recent developments in single-crystal inorganic nanotubes

730 Int. J. Nanotechnol., Vol. 4, No. 6, 2007

Copyright 2007 Inderscience Enterprises Ltd.

Recent developments in single-crystal inorganic nanotubes synthesised from removable templates

Guozhen Shen*, Yoshio Bando and Dmitri Golberg Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Fax: +81-29-851-6280 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: Since Sumio Iijima identified the hollow Carbon Nanotubes (CNTs) in the early 1990s, there have been significant research efforts to synthesise inorganic nanotubes of various solids. Generally, the formation of tubular nanostructures requires layered, anisotropic, or pseudo-layered crystal structures. Inorganic nanotubes, which typically do not possess such structures, are usually synthesised using template-based methods. However, these nanotubes are either amorphous, polycrystalline or exist only in ultrahigh vacuum. Recently, an epitaxial casting method using removable ZnO nanowires as templates has been developed to synthesise single-crystal GaN nanotubes. This opens up an exciting field of research on the synthesis of single-crystal nanotubes with non-layered or non-anisotropic structures. In this paper, we review recent research activities on single-crystal inorganic nanotubes synthesised using removable templates via vapour phase methods based on similar epitaxial casting concepts. The applications and nanodevices that have been built using these novel inorganic nanotubes are also presented.

Keywords: semiconductors; nanotubes; sulphides; phosphides; chemical vapour deposition; CVD; templates.

Reference to this paper should be made as follows: Shen, G., Bando, Y. and Golberg, D. (2007) Recent developments in single-crystal inorganic nanotubes synthesised from removable templates, Int. J. Nanotechnol., Vol. 4, No. 6, pp.730749.

Biographical notes: Guozhen Shen received a BS Degree in Chemical Education from Anhui Normal University in 1999, and a PhD Degree in Chemistry (with Professor Yitai Qian) from University of Science and Technology of China, and then he worked as a Postdoctoral Researcher at Hanyang University, Korea. He joined National Institute for Materials Science as a postdoctoral researcher in 2005. His most recent research interests include fabrication of semiconductor nanowires, nanotubes, and 1-D heterostructures; nanoelectronics, and devices applications of semiconductor nanowires like Field Effect Transistors (FET) for biosensing. He has authored and co-authored over 80 journal papers, eight review papers and book chapters, and holds three issued Japan patents.

Recent developments in single-crystal inorganic nanotubes synthesised 731

Yoshio Bando obtained a PhD Degree in Inorganic Chemistry from Osaka University. He is currently director-general of International Center for Young Scientists, Fellow of National Institute for Materials Science (NIMS), group leader of Nanomaterials Synthesis and Analysis Group, NIMS, and Professor of University of Tsukuba. He has published over 400 original papers in international journals.

Dmitri Golberg obtained a PhD Degree in Solid State Physics from Bardin Central Institute for Ferrous Metallurgy, Moscow, Russia in 1990. He is currently a Nanotube Group leader within NIMS. His research interests include synthesis, electron microscopy analysis and physicochemical property studies of boron nitride, boron carbonitride, various metal oxide, carbide, boride, phosphide, nitride, sulphide and selenide nanotubes, metal filled nanotubes, diverse nanowires and nanobelts, and nanostructure formation, and phase transitions under non-equilibrium conditions at the nanoscale. He has authored over 250 original papers in international journals and more than 70 Japanese patents.

1 Introduction

Since the identification of Carbon Nanotubes (CNTs) in 1991 by Iijima [1], there have been enormous research efforts to synthesise various kinds of CNTs and to test their unique functions such as hydrogen storage, field-emission displays, field-effect transistors, diodes, sensors, and actuators [26]. To date, CNTs have successfully been synthesised using a variety of methods, such as arc-discharge, laser vapourisation, hydrocarbon pyrolysis, solvothermal methods, and Chemical Vapour Deposition (CVD) [712]. In addition to CNTs, there have also been great research endeavours to fabricate nanotubes of other solids. Some layered inorganic solids similar to graphite, in which the atoms are covalently bonded to form two-dimensional layers that are stacked together through van der Waals interactions, such as WS2, MoS2, and NiCl2 can also be rolled up to form seamless nanotubes. In fact, the formation of tubular nanostructures generally requires a layered or anisotropic crystal structure. Pioneered by the work on WS2 nanotubes in 1992 by Tenne et al. [13], inorganic nanotubes have been synthesised from a wide range of compounds possesing layered or anisotropic structures, such as MoS2 [14], TiS2 [15], ZrS2 [16], HfS2 [16], BN [17], VOx [18], NiCl2 [19], NbSe2 [20,21], NbS2 [22], TaS2 [22], Bi [23], Se [24], Te [25], Bi2S3 [26] and Sb2S3 [27].

Synthesis of inorganic nanotubes that do not have a layered crystal structure has also attracted considerable attention. Recent reports indicated that under appropriate experimental conditions, geometrically closed, concentric nanotubes could also form. However, compared with layered compounds, formation of nanotubes from solid materials without layered structures requires much more effort to bring together the atoms or small particles into hollow tubular structures. As a result, templates like CNTs, and porous membranes (AAO etc.) are usually needed to assist the growth into cylinder tubes. Another method relies on thin film rolling of lamellar precursors. Many kinds of inorganic nanotubes have been synthesised from non-layered inorganic materials using the so-called template-assisted methods, such as TiO2 [28], SiO2 [29,30], Al2O3 [31], SrAl2O4 [32], In2O3/Ga2O3 [33], ZrO2 [34] and metals [35,36]. However, typically, all the produced nanotubes are either amorphous, polycrystalline or exist only in ultrahigh

732 G. Shen, Y. Bando and D. Golberg

vacuum. In fact, the growth of single-crystal hollow nanotubes would be more advantageous for potential practical applications in nanoscale electronics, optoelectronics, and biochemical sensing. Thus the synthesis of single-crystal nanotubes with non-layered structures still remains a challenging task.

Goldberger et al. [37] reported on the synthesis of single-crystal GaN nanotubes by epitaxial casting, in which hexagonal ZnO nanowires had served as templates for epitaxial overgrowth of thin GaN sheaths. The ZnO nanowire template were then subsequently removed by thermal reduction and evaporation with formation of GaN nanotubes [37]. This work has opened up an exciting new field of research on the synthesis of single-crystal nanotubes with nonlayered or non-anisotropic structures. In this paper, we review the recent research activities and breakthroughs with respect to single-crystal inorganic nanotubes synthesised using removable templates via vapour phase methods based on similar epitaxial casting concepts. Firstly, the synthetic concepts are introduced. Then the inorganic nanotubes synthesised through these approaches are surveyed based on two different sub-concepts of the template-based methods: the physical templating and chemical templating. Finally, the applications and nanodevices that have been built using these novel inorganic nanotubes are highlighted.

2 Synthesis concepts

There are many kinds of methods developed for the vapour-phase syntheses of single crystal inorganic nanotubes. Templating against pre-existing nanostructures (e.g., nanowires, nanobelts) in the vapour phase processes is one of the most recently adopted techniques. It represents a straightforward and efficient route towards single-crystal nanotubes.

In such process, the first step is either pre-formation or in situ formation of easily removable 1-D nanostructures in vapour phase conditions. These 1-D nanostructures are used as templates for the next deposition of target material coatings to finally form 1-D core/shell nanostructures. In the last step, by choosing a proper etching technique, the inner core nanowires are removed, which resulted in the formation of single crystal nanotubes.

To this end, two different strategies have been developed, namely, the physical templating and chemical templating approaches, as indicated in Figure 1. In the physical templating approach, a 1-D single crystal nanostructure serves as a substrate for the epitaxial growth of another composite to obtain core/shell nanowires containing sharp structural and compositional interfaces. Selective etching of the inner nanostructures leads to the formation of single crystal nanotubes with well-controlled inner and outer diameters. In the chemical templating method, a 1-D single-crystal nanostructure serves as not only a substrate, but also reacts with proper chemical reagents to be partially or completely converted into the target nanotubes.


Figure 1 Schematic illustration of the removable template method towards single-crystal nanotubes via a vapour phase process

3 Physical templating approach

3.1 GaN Nanotubes

Wurtzite GaN, a particularly important IIIV semiconductor with a direct band-gap of 3.4 eV, is one of the most promising semiconductors suitable for designing and fabricating optoelectronic devices in the violet and blue region, in which Si and conventional IIIV semiconductors are not applicable. So far, considerable efforts have been directed towards the synthesis of one-dimensional GaN materials, such as nanorods, nanowires, and nanobelts [3840]. The well-established methods include a carbon nanotube confined reaction, arc discharge, laser ablation, sublimation, pyrolysis, and CVD. However, these methods failed with respect to the synthesis of single-crystal GaN nanotubes, though they are useful for the fabrication of amorphous or polycrystalline GaN nanotubes when combined with template-confined techniques.

Synthesis of single-crystal GaN nanotubes was accomplished for the first time using the epitaxial casting technique. In this technique, arrays of wurtzite ZnO nanowires on (110) sapphire wafers were utilised as templates for the epitaxial overgrowth of thin GaN layers in a CVD system. Trimethylgallium and ammonia were used as the precursors for GaN layers. Figure 2(a) is the SEM image of the ZnO nanowire arrays used as templates in this process, which were synthesised using a vapour deposition process. These nanowires grew along the [001] direction and have uniform lengths of 25 m and diameters of 30200 nm with well faceted hexagonal cross-sections. After coating these ZnO nanowires with GaN layers, the inner ZnO cores were either chemically etched by ammonia or thermally reduced at high temperatures. Figure 2(b) shows the SEM image


of the resultant GaN nanotube arrays. It can be seen that the morphology of the initial nanowire arrays was well preserved. The hollow cavities of these GaN products are visible in some broken tubes, the inset in Figure 2(b). The studies revealed that all the synthesised GaN nanotubes are single crystals with the preferred [001] growth directions.

Figure 2 SEM images of the: (a) ZnO nanowire arrays; (b) GaN nanotube arrays and (c,d) TEM images of arrays of GaN nanotubes with their ZnO nanowire templates partially removed (from [37], with permission)

For the formation of single-crystal GaN nanotubes using ZnO nanowires as templates, the most important factor is the perfect epitaxial relationship between wurtzite GaN and ZnO, which was indeed confirmed while studying the microstructures of GaN nanotubes with their ZnO nanowire templates partially removed (Figure 2(c) and (d)). As known, both GaN and ZnO have the wurtzite crystal structures and very similar lattice constants of: a = 3.249 , c = 5.207 for ZnO and a = 3.189 , c = 5.185 for GaN. Thus it is easy for GaN to epitaxially coat on the side {110} plane surfaces of ZnO. It is in fact, a perfect epitaxial casting mechanism.

3.2 Si nanotubes

Due to the key role of silicon in interconnections and basic components for the future nanoelectronic and especially optoelectronic devices, 1-D Si nanostructures have recently become of special interest. Different methods including laser ablation, CVD, oxide-assisted method and solution process have been developed for Si nanowire fabrication. Though these methods have also been used to make Si nanotubes, all the obtained Si nanotubes are either polycrystalline or amorphous [4143].

Recently, we developed an efficient CVD process to synthesise single-crystal Si nanotubes using ZnS nanowires as the removable templates. ZnS and SiO powders were utilised as the source materials in a vertical induction furnace [44]. The experimental procedure was as follows: firstly, single-crystalline ZnS nanowires were synthesised from ZnS powders at 1200C. Then, the reaction temperature was increased to 1450C.


During this step, core/shell ZnS/Si nanowires formed. Finally, the core/shell nanowires were etched in a HCl solution, which resulted in the removal of inner ZnS cores.

ZnS nanowire templates used in the process have diameters of 2060 nm and grow along the [0001] directions. Figure 3(a) shows the SEM image of the resultant Si nanotubes. They have very thin walls and tip-ends either closed or open (Figure 3(b) and (c)). These Si nanotubes are well-structured single crystals growing along the [111] direction (Figure 3(d)).

Figure 3 (a) Low-magnification TEM image of Si nanotubes; (b,c) TEM images showing the thin wall and open tip of a Si nanotube and (d) HRTEM image of a Si nanotube (from [44], with permission)

The formation of single-crystal Si nanotubes was primarily influenced by the initial ZnS nanowire templates. In fact, the [0001] grown ZnS nanowire and the [111] grown Si nanotube have well-defined epitaxial relationships as (111)Si//(111)ZnS and [111]Si//[111]ZnS, which results in the formation of final single crystal Si nanotubes in a way similar to single-crystal GaN nanotubes prepared using ZnO as removable template.

3.3 IIBVI nanotubes

Group IIBVI semiconductors (ZnS, ZnSe, CdS, CdSe) are of great importance in many fields. Their common characteristic is a propensity to form a wurtzite crystal structure if the right conditions are chosen. Significant efforts have been paid to shape IIBVI


semiconductors into 1-D morphologies, such as nanorods, nanowires and nanobelts using solution and CVD processes, or laser ablation [4548]. However, no success has been claimed in fabricating single crystal IIB-VI nanotubes using these conventional methods. The pioneering work by Goldberger et al. [37] on GaN nanotubes has opened up a new horizon in the synthesis of single-crystal nanotubes and makes it possible to prepare single crystal IIB-VI nanotubes on a large scale.

The first example is single crystal ZnS nanotubes [49]. These nanotubes were synthesised in a horizontal high-temperature resistance furnace using ZnS and SnO as the sources. The reaction temperature was 1150C; N2 was used as a protection gas. Figure 4 shows the typical TEM images of a synthesised ZnS product. It is apparent that all structures have tubular shapes. Some tubes have uniform diameters and wall thicknesses along the whole tube, while the others have tapered diameters from ~250 nm to ~50 nm. These ZnS nanotubes are single crystals with the growth directions parallel to the [120] crystallographic orientation of a wurtzite ZnS crystal. A common feature of the synthesised ZnS nanotubes is that all of them are entirely or partially filled with Sn. It is deduced that the formation of ZnS nanotubes is a Sn nanowire-templated process. During this process, the first step is the reduction of SnO by carbon to generate Sn, which resulted in the self-catalyzed growth of Sn nanowires. During the second stage, a thin layer of ZnS deposited on the surface of Sn nanowires to form a Sn/ZnS core/shell nanostructure. After evaporation at high temperature the inner Sn nanowires were etched. As a result single-crystal ZnS nanotubes were formed.

Figure 4 Typical TEM images showing filling characteristics of Sn inside the ZnS nanotubes: (a) ZnS nanotubes with uniform diameters throughout the whole lengths and (b) tapered ZnS nanotubes (from [49], with permission)

Similarly, single crystal ZnSe nanotubes/submicrotubes were synthesised, which also grew along the [120] orientation of a wurtzite ZnSe crystal [50].

CdS and CdSe compounds with crystal structures similar to ZnS or ZnSe are particularly important. Wurtzite is the most stable structure for CdS and CdSe. Both materials have many potential applications in optical, electric and optoelectronic fields. Though many kinds of 1-D CdS or CdSe nanostructures have been fabricated using VLS or VS processes, single crystal CdS and CdSe nanotubes have never been obtained using these techniques. The only success was achieved through a physical templating approach using in situ formed Sn nanowires as the removable templates [51].


The formation of CdS and CdSe nanotubes is generally similar to that of ZnS and ZnSe ones. In a typical process, CdS or CdSe, SnO, SnO2 and activated carbon powders were thermally evaporated at high temperature in a horizontal resistance furnace. The products deposited on graphite wafers in a low temperature region. Figure 5 depicts a series of SEM images of CdS nanotubes (left-hand-side) and CdSe nanotubes (right-hand-side). Both CdS nanotubes and CdSe nanotubes have the following features: parts of them have uniform diameters along the whole lengths and the others have tapered diameters. All tubes are fully or partially filled with Sn and have large Sn particles attached to their tips. All the nanotubes are single crystals with the preferred growth directions along the [001] crystallographic orientations. Similar to single-crystal ZnS nanotubes, the formation mechanism is a two-step Sn nanowire templated process.

Figure 5 (ae, left) TEM and HRTEM images of CdS nanotubes. (ae, right) TEM and HRTEM images of CdSe nanotubes (from [51], with permission)

3.4 Magnetite nanotubes

Magnetic nanotubes, such as FePb, Fe3O4, and LPMO, may potentially serve as tunable fluidic channels for tiny magnetic particles, data storage devices in nanocircuits, and scanning tips for magnetic force microscopes. Single-crystal magnetite (Fe3O4) nanotubes were synthesised by a three-step process [52]. Single-crystal MgO nanowires (30100 nm in diameter) were first grown on a gold-coated Si/SiO2 substrate from Mg3N2 powders by thermal evaporation. Then a layer of Fe3O4 was deposited on these MgO nanowires using the pulsed laser deposition technique to form the MgO/Fe3O4 core/shell nanowires. Being etched in a (NH4)2SO4 solution at 80C, the inner MgO nanowires were removed and single-crystal Fe3O4 nanotubes were formed. Figure 5(a) shows the TEM image of a single Fe3O4 nanotube with an outer diameter of ~30 nm.


The upper inset SAED pattern clearly indicates that the tube is a single crystal. Combined with the HRTEM image shown in the lower inset it reveals that the growth direction was along the [100] orientation. These free-standing Fe3O4 nanotubes rendered a unique opportunity to investigate the electron transport through Fe3O4 in a quasi-one-dimensional form.

Similar technique can be extended to synthesise core/shell transition metal oxide nanowires, such as MgO/YBa2Cu3O6.66, MgO/La0.67Ca0.33MnO3, and PbZr0.58Ti0.42O3 nanowires [53,54]. By efficient selection of proper etching methods, single crystal transition metal oxide nanotubes, YBa2Cu3O6.66, La0.67Ca0.33MnO3, PbZr0.58Ti0.42O3, are expected to also form. If these kinds of transition oxide nanotubes can be successfully synthesised, novel chemical and physical properties, such as high Tc superconductivity, colossal magnetoresistivity and ferroelectricity are envisaged.

4 Chemical templating approach

4.1 II3V2 nanotubes

Narrow band gap II3V2 semiconductors are of prime scientific and technological importance. They may exhibit more pronounced size quantisation effects than the IIVI or IIIV semiconductors due to their large excitonic radii. However, it is quite difficult to synthesise one-dimensional II3V2 nanostructures due to the lack of general synthetic methods. Recently, we obtained single-crystal II3V2 nanotubes using the in situ formed IIB metal nanowires as removable templates [55]. Single-crystal II3V2 nanotubes (Zn3P2, Cd3P2) were synthesised at 1350C under ambient pressure in a vertical induction furnace from a mixture of ZnS (or CdS), P and Mn3P2 powders. Figure 6 gives the typical SEM images of the synthesised Zn3P2 and Cd3P2 nanotubes. It can be seen that using the removable template method, in fact, the nanotubes can be obtained on a large scale. By controlling the experimental conditions, the product morphology can also be well controlled. For instance, if the reaction temperature was increased, Zn3P2 and Cd3P2 microtubes were synthesised at the expense of nanotubes. When excessive ZnS or CdS were taken as the source material, hierarchical Zn3P2/ZnS or Cd3P2/CdS heterostructures were prepared instead.

Figure 6 Low-magnification TEM image of a Fe3O4 nanotube, with SAED pattern and HRTEM image shown in the upper and lower inset, respectively (from [52], with permission)


Microstructures of the synthesised nanotubes have clearly been verified using HRTEM imaging. From the low-magnified TEM images, the hollow cavities of the products are visible (Figure 7(a)(c)). All the nanotubes have very thin walls of ~4050 nm and open tips (Figure 7(d), (f) and (g)). Both the Zn3P2 and Cd3P2 nanotubes are single crystals with the preferred growth directions perpendicular to the (101) planes (Figure 8). The growth of the nanotubes is a three-step process: firstly, metallic Zn or Cd nanowires were formed through a Vapour-Solid (VS) process via ZnS + C Zn + CS2 or CdS + C Cd + CS2. In the second step, the generated phosphorus (P) gases partially reacted with these Zn or Cd nanowires to form Zn/Zn3P2 or Cd/Cd3P2 core/shell nanowires. Finally, the inner Zn or Cd were consumed during evaporation of the coaxial nanocables, whereas the Zn3P2 or Cd3P2 shell remained intact as the final nanotubes.

Figure 7 SEM images of the synthesised Zn3P2 nanotubes (upper) and the Cd3P2 nanotubes (lower). The inset displays a magnified image; that clearly shows a hollow structure

Due to their extremely thin walls, these nanotubes are expected to show size quantisation effects. This idea was checked during the Cathodoluminescence (CL) measurements. It is known that a bulk Zn3P2 crystal has an emission at ~802 nm. The synthesised Zn3P2 nanotubes with wall thickness of ~45 nm emit at 796 nm, which is similar to that of bulk crystal since the wall thickness (45 nm) is much larger than the excitonic radii of Zn3P2. However, when the wall thicknesses became 20 nm and 10 nm, emissions centred at 711 nm and 491 nm appeared, respectively. These values show notable blue-shifts with respect to a bulk crystal due to predicted quantisation effects.


4.2 ZnAl2O4 nanotubes

Spinel oxides have the composition AB2O4, where A and B may represent divalent and trivalent cations, respectively. Spinel oxides have very important technical applications. For example, iron-containing spinels (ferrites) are well-known magnetic materials. Strontium aluminate (SrAl2O4) is one of the most studied and most efficient host materials for long-lasting phosphorescence [56].

The first example of a single crystal spinel nanotube using a removable template is a ZnAl2O4 nanotube [57]. Fan et al. synthesised single crystal ZnAl2O4 nanotubes using ZnO nanowires as templates. Single-crystal ZnO nanowires were synthesised using vapour phase transport methods. Then a thin layer of Al2O3 was deposited on these ZnO nanowires via atomic layer deposition to form core/shell nanowires. These nanowires were annealed at 700C and final ZnAl2O4 nanotubes were fabricated.

After annealing, the remained 1-D nanostructures are freestanding, narrow in diameter (3040 nm) and wall thickness (10 nm) and are hollow from one end to the other, as shown in Figure 9(a) and (b). The nanotubes are single crystals and most of them have crystal orientations parallel to the (111) spinel planes. By careful choosing of ZnO template with different morphology, different kinds of tubular ZnAl2O4 nanostructure can be fabricated. For instance, saw-like branched ZnAl2O4 nanotubes were synthesised using saw-like ZnO nanowires as templates.

Figure 8 (a)(c) TEM images of Cd3P2 nanotubes; (d,e) HRTEM image of a Cd3P2 nanotube, revealing its single-crystal nature and (f)(h) TEM and HRTEM images of single-crystal Zn3P2 nanotubes (from [55], with permission)


Figure 9 (a,b) SEM and TEM images of the ZnAl2O4 spinel nanotubes and (c) a branched ZnAl2O4 spinel nanotube (from [57], with permission)

Single-crystal ZnAl2O4 nanotubes were formed via an interfacial solid-state reaction between ZnO and Al2O3 involving the Kirkendall effect, which is a classical phenomenon in metallurgy and was applied to explain the formation of hollow spherical nanocrystals [58,59]. It is an interesting approach since the Kirkendall effect allows one to develop a rational design of nanoscale tubular objects based on any appropriate choice of materials and different reaction properties. It also constitutes a systematic pathway to prepare single-crystal nanotubes made of diverse range of materials.

4.3 MgAl2O4 nanotubes

Fabrication of single crystal MgAl2O4 nanotubes was based on the scheme shown in Figure 10 [60]. In this process, reactive MgO nanowires were chosen as templates. As we have discussed above, a MgO nanowire is a very commonly used epitaxial substrate for depositing various transition oxides including Fe3O4, YBa2Cu3O6.66, La0.67Ca0.33MnO3, and PbZr0.58Ti0.42O3 and forming either single crystal nanotubes or core/shell nanowires. Due to a high reactivity of MgO during spinel syntheses through the topotaxial solid state reactions, MgO nanowires are assumed to be proper sacrificing templates for MgAl2O4. A work carried out by Fan et al. [60] did confirm such assumption. Firstly, single-crystal MgO nanowires were synthesised inside a horizontal quartz tube furnace using a Mg3N2 powder as the source material. Then using the precisely controlled atomic layer deposition, a thin layer of ~10 nm Al2O3 was deposited on the MgO nanowires to form core/shell MgO/Al2O3 nanowires. In order to transform the core/shell MgO/Al2O3 nanowires into spinel MgAl2O4 nanotubes, the sample was first annealed at high temperature and then etched in a (NH4)2SO4 solution. The whole process is depicted in Figure 10.


Figure 10 Schematics of the fabrication process of MgAl2O4 spinel nanotubes (from [60], with permission)

After etching, MgAl2O4 nanotubes were obtained, as shown in Figure 11(a). Typical nanotubes have wall thicknesses of 1112 nm. All these nanotubes are single crystalline (Figure 11(b)) with a longitudinal direction along the (001) planes of a spinel MgAl2O4 crystal (Figure 11(c)).

Figure 11 HRTEM images of the MgAl2O4 nanotubes, showing their single-crystalline characteristics (from [60], with permission)

In the regarded process, the tube wall thickness and diameters can be efficiently controlled through two different ways:

a control in the deposited Al2O3 layer thickness

a control in the diameter of a MgO nanowire template.

4.4 GaN nanotubes

Although single-crystal GaN nanotubes have been synthesised by Goldberger et al. [37] based on the physical templating method and using ZnO nanowires as the removable templates, it is still a challenge to find a pathway towards single-crystal GaN nanotube fabrication using chemical templating [61].


Single-crystal GaN nanotubes using this concept were crystallised via a two-stage process based on a well-controllable conversion of amorphous gallium oxide (Ga2O) nanotubes. During the first stage, amorphous Ga2O nanotubes were prepared by carbon-thermal reduction of Ga2O3 powders under N2 at 1250C. The involved reactions were Ga2O3 + 2C Ga2O + 2CO, and 2Ga2O + 4CO 4Ga + C + 3CO2. These reactions resulted in the formation of Ga-filled amorphous Ga2O nanotubes (Figure 12(a), (b)). During the second stage, N2 gas was changed to NH3 gas and the reaction temperature was increased to 1400C. In this stage, the involved reaction was Ga2O + 2NH3 2GaN + H2O + 2H2. It led to the formation of single-crystal GaN nanotubes (Figure 12(c) and (d)). The GaN nanotubes have preferred growth direction along the [101] orientation, which is quite different from GaN nanotubes synthesised using ZnO nanowires as removable templates. The possible reason may be that the present GaN nanotubes were formed via a chemical conversion method while the former GaN tubes were fabricated using epitaxial casting of GaN on ZnO nanowires.

Figure 12 (a,b) TEM images of amorphous Ga2O nanotubes and (c,d) TEM images of single-crystal GaN nanotubes converted from Ga2O nanotubes (from [61], with permission)

The regarded single-crystal GaN nanotubes showed different cathodoluminescence properties compared with a bulk GaN crystal. A green emission centred at 505 nm was observed for GaN nanotubes, which displays a blueshift of ~40 nm compared with a bulk GaN crystal. In the templating conversion process, GaN nanotubes were prepared from Ga2O. Therefore, it was thought that a blueshift may be attributed to some intrinsic point defects, such as Ga vacancies, or O impurities.


5 Applications and nanodevices

1-D nanostructures have been in the focus of recent extensive studies worldwide due to their unique physical properties and potential to revolutionise broad areas of nanotechnology compared with standard bulk crystals. 1-D nanostructures represent the smallest dimension structure that can efficiently transport electrical carriers. They are ideally suited to the critical and ubiquitous task of moving and routing charges in nanoscale electronics and optoelectronics. Furthermore, 1-D nanostructures also exhibit device functions, and thus can be exploited as both the wiring and device elements for diverse functional nanosystems [6266].

Similar to nanowires, inorganic nanotubes represent important types of nanometer scale 1-D structures. Use of removable templates via vapour phase methods makes it possible to rationally and predictably synthesise single-crystal nanotubes with controllable chemical composition, diameter, length and doping levels. Thus it is possible to assemble nanotube devices including field-effect transistors, p-n and light emitting diodes, complex logic gates and so on.

Electrical transport measurements provide information about the electronic structure and the behaviours of carriers under electric field in 1-D nanoscale materials. One powerful configuration for studying electrical transport is a Field-effect Transistor Setup (FET). Single-crystal Fe3O4 nanotubes synthesised using MgO nanowires as templates rendered a unique opportunity to investigate the electron transport in a quasi 1-D form [52]. Figure 13 shows the SEM image of a device based on single-crystal Fe3O4 nanotubes and used for the analysis of carrier transport properties. The device consists of a Fe3O4 nanotube (~5 m) and four metal electrodes evenly distributed on the top. Figure 14(a) shows the IV curves recorded at different temperatures (290, 180 and 77 K). They exhibit rather linear features and the nanotube resistivity increases monotonically with decreasing temperature. Similar with that of an epitaxial Fe3O4 film, room temperature resistivity was deduced to 4 102 . Figure 14(b) demonstrates the magnetoresistance measurement at 77 K. Magnetoresistance was clearly observed for the Fe3O4 nanotube due to the spin-polarised transport across the structural domain boundaries.

Figure 13 SEM image of a Fe3O4 nanotube device used in the transport study


Figure 14 Electrical measurements of individual Fe3O4 nanotubes (from [52], with permission) (for colours see online version)

As another example, the electron transport properties of single-crystal GaN nanotubes are also highlighted here [37]. Figure 15 displays the temperature-dependent IV curves recorded on a GaN nanotube. The electrodes (20 nm Ti and 80 nm Au) for the electrical measurements were fabricated using e-beam lithography and thermal evaporation. They indicate that the resistances of the GaN nanotube are of the order 10 M at room temperature, and increase with decreasing temperature.

Figure 15 Temperature dependence IV curves of a GaN nanotube (for colours see online version)

6 Summary

Synthesis of inorganic nanotubes that do not have a layered crystal structure has attracted considerable attention in recent years due to their interesting and important properties and applications in electronics, optoelectronics, drug release and bio-sensing. Compared with layered compounds, formation of nanotubes from solid materials without layered structures requires much more effort to bring together the atoms or small particles into


hollow tubular structures. As a result, templates are usually required to assist the growth into cylinder tubes. In this paper, a comprehensive survey of work on single-crystal inorganic nanotubes synthesised from removable templates in the vapour phase processes was reviewed. Such processes can be divided into two categories: a physical templating approach and a chemical templating approach. For the physical templating approach, a good example is single-crystal GaN nanotubes synthesised from ZnO nanowire templates. In such a process, ZnO nanowire only acts as a substrate for the deposition of a GaN layer. Other examples contain single-crystal Si nanotubes, IIBVI nanotubes, and magnetite nanotubes. In the chemical templating approach, preformed or in situ formed nanowires are used not only as a substrate for the deposition of different chemicals but they also thoroughly or partially react with the chemicals to form the target composite nanotubes. Single-crystal nanotubes obtained using such concept include II3V2 nanotubes, ZnAl2O4 nanotubes, MgAl2O4 nanotubes and GaN nanotubes. We believe that these templating approaches can be readily extended to produce a variety of single-crystal nanotubes with different compositions if a proper choice of experimental conditions and reactants is made.

Single-crystal hollow nanotubes are more advantageous for potential practical applications in nanoscale electronics, optoelectronics, and biochemical sensing applications than their polycrystalline or amorphous forms. They represent exciting systems to probe fundamental questions about localisation of electrical carriers and optical excitons in one dimension. In this paper, single-crystal nanotube applications and devices were also demonstrated. For example, the four-probe device based on a single-crystal Fe3O4 nanotube was fabricated in order to investigate its electron transport properties. The field-effect transistor based on a single-crystal GaN nanotube was also prepared and the regarded studies showed that the GaN nanotube resistance increased with decreasing temperature.

Generally, it is worth noting that further continuing efforts are required to achieve much better control of single-crystal nanotube syntheses and nanodevice fabrication in order to uncover novel inorganic nanotube intriguing and exciting functional properties.

Acknowledgements

We thank Dr. D. Chen, Dr. J.Q. Hu, Dr. X.S. Fang, and Dr. C.H. Ye for helpful discussions.

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