Considerable attention has been paid on semiconducting nanotubes since the first report of carbon nanotubes by Iijima in 1991 11 . Nanotubes are often obtained for materials with layered or pseudolayered structures 13 . Without these kinds of structures, templates (hard templates or soft templates) are usually used to promote the formation of tubular structures 21 51. Recently, we developed an in situ nanorod template method to synthesize high-quality single crystalline II–V nanotubes, including Zn₃P₂ nanotubes and Cd₃P₂ nanotubes 52 . The whole process can be expressed as shown in Fig. 1. This process was performed in a vertical induction furnace at high temperature and the mixture of ZnS (or CdS), Mn₃P₂ and P powders were used as the evaporation sources. At high reaction temperature, ZnS or CdS was evaporated and reacted with graphite to generate Zn or Cd vapors, which were transferred to low temperature region and deposited on the surface of graphite crucible, resulted in the formation of Zn or Cd nanorods due to the anisotropic nature of wurtzite Zn or Cd phases. At the same time, Mn₃P₂ was thermally decomposed to generate P gases. These newly generated P gases deposited on the surface of Zn or Cd nanorods and reacted with them to generate the Zn₃P₂ or Cd₃P₂ shells. After a reaction time, the inner Zn or Cd nanorods were consumed and finally only Zn₃P₂ or Cd₃P₂ nanotubes were formed.

Figure 2a is a SEM image of the Cd₃P₂product obtained in the in situ nanorod template process. 1-D nanostructures can be found deposited on the graphite crucible on a large scale. A magnified SEM image shown inset clearly reveals that they are hollow nanotubes. Figure 2b and c show typical TEM images of the obtained Cd₃P₂nanotubes. The clear brightness contrast confirms that they are hollow nanotubes. Typical nanotubes have diameters of about 100 nm and the wall thickness of several nanometers. As-synthesized Cd₃P₂nanotubes are of single crystalline nature with the preferred growth direction perpendicular to the (101) planes as revealed in Fig. 2d. Similarly, the Zn₃P₂products obtained in the in situ template process are also single crystalline nanotubes as shown in Fig. 2e. We should mention that defects existed in the as-synthesized Cd₃P₂and Zn₃P₂nanotubes because of the release of built-in strain.

Nanowire is one of the most common 1-D structure and many kinds of materials can form into the nanowire structures. Till now, Cd₃P₂, Zn₃P₂, and Cd₃As₂ among the II–V group semiconductors were found to be able to form into the nanowire structures. Omari et al. 53 reported the synthesis of single-crystalline Cd₃As₂ nanowires by thermal evaporation of Cd₃As₂ powders at 750 °C. Figure 3a is a SEM image of the Cd₃As₂ nanowires. Studies found that these Cd₃As₂ nanowires are single crystals with the growth directions along the (112) crystal planes. Omari et al. also investigated the optical properties of these nanowires and found that they have IR active direct type absorption transitions at 0.11, 0.28, and 0.54 eV, which make it possible to use these nanowires as low cost optoelectronic devices and photodetectors operating in the long wavelength range.

Liu et al. 54 reported the synthesis of Zn₃P₂ nanowires by the reaction between Zn and InP powders at 850 °C. These Zn₃P₂ nanowires are also single crystals and have typical diameters of about 100 nm and lengths of tens of microns, as revealed in Fig. 3b. During this process, Au was used as catalysts to direct the nanowire growth and it is obviously a vapor–liquid–solid (VLS) process.

We were able to synthesize high quality Zn₃P₂ and Cd₃P₂ nanobelts on a large scale using a method similar to the synthesis of Zn₃P₂ and Cd₃P₂ nanotubes 55 . Zn₃P₂ and Cd₃P₂ nanobelts were synthesized by using a mixture of ZnS (or CdS) and Mn₃P₂ powders as the evaporation source. The reaction was performed at 1350 °C for about 1 h. Figure 4a is a SEM image of the synthesized Zn₃P₂ nanobelts with rectangular cross-sections, which have diameters of about 100–200 nm and thickness of 40 nm. A TEM image of a single Cd₃P₂ nanobelt is depicted in Fig. 4b. The twisted structure confirms it is a nanobelt and the thickness is about 70 nm. The microstructures of the Cd₃P₂ nanobelts were studied by HRTEM and SAED. Figure 4c and d are the HRTEM images taken from the surface and body of a Cd₃P₂ nanobelt, respectively. The clearly resolved lattice fringes perpendicular to and along the longitudinal axis of the nanobelt are 0.71 and 0.87 nm, respectively, corresponding to the (101) and (010) lattice planes of tetragonal Cd₃P₂ phase. Studies revealed that these Cd₃P₂ nanobelts have the preferred growth directions along the [010] crystallographic orientations. For the synthesis of Zn₃P₂ and Cd₃P₂ nanobelts, no catalyst was used, indicating that it was governed by the vapor–solid (VS) mechanism. In fact, we found that Zn₃P₂ and Cd₃P₂ nanobelts can also be synthesized via the VLS mechanism if suitable catalysts, such as indium, were selected.

Synthesis and assembly of 1-D nanostructures with special morphologies, shapes, and compositions have attracted great interests very recently because they may process interesting physical and chemical properties associated with their specific characteristics. They may also be used to fabricate special electronic and optoelectronic devices which cannot be fulfilled using simple 1-D nanostructures.

Wang et al. 56 recently succeeded in synthesizing three-dimensional (3-D) branched tree-like Zn₃P₂ nanostructures as shown in Fig. 5. These 3-D Zn₃P₂ nanostructures were synthesized in a thermal-assisted pulse-laser-deposition (PLD) system using Zn₃P₂/ZnO/Zn as the target. A top view image shown in Fig. 5a demonstrated that these 3-D nanostructures have a sixfold symmetry. The tree-like branched shapes were confirmed by the side view SEM image depicted in Fig. 5b. The constituent branches of the tree-like structures were found to be as long as several tens of micrometers, with the diameters lying in the range of ten to several hundred nanometers. During this process, it was found that nanobelts were formed once the branches grew much longer as indicated in Fig. 5c. The composition of the tree-like Zn₃P₂ structures was checked using energy-dispersive spectrometry (EDS) and the results confirm they are pure Zn₃P₂ (Fig. 5d).

1-D bicrystalline nanostructures have received great attention because of their peculiar structures and structure-related properties. Several kinds of 1-D bicrystalline nanostructures have been obtained for ZnS, InP, etc. 57 58 59 60 61 62. We synthesized bicrystalline Zn₃P₂ and Cd₃P₂ nanobelts via a VS process in the vertical induction furnace system 63 . The source materials used are a mixture of Zn (or Cd), ZnS (or CdS), GaP, and Mn₃P₂ powders. Figure 6a is a SEM image of the Zn₃P₂ product, which clearly shows that long and straight nanobelts are obtained on a large scale. Each nanobelt has a uniform width of 100–200 nm and length in the range of tens of micrometers. A high-magnification SEM image shown in Fig. 6b clearly reveals that these nanobelts are bicrystals with distinct grain boundary along the direction parallel to the growth direction. TEM images of several typical Zn₃P₂ bicrystal nanobelts were depicted in Fig. 6c and d. Grain boundaries were clearly observed in the middle of the nanobelts, indicating the bicrystal nature. These Zn₃P₂ bicrystal nanobelts are composed of two single-crystal with the preferred growth axis along the [101] directions. HRTEM image taken from a single Cd₃P₂ bicrystal nanobelt was shown in Fig. 6e. The marked lattice fringes in each part of the Cd₃P₂ nanobelt are 0.71 nm, in accordance with the (101) plane of tetragonal Cd₃P₂. The correspondence Fast Fourier Transform (FFT) patterns taken from the two parts within a single bicrystal nanobelt were demonstrated in Fig. 6f and g, which verified the formation of bicrystalline Cd₃P₂ nanobelts with the preferred growth along the [101] directions.

Besides the above discussed special 1-D II–V semiconducting nanostructures, we were also able to get other kinds of interesting 1-D II–V semiconducting nanostructures. For instance, Fig. 7a and b show the SEM and TEM image of zigzag twinned Zn₃P₂ nanowires, which were also obtained in the vapor phase process 64 . Spherical indium nanoparticles were found attached to the zigzag nanowires, indicating that the process was governed by the VLS mechanism. These zigzag Zn₃P₂ nanowires typically have periodic twins with a period of 50–120 nm along the whole nanowires. Lots of defects were found in the twin boundaries area, similar to previous reports on twinned nanowires 57 58 59 60 61 62. If we did not use indium catalysts and kept other conditions constant, we synthesized zigzag single crystal Zn₃P₂ nanowires as shown in Fig. 7c and d. The angle between two neighboring kinks is about 120°, consistent with that between the (001) and (101) planes of tetragonal Zn₃P₂ phase. Figure 7e–g are SEM images of trumpet-like Zn₃P₂ nanowires, which are composed of a hollow cone supported by a nanowire 65 . The trumpet-like Zn₃P₂ nanowires are single crystals with preferred growth directions along the [010] orientations. Other special II–V semiconducting 1-D nanostructures include 1-D hierarchical Zn₃P₂/ZnS nanotube/nanowires heterostructures, which consist of main Zn₃P₂ nanotube wrapped with high density ZnS nanowires 66 .

We have discussed above several kinds of 1-D II–V semiconducting nanostructures obtained till now. By carefully controlling the experimental parameters, such as evaporation sources, temperature, carrier gases, etc., more 1-D nanostructures are expected to be obtained for II–V group semiconductors.

Figure 8a inset is a schematic illustration of a MIS-FET built on a single Zn₃P₂nanowire. Basically, the MIS-FET is supported on an oxidizedp-type silicon substrate with the underlying conducting silicon as the back gate electrode to vary the electrostatic potential of the nanostructure. Two metal contacts, such as Au and Ti/Au, corresponding to the source and drain electrodes, are defined by either photolithography or electron beam lithography, followed by evaporation of suitable metal contacts. TheI _ds–V _dscurves of the MIS-FET are illustrated in Fig. 8a, showing typicalp-type behavior of the Zn₃P₂nanowire. From theI _ds–V _dsmeasurement, the resistivity of the nanowire is calculated to be about 1.96 Ωcm. We also built MIS-FET using zigzag Zn₃P₂nanowire as shown in Fig. 8b inset. As-fabricated MIS-FET also confirms thep-type behavior of the zigzag nanowire.

To investigate the electronic transport behaviors of 1-D II–V semiconducting nanostructures, we fabricated MIS-FET built on single Zn₃P₂ and Cd₃P₂ bicrystal nanobelts and explored the electronic transport behaviors as a function of temperature in vacuum 63 . A SEM image of the Zn₃P₂ MIS-FET is depicted in Fig. 8c inset. Figure 8c displays the I–V curves of a Zn₃P₂ bicrystal nanobelt MIS-FET device measured in the temperature region of 100–300 K without applying gate voltage. The conductance of the device continuously decreased as the temperature decreased. The zero-bias conductance at 300 K is calculated to be 27.75 nano-Siemens (nS) and it decreases to 0.01 nS at 100 K. Plotted the zero-bias conductance in a logarithmic scale as a function of 1000/T gives a linear behavior within the temperature region investigated. All these results suggested that the thermal activation of carriers is the dominant transport mechanism for the Zn₃P₂ bicrystal nanobelt MIS-FET. The electronic transport behavior of Cd₃P₂ bicrystal nanobelts was also investigated at different temperatures and the results (Fig. 8d) also suggested a dominant thermal activation of carriers transport mechanism.

Liu et al. 67 fabricated MS-FET using single Zn₃P₂ nanowire as the active material. Basic structure of the MS-FET is demonstrated in Fig. 9a, in which Ni/Au was used as electrodes and Al as the top gate electrode across the nanowire. The typical I _SD V _SD characteristics of a p-type Zn₃P₂ nanowire based MS-FET measured at room temperature under gate biases ranging from −0.5 to 0 V with a step of 0.1 V is depicted in Fig. 9b. As-fabricated MS-FET shows p-type conductance behavior and it is turned off at zero gate bias, indicating that the MS-FET was in the E-mode.

Very recently, Wang et al. fabricated p n heterojunction photodiode using a crossed heterojunction made of p-type Zn₃P₂ nanowire and n-type ZnO nanowire 56 . The device structure is displayed in Fig. 10a inset and the I–V curve at reverse and forward bias is illustrated in Fig. 10. Wang et al. studied the device behaviors by checking the photodiode under reverse bias and in the dark and under the illumination of light with wavelength of 532 or 680 nm. The results are presented in Fig. 10b, which show apparent current enhancement by the light. The increase of the current results from the generation of electron-hole pairs inside the depletion region and nearby under the excitation of the light. The p n heterojunction photodiode gives rapid response and very high on/off ratio upon light illumination.