before and after lithiation, respectively. The insets in the HRTEM images are the corresponding selected area electron diffraction (SAED) patterns. Reprinted from29. © Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

MoO2 is another important molybdenum oxide. The electronic configuration of Mo4+ is 4d2, and MoO2 possesses a monoclinic structure32. MoO2 is used extensively, in Li-ion batteries, field emission devices, catalysts, sensors, photochromic devices, and electrochromic devices, etc. In 2000, Satishkumar et al.26 prepared MoO2 nanowires using carbon nanotube templates. In this experiment, MoO3 was first produced, and then heated in H2 at 500 °C for 48 hours to prepare MoO2 nanowires. Zhou et al.33 produced nanowire arrays of MoOx on silicon substrates based on a process of thermal evaporation followed by further oxidation (for the oxide ensembles). MoO2 nanowire arrays were first grown on silicon substrates by heating a Mo boat under a constant flow of argon for 60 minutes in a vacuum chamber.

In last decade MoO2 nanowires have mainly been synthesized by reducing MoO3 nanowires, but the structural transformation from MoO3 to MoO2 has been seldom studied. In 2009, Hu et al.34 prepared MoO3 nanowires using a MoO3nH2O solution through the hydrothermal technique, and obtained MoO2 nanowires via hydrogen reduction of the as-synthesized MoO3 nanowires. They then

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Fig. 3 (a) SEM image of MoO3 nanowire templates. (b) Low and (c) high magnification images of MoO2 nanowires. (d) Schematic illustration of the transformation from MoO3 nanowires to MoO2 nanowires and (e) a different top view of a MoO3 layered structure with the cleavage direction. Reprinted with permission from34. © 2009 American Chemical Society.

investigated the growth mechanism of one-dimensional MoO2, which can be explained by the cleavage process, due to defects in the MoO3. The decrease of length and width was attributed to the cleavage mechanism and the presence of defects in the MoO3 nanowire. The morphology and transformation mechanism are shown in Fig. 3.

Several molybdenum oxides also have important properties, and thus it has been necessary to explore approaches for preparing nanowires from multiple molybdenum oxides. The liquid phase method is a traditional approach for preparing multiple molybdenum oxide nanowires, and it is hoped that other kinds of flexible method will be available in the future. Dong et al.35 synthesized large scale and homogeneous bunched lead molybdate nanowires via a vertically supported liquid membrane system in the presence of ethylenediamine. Ghorai et al.36 prepared bismuth molybdate (a-Bi2Mo3O12 phase) nanowires through a pyridine intercalative sonochemical route. Thongtem et al.37 reported reactions using microwave radiation to synthesize multiple molybdenum oxide nanowires.

By reviewing the synthesis of MoOx nanowires, it has been shown that molybdenum oxide nanowires can be synthesized using many different techniques, such as the gas and liquid phase methods. Template technology can be adopted for controlling the morphology and orderliness of the MoOx nanowires. Self-assembled nanowires are of great importance in the design and fabrication of nanowire based devices. However, optimizing nanowire positioning, growth direction, morphology, and crystal structure (hexagonal/cubic) requires greater

knowledge of the conditions under which one-dimensional crystals preferentially form, and how the various growth mechanisms differ from each other.

Device assembly and properties

Once a nanowire has been grown it must be connected to the outside world. Thanks to the unique properties of MoOx nanowires, basic device functions are readily achievable with this material, such as energy storage, photocalalysis, field-emission displays, electronic transport, and photodetection.

Battery device assembly and Li storage performance

Li and Li-ion batteries for portable electronic devices and hybrid electric vehicles are becoming increasingly important in today's society38-42. Nanostructured MoO3 has been extensively investigated as a key material for fundamental research and technological applications in electrochemical storage. The reversible electrochemical behavior of the crystalline MoO3 can be explained by the following redox reaction, which is known to be topotactic43:

xLi+ + xe- + MoO3 = LixMoO3

Orthorhombic molybdenum trioxide is attractive due to its layered crystal structure. The asymmetrical MoO6 octahedral are interconnected through corner linking along the [100] direction and edge sharing along the [001] direction to form double-layer sheets parallel to the (010) plane. The interesting host lattice is particularly suitable for the Li+ insertion reaction and its application in secondary lithium batteries has been investigated extensively.

To investigate the Li storage properties of molybdenum oxide nanowires, Li-ion batteries have been assembled. Fig. 4a shows the assembly process of coin cells, which are assembled in an argon filled glove box, with Li metal as a counter electrode and 1 M LiPF6/ethylene carbonate-diethyl carbonate (w/w = 1:1) as the electrolyte.

Mai et al.29 found that the first discharge capacity of MoO3 nanowires is 301 mAh-g-1, which is higher than that for bulk MoO3 measured under the same conditions. Hexagonal molybdenum trioxide (h-MoO3) is a metastable phase of molybdenum oxides. However, the basic structural unit is still a distorted octahedron of MoO6 of ReO3-type, consisting of a three-dimensional array of corner-sharing MoO6 octahedra. This open structure not only permits the ready intercalation of some monovalent cations including Na+, K+, and NH4+, but also allows them to be readily mobile in the tunnels. Song et al.7 reported that h-MoO3 exhibited excellent electrochemical performance: the first reversible discharge specific capacity can reach 402 mAh-g-1, versus Li metal at 0.1 mA-cm-2 (voltage range 1.2 - 4.0 V).

Layered MoOx has the potential to offer much higher capacities compared to other metallic oxides. Unfortunately, these materials suffer from poor kinetics and/or a serious capacity fade with cycling,

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especially at higher rates. Extensive investigations are presently being devoted to overcome these problems through doping44, as well as by using conductive polymers45 and carbon coatings46. However, adding conductive polymers will make the battery less stable at higher temperatures and carbon coatings will lower the volumetric energy density. Lithiation is considered an effective way of increasing the cycling stability of the cathode and/or anode materials as well as investigating structural changes of the electrode materials during lithium ion insertion. Compared with pristine materials, the lithiated samples exhibit a better cycling capability11.

Mai etal.29 reported the electroactivity of a-MoO3 nanowires after lithiation, which demonstrated superior performance to the non-lithiated a-MoO3 nanowires. Fig. 5 shows the curves of discharge capacity versus the cycle number for the non-lithiated and lithiated MoO3 nanowire at a current density of 30 mA-g-1 and at a temperature of 25 °C. For the non-lithiated MoO3 nanowire, the discharge capacity decreased to 180 mAh-g-1 after 15 cycles, corresponding to a capacity retention of 60 %. However, the discharge capacity of the lithiated MoO3 nanowire decreased to 220 mAh-g-1 after 15 cycles, corresponding to a capacity retention of 92 %, showing the stability and enhanced performance of the lithiated nanowire.

There are only a handful of studies in the literature on MoO2 despite reports that MoO2 may be useful as an anode material in Li-ion batteries, as it has demonstrated47 a relatively large capacity of about 400 - 600 mAh-g-1. Shi et al. found that mesoporous MoO2 material exhibits a reversible electrochemical lithium storage capacity as high as 750 mAh-g-1 at C/20 after 30 cycles, making it a promising anode material for lithium ion batteries6.

The lithium storage capabilities of multiple molybdenum oxides have received the least attention by far13-15. Most research has focused on their possible application as anode materials, but their possible use as cathode materials still remains largely unexplored. Xiao et al.

Fig. 5 Curves of discharge capacity versus the cycle number for the non-lithiated and lithiated MoO3 nanowire. Reprinted from29. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

reported the lithium storage capability of AMoO4 (A = Ni, Co) nanowires: when cycled in the voltage window of 1.5 - 3.5 V, NiMoO4 nanowires manifest a reversible capacity of 100 mAh-g-1 after 70 cycles15.

Photocatalysis

The applications of solar energy conversion and degradation of pollution using semiconductor photocatalysts have received a great deal of attention. To utilize solar energy more effectively, the development of efficient, visible-light-active photocatalysts has attracted worldwide interest48. Generally speaking, a photocatalytic reactor consists of two parts: a glass reactor and a lamp. The lamp is positioned above the reactor shown in Fig. 4d. The reaction suspension is prepared by adding nanowires into the simulated wastewater. The

Table 1 Different decolorization rates of bulk MoO3 and MoO3 nanowires

2.00E-008

Dyes Methyl violet Malachite green Safranine Rhodamine B

Bulk MoO3 89.4 86.7 92.6 90.5

MoO3 Nanowires 99.4 95.3 95.4 97.3

suspension is stirred in the dark to ensure an adsorption/desorption equilibrium before being exposed to UV radiation. The suspension is then irradiated using UV light while being stirred continuously. Analytical samples are removed from the reaction suspension after various reaction times and then the particles are removed using a centrifuge. The supernatant liquor is analyzed by a UV/VIS spectrophotometer.

The photocatalytic properties of MoO3 nanowires have been studied and the results indicate that MoO3 nanomaterials demonstrate good photocatalytic ability. Qi etal.49 examined the photocatalytic properties of MoO3 nanowires by degradation of methyl violet, malachite green, safranine, and Rhodamine B dye solutions. The photocatalytic ability was measured by D = (A0 - A) / A0 x 100 %, where A0 and A are the absorbance of the dye solution before and after illumination, respectively. The results are shown in Table 1, which clearly demonstrate that MoO3 nanowires exhibit superior photocatalytic abilities over bulk MoO3. This may be attributed to the large surface area and an extremely high absorption capacity of the nanowires. Moreover, as the energy gap of MoO3 nanowires turns out to be wider than that of the bulk materials, electrons excited by photons have a more negative potential, which heightens their reducibility. So too, for the oxidability of excited holes, which may be another reason for the enhanced photocatalytic properties of MoO3 nanowires.

Field emission devices

In 1928 Fowler et al. established metallic field emission theory. Investigations associated with field emission theory and field emission devices (FEDs) aroused great interest. The structural diagram of an FED is shown in Fig. 4c. Under the excitation of high-energy electrons the nanowires will emit electrons that can be collected to form a cathode luminescence (CL) image. The Fowler-Nordheim (F-N) formula can be applied to describe the characteristics of field emission materials.

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Here, J is the current density, E is the local electric field near the emitter tip, and O is the work function of the nanowire. p is the field enhancement factor, which is related to the real electrical field at the tip.

Zhou et al.33 found that MoOx nanowires exhibited excellent field emission properties. The field emission measurement was carried out in a vacuum chamber of -2.0 x 10-7 Torr at room temperature.

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The turn-on field (Eto) and threshold field (Ethr) are defined to be the macroscopic fields required to produce a current density of 10 pA-cm-2 and 10 mA-cm-2 respectively. Ethr of the MoO2 and MoO3 nanowire arrays was 5.6 MV-m-1 and 7.65 MV-m-1, respectively. These values are higher than the best data from carbon nanotubes and SiC nanowires.

In 2006, an investigation on the field emission performance of the MoO2 nanowires was carried out in a chamber with a vacuum level of -2 x 10-7 Pa at room temperature50. Liu et al. found that the alignment of the nanorods played a significant role in the field emission properties. The increase in field emission of the nanorods can be attributed to their sharpened tips. For a given material, it has been demonstrated that a material with high aspect ratio, sharp tips, and abundant edges can greatly increase FE performance. Therefore, investigations on the effects of the factors influencing the field emission properties and successful control of these factors are essential for the application of these nanostructures in real devices.

Single nanowire devices and electrical transport

Nowadays, investigations on the electrical properties of one-dimensional nanomaterials are focused on bulk-forms for order/disorder nanotubes and nanowires51-56. As the critical scale of an individual device becomes smaller and smaller, the electron transport properties of their components become an important aspect to study57. Mai et al.58 studied the intrinsic reasons for electrode capacity fading in Li-ion based energy storage devices using in situ probing of single nanowire electrode devices in 2010. In these devices, a single nanowire was used as a working electrode, and the electrical transport of the single nanowire was recorded in situ to detect the evolution of the nanowire during charging and discharging. The work demonstrates the direct relationship between electrical transport, structure, and electrochemical properties of a single nanowire electrode, which will be a promising and straightforward way for nanoscale battery diagnosis.

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Fig. 7 (a) I-V characteristics of an individual MoO2 nanowire at 1 V. Inset: schematic view of an individual nanowire device. (b) I-V curves of MoO2 and MoO3 nanowires at 3 V. (c) Positive part of the I-V characteristics of the MoO2 transversal system in (b), shown as a function of log(I). Inset: experimental plot of ln(J) versus E1/2 with the electric field above 2500 V/cm. (d) The conductivity of the MoO2 nanowire changed with the increase of the sweeping time at 5 V. Reprinted with permission from34. © 2009 American Chemical Society.

In 2007, Mai et al.29 assembled a single nanowire device to measure the electrical transport through a single MoO3 nanowire before and after lithiation, to understand the superior performance of lithiated nanowires for Li+ storage. A single nanowire was placed across two gold electrodes, and the final contacts were improved by Pt deposition at the two ends. Before lithiation, the I-V characteristics of the nanowire showed asymmetric Schottky barriers at the two ends (the solid curve in Fig. 6), as created between the semiconductor MoO3 (with a band gap of 3.1 eV) and Au/Pt electrodes; the transported

current was on the order of ca. 300 pA at ca. 2 V. After lithiation, the I-V curve demonstrated Ohmic behavior (the dashed curve in Fig. 6), and the transported current was of the order of 10 nA at a bias of ca. 2 V. This result suggests that the Li+ ions introduced during lithiation effectively converted the MoO3 nanowire behavior from semiconductor to metallic. Using the measured resistance, the effective length, and cross-section of the nanowire, the conductivity was evaluated to be approximately 10-4S-cm-1 and 10-2S-cm-1 before and after lithiation, respectively. Because the nanowire grows along [001], the increase of conductivity along the nanowire implies an increase of carrier density in the MoO6 octahedral layers. This suggests that Li+ ions have been introduced as interstitials into the layers. The work provides a possible explanation as to why the performance of lithiated nanowires is superior to that of non-lithiated ones.

To understand the performance of MoO2 nanowire, Hu et al.34 measured the electrical transport through an individual MoO2 nanowire for the first time. The inset in Fig. 7a shows a schematic view of the device. The I-V characteristics were measured by sweeping the bias voltage from negative to positive for five cycles, as shown in Figs. 7a,b,d. The observed behavior is symmetrical and behaves linearly dependent for I vs V in the low voltage range (Fig. 7a), which is in agreement with Ohm's law, and the voltage scan for five cycles does not change the I-V curve. The Ohmic behavior at electric fields below 2500 V-cm-1 is due to the existence of delocalized electrons in the conduction band, and the conductivity of an individual MoO2 nanowire is estimated to be 190 S-cm-1 at room temperature. Schottky emission is responsible for the behavior above 2500 V-cm-1. The investigation of the electrical transport along a single MoO2 nanowire lays the foundation for further investigations toward exploiting MoO2 nanodevices.

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Fig. 8 (a) High magnification TEM image of MoO3 nanowires, indicating the crumpled characteristics of the MoO3 nanowires.(b,c) Two typical AFM images of the MoO3 nanowire samples. The big solid arrowheads indicate the locations of where the forces were applied. The distance between two small dashed arrowheads shows the equivalent length of the nanowires over the trenches. (d) Two typical force-distance curves on the center point of a nanowire and on the silicon substrate, respectively.

Mechanical properties of single nanowires

To integrate the nanowire into functional nanodevices, the mechanical properties must be accurately known, as failure of these building blocks may lead to the malfunction or failure of entire devices59-61. Recently our group investigated the intrinsic mechanical performance of individual MoO3 nanowires, and an interesting relationship between the crumpled nanowires and their elastic properties was found. To perform three-point bending tests on MoO3 nanowires, well-defined trench patterns were fabricated on a silicon substrate, and both ends of the nanowires were held by two other nanowires (Fig. 8a,b).

Fig. 8c shows two typical force-distance curves on the center point of the suspended nanowire and on the Si substrate. The clamped-clamped beam model (CCBM) is used to simulate the whole mechanical response of the nanowire62. According to the CCBM formula:

48Iv 48I

Where E is the bending modulus, F the force applied, I the moment of inertial given by wh3/12 for the rectangular beam, v the deflection of the suspended nanowire. Also,

Ssample

where Klever is the spring constant of the cantilever, S is the sensitivity, which is the inverse of the slope of the F-d curve63, the bending modulus E can be calculated. For the MoO3 nanowire, the average value of the elastic modulus is 31 GPa. Compared with bulk MoO3, with an elastic modulus value of 540 GPa, the elastic modulus has decreased approximately by 94 %. The decrease in elastic modulus is probably attributed to the high surface-to-volume ratio of the MoO3 wire64. Unlike atoms locked in the lattice, surface atoms are less constrained, thereby making the MoO3 nanowire easier to deform in the elastic regime, and consequently leading to a lower elastic modulus. The low elastic modulus and the nature of layered structure of MoO3 nanowires is a reasonable explanation for the formation of the crumpled morphology observed in Fig. 8a. These results provide some direct evidence for the mechanism responsible for the plastic deformation of a nanowire when it undergoes mechanical stretching or compression. This approach can be further extended to examine the atomic events occurring during the plastic failure of various metals and their alloys57.

Photodetection

Photodetectors are devices that detect light by converting optical signals into electrical current. The schematic of a photodetector device is shown in Fig. 4e. Si/SiO2 substrates with predeposited Ti/Pt gap-cell electrodes are used for electrical characterization. A nanowire is deposited and then connected to the predeposited electrodes. Lead wires of Au were joined

to the Pt/Ti electrode pads with Ag paste. No photocurrent was detected between the blank electrodes (i.e., without a nanowire) or between the electrodes and the Si layer. The white light of a light-emitting diode and a Xe lamp were used as light sources to observe the photoconductance as a function of turning the light on and off.

So far, most of the reported nanowire photodetectors are terminal devices. In general, for this type of photodetector there exists a tradeoff between the performance parameters, such as current responsivity (Rx), photoresponse ratio (Ilight/Idark), and the photoresponse time (rise and decay times). For practical use, exploring new ways to develop single photodetectors with an overall enhancement in the performance is critical65,66.

Makise et al.67 have fabricated MoOx nanowires that are grown on a SiO2 substrate by electron-beam-induced deposition (EBID) using a Mo(CO)6 precursor with O2. They found that the nanowire can be reversibly switched between low and high conductivity. As the Xe lamp (the wavelength is 400 nm) was switched on, the conductance jumped from 2.2 x 10-4 to 3.6 x 10-4 S in 5.9 s. The characteristics of the MoOx nanowires suggest that they are promising candidates for photodetectors and optical switches. Optimization of nanowire composition may improve the optical sensitivity and photo-response.

Conclusions and outlook

This article provides an overview of a variety of chemical methods that have been developed for generating MoOx nanowires. We have also discussed a range of interesting properties associated with MoOx nanowires. Preliminary results from many research groups have explored how MoOx nanowires may be used as a functional building block for electronics, photonics, and mechanical sensors. MoOx nanowire devices have been fabricated, and the novel physical/chemical properties investigated68-70.

MoOx nanowires and their nanowire device applications are still in the early stage of technical development. To be of real interest to industry, a number of issues must be addressed. First of all, generic issues related to epitaxy still need to be systematically studied. For example, improving the crystal quality of the desired MoOx nanowire with high power and high rate performance represents a significant step toward practical application in lithium-ion batteries. Also, searching for new molybdenum oxides with special coordination

Instrument citation

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characteristics is also expected to significantly enhance its use in energy-saving devices. The second challenge faced by chemically synthesized nanowires regards the modification. MoOx is a kind of semiconductor; it can be doped to conduct with negative electrons or positive holes, enabling p-n junctions and transistors to be formed. The possibility of controlling the conductivity and carrier type of MoOx by adding impurities is the subject of forthcoming investigations. The third challenge is the self assembly of MoOx into complex structures or device architectures. For example, hierarchical structured nanowire networks, which can be based on functional MoOx nanowires, are necessary for creating devices. Recent advances in nanoscience and nanotechnology open up a myriad of opportunities for a new generation of devices based on MoOx nanowires. Looking forward, the future appears remarkably bright, and the application of MoOx

nanowire devices is expected to greatly expand the impact of these materials on science and industry. d

Acknowledgement

This work was partially supported by the National Nature Science Foundation of China (50702039, 51072153), program for New Century Excellent Talents in University (NCET-10-0661), the Research Fund for the Doctoral Program of Higher Education (20070497012), the Fundamental Research Funds for the Central Universities (2010-II-016), and Scientific Research Foundation for Returned Scholars, Ministry of Education of China (2008-890). We express our deep thanks to Professor C. M. Lieber of Harvard University, Professor Z. L. Wang of Georgia Institute of Technology, Professor W.J. Mai of Jinan University and Dr. Y.J. Dong, Professor Y. Shao of Massachusetts Institute of Technology for fruitful collaboration and stimulating discussion.

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