efforts in nanomaterials have rapidly expanded into the assembly of well - ordered two- and/or three - dimensional ( 2d and/or 3d ) superstructures.the 3d superstructures provide possibilities to probe brand - new properties and applications due to the spatial orientation and arrangement of the nanocrystals [ 1 - 3 ] .
different methods have been used to fabricate preferentially oriented nanowire arrays , such as , electrodeposition [ 4 - 6 ] , vapor  liquid 
solid ( vls ) , thermal evaporation [ 9 - 11 ] , lithography , aao , or mesoporous silica sba-15 -assisted methods . in these methods , sba-15 template shows a promising technique in controlling the preferential orientation without changing the nanowire morphology , and it has been explored for synthesis of ag , au , pt , ph , and si nanowires   as well as binary semiconductor nanowires [ 19 - 22 ] .
as previously reported , after removal of the silica template , the ordered mesostructure is seldom maintained , because the inorganic precursors are inclined to be absorbed on the external surface of templates and the channels are not completely filled up , which causes the framework formed inside the pores to be lacking in sufficient internal cross - linkage .
recently , a single - source precursor , cadmium thioglycolate , was used to synthesize crystalline mesoporous cds nanoarrays through sba-15 silica template technique .
the results demonstrated that such mesoporous semiconductor nanoarrays with high crystallinity were exactly an inverse replica of sba-15 .
these nanoarrays provide many opportunities for new applications as advanced materials ; however , the systematic studies of the transport , optical , and electrical properties of these nanoarrays were not reported till now .
recently , some research has focused on rectification properties in the nanoscale [ 25 - 27 ] .
the electrical properties of cds with different morphologies , such as nanoparticles , nanorods , and nanowires , were reported recently .
the i  v characterization of these cds systems demonstrated good ohmic contacts   or highly insulating , but the rectification was not observed in these pure cds systems .
the rectification can be obtained only when cds form heterostructures with others , such as , nanoparticles , polymer   or si . here , the cds nanoarrays were synthesized through single - source precursor , metal alkyl xanthate .
the remarkable performance on rectification within a bundle of cds nanoarrays was characterized by semiconductor characteristic measurement system ( keithley 4200 scs ) , and the mechanism of the rectification of the cds nanoarrays was discussed .
mesoporous silica sba-15 was prepared by a triblock copolymer under hydrothermal treatment at 130 c for 48 h following the general procedure reported by zhao et al . .
for synthesis of cds nanoarrays , typically , 0.05 g sba-15 was added to a solution obtained by dissolving 0.34 g of cadmium alkyl xanthate in a certain amount of tetrahydrofuran , and then the mixture was kept stirring at room temperature until the solvent was completely vaporized .
the residual powders were dried and then heated to160 c at a rate of 1 c / min and maintained at this temperature for 10 h under argon .
silica composites were soaked in 2 m naoh for several hours to remove the silica template .
the template - free cds products were recovered by centrifugation , washed with water , and dried at room temperature .
the morphology of the samples was characterized by transmission electron microscope ( tem , jem-100cx ) at 100 kv .
structural characterization was performed by x - ray diffraction ( xrd , xpert pro mpd , with cu k radiation ,  = 1.54060  ) at 40 kv and 40 ma .
scanning electron microscope ( sem ) measurement was carried out using a jsm-5600 lv equipped with edx ( oxford isis ) at 20 kv .
 vcurves of assembled cds nanowire arrays were measured by semiconductor characteristic system ( keithley 4200-scs ) at 350 nm illumination .
desorption isotherms were measured on a micromeritics tristars 3000 analyzer at 196 c . before the measurements
the sba-15 was synthesized at a high hydrothermal temperature of 130 c to increase the mesotunnels , which are beneficial for the production of high - quality replica materials .
figure 1a exhibits the small - angle x - ray diffraction of the template - free cds nanoarrays .
it indicates a highly ordered 2d hexagonal mesostructure ; at the same time , it implies that cds nanoarrays replicate well the ordered mesoporous of the sba-15 template and confirms the ordered arrangement of cds nanowires .
the edx spectrum of the nanowire arrays confirms that these nanowire arrays consist of stoichiometric cds with a cd / s ratio of 1.05:1 and displays strong signals from cd to s elements without the detection of silicon element .
asmall - angle diffraction pattern of template - free cds nanoarrays.beds pattern of the template - free cds nanoarrays.cwax diffraction curves of cds nanoarrays present in sba-15 pore channels and silica - free cds nanoarrays the wide - angle xrd patterns of the cds nanoarrays ( fig .
1c ) before and after removal of sba-15 show the ( 100 ) , ( 002 ) , ( 101 ) , ( 110 ) , ( 103 ) , and ( 112 ) planes at 2 values 24.8 , 26.5 , 28.2 , 43.7 , 47.8 , and 51.8 , respectively , which match those of the hexagonal wurtzite structure of cds crystallite .
the same xrd patterns of cds nanoarrays before and after removal of sba-15 template demonstrated that the structure of the cds nanoarrays is completely maintained during the etching . on the basis of the width of the diffraction peaks ,
the size of the cds nanocrystals is on the nanometer scale and the average particle size is calculated to be 7.1 nm , which is consistent with the pore diameter of the host sba-15 .
2a ) show typical type - ivcurves with an h1-type hysteresis loop , attributed to perfect cylindrical mesopore channels .
a calcined mesoporous silica sample exhibits a high surface area of 560 m / g , the pore volume of 1.23 cm / g , and a narrow pore size distribution with a mean value of 7.3 nm .
the surface area and pore size distribution of the cdr2/sba-15 nanocomposite are 28 m / g and 5.4 nm , respectively , and the adsorption volume is 0.06 cm / g , suppressed by 20 times after the one - step nanocasting process . upon the calcination at 160 c for cds@sba-15 , the surface area and the pore volume decrease to 38 m / g and 0.04 cm / g , respectively , suggesting that the pores have been filled up by cdr2 .
desorption isotherms for the template - free cds sample exhibit a surface area of 100 m / g and a pore volume of 0.11 cm / g .
a relatively narrow pore size distribution of around 3.3 nm is also exhibited , further demonstrating the high - quality mesoporous cds replicas .
anitrogen sorption isotherms of the samples successively produced in the synthesis : the sba-15 hard template (  ) , cdr2@sba-15 composites (  ) prepared by the impregnation of cdr2into sba-15 , cds@sba-15 (  ) obtained from the cdr2@sba-15 calcined at 160 c , and cds - sba-15 replicas (  ) etched by naoh solution.bthe corresponding pore - size distribution curves ofa figure 3demonstrates the tem images of the sba-15 template , and the cds nanoarrays after the removal of sba-15 templates ( fig .
3a ) templates have a uniform fiber - like morphology and well - ordered 2d mesoporous structures with uniform pore size .
the high - magnification tem shows the pore size of sba-15 to be approximately 7.3 nm ( fig .
3b ) demonstrate that the as - prepared cds nanowire arrays have a uniform fiber - like morphology , and that the length of the cds nanowire arrays to be more than 2 m . from the insert of fig .
3b , it is obviously observed that these cds nanowire arrays are composed of uniform nanowire , and the diameter of each nanowire is about 7.3 nm , which is in reasonable agreement with the mean pore size of the silica template .
these phenomena may reflect that the cds nanowire arrays are perfect duplication of the hard template and a high yield for the incorporation of cds .
3e ) observed from this area can be indexed as the hexagonal wurtzite cds structure , indicating that the products consist of wurtzite cds nanocrystals , which is in agreement with the xrd data .
atem images of the sba-15 sample along the direction of the hexagonal pore arrangement.btem images of the template - free cds sample .
inseteis the corresponding saed pattern ofb in recent years , ac electric field assembly is widely used to construct nanodevices from nanowires   and carbon nanotubes [ 37 - 39 ] . here , the cds nanowire arrays were ultrasonically dispersed in ethanol . after applying a droplet of the nanowire arrays suspension onto the electrodes ,
the pt electrodes were connected to an 8-vpp ( peak - to - peak ) and 10-khz ac signal for 30 s. this signal generated an alternating electrostatic force on the nanowire arrays in the solution . under the electrical polarization force ,
the 10-nm - thick pt electrodes were patterned on an oxidized silicon wafer with 200-nm silicon dioxide using photolithography .
4a shows the sem image of a bundle of cds nanowire arrays on pt electrodes .
the electrical transport property was carried out through semiconductor characterization system ( keithley 4200-scs ) .
little current flows in reverse bias , whereas , the current increases rapidly when the bias voltage is more than 4 v. the most striking feature ( fig .
4 ) is that both of the curves display rectifying behaviors at room temperature in air environment . under dark condition , this rectifying diode - like behavior has a threshold voltage of ~3.9 v , and no obvious breakdown current is observed even at 11 v bias .
4b ) ; at the same time , the forward current is 25 na at 11 v , and their rectification ratio is about 625 . under uv
(  = 350 nm)-illumination the turn - on voltage was dropped from 3.9 v to 3.4 v , and the breakdown current is not observed even at 11 v bias .
the reverse current is 0.42 na at 11 v , the forward current is 64 na at 11 v , and their rectification ratio is about 152 . the rectifying behavior is an important application of a diode [ 25 - 27 ] .
recently , the rectifying behaviors of electric field - assembled zno nanowire have been reported . to our knowledge ,
such rectifying behaviors were not reported in pure cds system . in order to confirm the validity of the rectification for different metal sulfides ,
under dark condition , we observed the threshold voltage of ~2.0 v , the forward current of 150 na at 10 v , and the reverse current of 27 na at 10 v. for cds nanoarrays ( fig .
4b ) , the reverse current is just 0.04 na at 11 v , and the reverse current for zns nanoarrays is much larger than that of cds nanoarrays , so the rectification ratio for zns nanoarrays is about six . but the obvious rectifying characterization was observed from the zns nanowire arrays .
it is a very effective method for preparing metal sulfides with rectification through sba-15 as hard template and using a single - source precursor .
thei  vcurves of cds ( a ) and zns ( b ) nanoarrays at dark condition according to our previous reports on the current transport behavior of semiconductor nanowires , the nonlinear transport behavior in fig .
4 indicates that the cds nanoarrays make two schottky barrier contacts with the two pt electrodes , and a back - to - back schottky barriers structure is formed . in brief , the back - to - back schottky barrier structure is composed of two inversely connected schottky barriers , and the current in this structure is dominated by the reverse current of the reverse - biased schottky barrier .
the detailed description of the properties of the back - to - back schottky barriers structure has been discussed in the previous reports of our group   and other groups . in this structure , there should be no rectifying behavior if the two schottky barrier were symmetric .
however , in the dielectrophoresis deposition process , the ac voltage introduces an electrostatic force on the nanoarrays , and then the nanoarrays move toward the electrodes until it gets finally deposited on them .
the contacts of both ends of the nanoarrays onto the electrodes occur in a consecutive order .
we consider that the side of the nanoarrays that touched the electrode first may have a firm contact with the electrode , thus forming a better contact with lower barrier height , whereas the other end that contacted later had a higher barrier , possibly leading to the formation of the schottky diode for our devices .
thus , the asymmetric contacts were formed between the cds nanoarrays and pt electrodes , and resulted in the rectifying behavior .
there occurs a current difference between the dark and uv illumination , because more number of electrons in the valence band are excited into the conduction band under uv illumination thereby increasing the carrier concentration of the cds nanowire arrays . as shown in fig .
4 , both thei  vcurves under dark and illumination conditions show linear behaviors when the bias is larger than 6 v. in this linear region , the current transport property is dominated by the nanowire itself , not by the schottky barrier .
the slope of this linear region is proportional to the carrier concentration of the cds nanowire arrays . as shown in fig .
4 , the slope of the current curve under illumination condition is three times higher than that of the one under dark condition .
in summary , highly ordered mesoporous cds nanoarrays have been achieved by sba-15 as a template and cadmium xanthate as a single - source precursor .
these materials exist as strongly nonlinear and asymmetrical , showing rectifying diode - like behavior , and provide opportunities for new applications as advanced nanodevices .
the synthesis and properties of other systems , such as , mns , zns / cds composite nanoarrays , are currently in process .
this study was supported by henan project for university prominent research talents ( haipurt ) ( grant no .
 highly ordered mesoporous cds nanowire arrays were synthesized by using mesoporous silica as hard template and cadmium xanthate ( cdr2 ) as a single precursor . upon etching silica , 
 mesoporous cds nanowire arrays were produced with a yield as high as 93 wt% . 
 the nanowire arrays were characterized by xrd , n2adsorption , tem , and sem . 
 the results show that the cds products replicated from the mesoporous silica sba-15 hard template possess highly ordered hexagonal mesostructure and fiber - like morphology , analogous to the mother template . 
 the current  voltage characteristics of cds nanoarrays are strongly nonlinear and asymmetrical , showing rectifying diode - like behavior . 
Introduction
Experimental
Results and Discussion
Conclusions
Acknowledgment