one - dimensional ( 1d ) nanostructures such as nanowires ( nws ) , nanobelts , nanorods , and nanotubes , have been the focus of intensive research due to their novel electronic properties and potential applications in nanoscale devices [ 1 - 6 ] . among them , semiconductor nws is investigated in more detail due to their important roles in fabricating nanoscale electronic or optoelectronic devices [ 7 - 11 ] . the growing interest in semiconductor nws for electronic and photonic applications makes rational control over their morphology , structure , and key properties more and more important . it also requires thorough understanding of the growth mechanisms in specific material systems and techniques . vi   semiconductor compounds nws had been fabricated via the vapor  liquid  solid ( vls ) mechanism successfully . by this method , a liquid metal alloy initiates the growth of a solid whisker from vapor reactants . compared to the vls method , solution phase reactions have the advantage that seeds are not restricted to a two - dimensional ( 2d ) growth plane , and copious quantities of well - defined nanostructures can be obtained easily compared to methods based on vapor - phase reactions . chemical solution nw growth for group vi semiconductor material systems was envisioned to occur via the solution  solid  solution mechanism , in which trigonal se or te seeds initiate the growth of solid se , te , or se / te alloy nws from solution reactants . traditionally , se [ 16 - 18 ] and te   nws have been synthesized by reduced selenious acid or orthotelluric acid at elevated temperatures , typically at 90100 c or by the reduction of metal  salt solutions with ascorbic acid at room temperature , or by a mild bio - molecule - assisted reduction method under hydrothermal conditions . as se and te have similar trigonal structures , it is possible that se / te alloy nws of a single crystalline nature can be obtained by reducing selenious acid and orthotelluric acid at the same time in solution . more importantly , as te tends to form in the trigonal phase more readily than se , one may fabricate se / te heterojunctions by using te nanorods as crystalline seeds . although xia et al .   had synthesized se  te alloy nanorods successfully by reducing selenious acid and orthotelluric acid with hydrazine at the temperature range of 90100 c , the lateral dimensions and morphology of the se / te nws could not be controlled in this case due to lack of any surfactant and exact experimental control . on the other hand , the use of a trigonal te ncs as a crystalline seed in se / te qian research group   had previously reported that by employing sodium dodecylbenzene sulfonate ( sdbs ) or other surfactants , te nanorods with well controlled diameters and lengths could be reproducibly produced , which made the fabrication of se / te nws by using te ncs as crystalline seeds possible . our research group   had further found that by using sdbs as the surfactant , the morphology and the lateral dimensions of se / te alloy nws could be easily controlled . following this , se  te alloy nws with v - shaped structure has been prepared for the first time successfully by our research group with sdbs as surfactant . in this article , we present new and simple methods for the fabrication of se / te nws with different morphologies by using te ncs seeds . we further investigated the nucleation and growth mechanism of se  te alloy with different morphology by controlling the experimental procedure . for the first time we have investigated the fabrication of se / te nws with v - shape morphology , u - shape morphology , or straight morphology in the presence of sdbs surfactant by using different te ncs seeds in detail . we prove here that such a method is a highly effective synthesis protocol to produce 1d nanostructures of se / te alloy nws with different morphologies . because of the mild reaction conditions and easily controlled synthesis , this method can be used in large - scale production of te and se / te nw materials . first , fabrication of te ncs by a chemical solution process similar to our previous report . typically , 2 mmol of orthotelluric acid and 0.5 g sdbs were added to 100 ml pure water . then , the resulting solution was heated up to 95 c at a rate of 10 c / min in an argon atmosphere . after 30 min , 1.5 ml of hydrazine was quickly injected into the solution through a syringe and the solution turned black and cloudy immediately . the solution was kept at 95 c for another 15 mins and then moved to an ice bath to quench the reaction to 0 c . the resulting solution was refluxed at room temperature for different time periods in order to get nws with different lengths . to obtain short te nanorods and nanoparticles , the reflux time is about 1 or 2 days while it takes at least 6 days to obtain long te nanorods . second , to obtain the v - shaped or u - shaped se / te nws , the resulting solution , which was refluxed for about 1 day , was heated to 95 c , and then a 15 ml solution containing 1 mmol , 2 mmol , or 4 mmol selenious acid was added drop by drop through a funnel into the resulting solution containing trigonal te nanorods and colloids . the corresponding feeding ratio between se to te is 1:2 , 1:1 , and 2:1 , respectively . the solution was refluxed at 95 c for another 3 h and cooled down to room temperature . on the other hand , to obtain se / te nw with a straight morphology , the resulting solution that had been refluxed for 4 days was heated to 95 c , and then a 15 ml solution containing 2 mmol selenious acid was added drop by drop through a funnel into the resulting solution which contains trigonal te nanorods and colloids . the solution was refluxed at 95 c for another 3 h and cooled down to room temperature . the growth of se / te nws was performed by using te nanorods as crystalline seeds through a chemical solution process . the detailed process and growth parameters for the nws growth can be found in the experiment procedure . specifically , we have found that te tends to form rod - shaped structures more easily than se and hence has the highest surface reactivity along its spiral chain direction . therefore , the se / te nws are expected to form a wire - like structure by using the te nanorod as the crystalline seed . in order to study the effect of feeding ratio ( molar ratio ) between se and te sources on the final morphology of se / te nws , we investigated the tem images of the se / te final product prepared by using short te nanorods prepared at the same condition as crystalline seeds with different se to te feeding ratios . figure 1a ( 1b , 1c ) shows the tem image of se / te nws fabricated under different conditions ( with se to te feeding ratios 1:2 , 1:1 , and 2:1 ) . figure 1a1 ( b1 , c1 ) shows the corresponding diameter distribution of se / te nws , while fig . 1a2 ( b2 , c2 ) shows the angle deviation from the growth direction ( showed in the inset of fig . we found that the morphology is different for different se to te feeding ratios used during the reaction . first , we observed that the morphology is different from that of the se / te nws with a straight morphology reported previously . v - shaped and u - shaped se / te nws were obtained in our case . second , se / te nws with a low se feeding ratio ( below 1:1 ) show an almost homogeneous distribution of v - shaped structures . no particles were found in the final product , which implies that all the amorphous se or te had been turned to triganol phase crystal structures during the reaction . in the case of v - shaped nws , we found that the average nw length and nw diameter were about 150 nm and 13 nm , respectively , for all se to te feeding ratios . third , for high se feeding ratios , se / te nws with all kinds of morphologies including v - shaped , u - shaped , and straight were obtained . from the distribution diagram of se / te nws , we found that the angle deviation from the growth direction is from 25 to 40 depending on the different se to te feeding ratio ( 1:2 or 1:1 ) . the angle distribution is in the range from 10 to 90 at high se to te feeding ratio ( 2:1 ) . this implies that with a high se feeding ratio , the strong competition between tellurium and selenium causes the growth direction to deviate from the regular direction . tem images of t - se / te nanowires with different se to te molar ratio : ase : te = 1:2,bse : te = 1:1 , andcse : te = 2:1 and their corresponding diameter ( a1 ) , ( a2 ) , ( b1 ) , deviate angle distribution schematic ( b2 ) , ( c1 ) , ( c2 ) as a comparison , we also synthesized se / te nws by a similar chemical solution process using long te nanorods as crystalline seeds ; that is , the te nanorods are prepared by refluxing the resulting solution containing te nanocrystalline for 6 days at room temperature . the feeding ratio of se to te is 1:1 in this reaction . shown in fig . 2a is the te nanorod crystalline seeds with a diameter of about 9.5 nm and length of about 150 nm . figure 2b shows the tem image of se / te nws prepared by using such te nanorod as the crystalline seeds . all the se / te nws exclusively show a straight morphology which is quite different from the v - shaped or u - shaped se / te nws prepared by using short te nanorods as crystalline seeds . the diameter of each se / te nw is about 13 nm , larger than that of the te crystalline seeds , while the length is about 300 nm , just two times that of the te nanorods . atem images of te nanorods crystalline seeds prepared by a chemical solution process;bse / te nws with straight morphology grown by using long te nanorods with crystalline seeds figure 3 is the xrd patterns of the products obtained from the as - synthesized sample of se  te alloy nws with different se to te ratio and pure te crystalline seeds prepared by hydrazine reduction with sdbs as surfactant . the main diffraction peaks can be assigned to ( 100 ) , ( 101 ) , ( 110/102 ) , ( 111 ) , ( 200 ) , ( 103 ) , ( 210 ) , ( 211 ) , ( 212 ) , ( 301 ) and ( 201/003 ) of se / te and ( 100 ) , ( 101 ) , ( 102 ) , ( 111 ) , ( 200 ) , ( 201 ) , ( 003 ) , ( 202 ) , ( 210 ) , ( 212 ) and ( 301 ) of neat te nws . we find that the composition of se / te alloy nws have little effect on the structure of as prepared samples . other se / te alloy nws prepared with different conditions has the same structure as those in fig . no peak of other impurities , such as amorphous se or te , is detected , implying the synthesis of high purity te / se and te products and indicating that these alloy wires had been crystallized in a trigonal lattice similar to that of crystalline se or te , as reported in our previous studies . xrd pattern of the se / te nws with different se to te ratio and pure te nws supported on a glass slide . all peaks can be indexed to the triangular se / te and te lattice we further performed a set of measurements including hrtem and edx to investigate the structure and se , te composition in different part of one single se / te nw . first , we characterized the composition of v - shaped and straight se / te nws in different parts by energy dispersive x - ray spectroscopy ( edx ) measurements ( fig . the atomic and weight percentage composition of se and te in different parts of a single nw is listed in table 1 . we noted that the composition of se and te in different parts of one single nw was quite different . in the middle part of v - shaped se / te nws , the content of te decreased almost linearly from 57.1 to 36.9% ( 35.8% on another side ) from the middle part to the end side , while se increased from 42.9 to 63.1% ( 64.2% on anther side ) . we checked several single nws and obtained similar results . in the case of straight se / te nws , similar se or te content changes were found in different parts of single se  te nw . the te content in the middle of the nws is about 85.7% , the value of which is much higher than that of se content ( 14.3% ) . the te content decreases rapidly from the middle part to the end part . in the end part of a straight se / te nw , the value of te content is 32.2 and 35.5% , respectively . on the other hand , se content increased from 14.3 to 63.1% ( 67.8% in another side ) from the middle part to the end part of se / te nw . all these observations clearly imply that the growth of the se / te nws may be initiated from the middle part of the se / te nws.shown in fig . 4c and d are the hrtem images and fast fourier transforms image patterns ( fft , inset of fig . 4 ) of v - shaped and straight nws taken from the middle part of nw (  ) . tem images of 1d single se / te nanostructures prepared with sdbs as surfactant.av-shape se / te;bstraight se / te nw and their corresponding hrtem image(inset show the sead pattern ) ; andc , dedx taken from different part of one single v - shaped and straight se / te nw (  ) , the results have been listed in table 1 eds analysis of se and te content at different part of v - shape and straight se / te nw the fft pattern was recorded by focusing a convergent beam on the nw . since this pattern remained unchanged along the length of the nw , we concluded that this nw was essentially single crystalline in nature . hrtem images show well resolved lattice fringes ( in the ( 001 ) planes ) of the se / te lattice , with the interplane spacing of 5.8  and 5.4  , respectively , which are between the values of trigonal se ( c = 4.953  ) and te ( c = 5.921  ) , indicating that the nws grow along the ( 100 ) direction . the pattern indicates that these se / te nws are single crystalline in nature and have predominantly grown along the   direction , with the helical chains of se / te atoms parallel to the longitudinal axis . to further investigate the nucleation and evolution of v - shaped se / te nws during growth , we monitored the morphological changes of the se / te nws by tem imaging for different growth times but otherwise identical growth conditions ( with a se to te feeding ratio of 1:1 ) . before the se sources addition , the product contained te nanorods and some nanoparticles ( fig . 5a ) . the diameter of a typical te nanorod is about 8 nm while the length is about 50 nm . after se source addition , in a short growth interval ( 1 min ) , the apparent nw diameter increased to ~12.3 nm while the length increased to about 170 nm due to amorphous se and te nanoparticle resolved and deposited on the end and side wall of the te nanorods . the length and diameter of the nws was observed to grow rapidly with increasing growth time . figure 5e and f show histograms of average nw length and average diameter for different growth times . it is evident that the nws start to grow with small diameters of about 8 nm . the diameter of each nw increases rapidly from 8 nm to 12 nm with 2 min of additional growth time after se source addition , and then it grows slowly in diameter when we further increase the growth time . the diameter of the final product is about 13.5 nm , a little larger than that formed with a 2 min reaction . the length of the nws increase rapidly from 50 nm to 170 nm , 288 nm , and 416 nm for growth times of 0 , 1 , 3 , and 5 min after se source addition . the increase in nw diameter and length indicates the formation of v - shaped se / te nws based on how short te nanorods finishes quickly upon se source addition . afterwards , there is almost no change even with an increase in the growth time . the only difference is that only straight morphologies were obtained in the final se / te nw product . these results further suggest a growth mechanism different from the se or te that was generated in the same reaction solution through homogeneous nucleation , which takes a long time to finish , and provides further evidence that te nanorods as crystalline seeds are necessarily initiating se / te nw growth , and that nw growth is nucleated by te nanorods in our study . tem images ( a  d ) of v - shape se / te nws at different growth time after the addition of selenious acid into a solution containing short te nanorods and nanoparticles and the statistical schematic of diameter and length of se / te nws with growth time diffraction patterns ( e , f ) that support the mechanism outlined in fig . 6.amonodispersed short te nanorods crystalline seeds and te nanoparticles , b1 min , c3 min , andd5 min after the addition of selenious acid.ethe length of se / te nws with growth time , andfthe diameter of se / te nws with growth time these experiments demonstrate that te nanorods and sdbs surfactant play important roles in fabricating v - shaped and straight se / te nws . specifically , the observations that the content of te in the middle part is higher than in the end part implies that se / te nws grow from the middle part of the short or long te nanorods . shanbhag et al .   prepared for the first time v - shaped te nanorods with a similar chemical solution process . they assumed that the incidence of nanoscale checkmark formation was governed by the frequency of proximal interactions between nanorod tips and based on the simulation results , they were able to explain why v - particles were observed for short nanorods while absent for long nanorods . we found that this mechanism can be used to explain the growth of se / te alloy nws with different morphology in our case . however , we must point out that there are still some differences among them such as the preparation method . the se and te source are used in our case while only the te source is used in their case and the using te nanorods as crystalline seeds . the fact that there are no se or te nanoparticles in the final product indicates that the presence of te nanorods is important for the single crystalline growth of se / te nws . study of the growth of se / te nws has shown that the formation of v - shaped and straight se / te nws in solution with sdbs as the surfactant involves several distinct stages : ( 1 ) the generation of short or long t - te nanorods by adding n2h4 to reduce te in the solution with different refluxing times , ( 2 ) the formation of se ncs when se source was added drop by drop in the reacting solution , and ( 3 ) the a - se and a - te nanoparticles and some small te nanorods dissolve into the solution during refluxing . the selenium and tellurium dissolved from a - se , a - te colloids and small t - te nanorods could subsequently compete against each together and deposited on the surfaces of t - te nanocrystallites ( seeds ) . ( 4 ) trigonal te nanorods(seeds ) absorbed selenium and tellurium and grew into uniform , v - shaped or straight single crystalline nws . the solid  solution  solid transformation , the anisotropic nature of the building blocks along the   direction of nanocrystalline se / te , and the surfactant - assisted preferentially unidirectional growth mechanism could be key factors in the formation of se / te nws . shown in fig . upon addition of hydrazine to the solution containing orthotelluric acid , the clear mixture immediately became black and opaque , indicating the formation of spherical colloids of te ncs ( fig . 6a ) . te ncs were produced through in situ reduction of orthotelluric acid with excess hydrazine : 2h6teo6 + 3n2h4  2te( ) + 3n2( ) + 12h2o . the formation of te nanorods is similar to the se / te nws reported in our previously published work . when the selenious acid was added to the solution containing te ncs and excess hydrazine , se nanoparticles were produced immediately with hydrazine : h2seo3 + n2h4  se( ) + n2( ) + 3h2o . the concentration of a - se increases rapidly and will slowly dissolve into the solution during refluxing at relative high temperature ( ~90 c ) due to their higher free energies as compared to those of t - se . the selenium and tellurium dissolved from a - se , a - te colloids , and some small t - te nanorods can be subsequently deposited on the surfaces and the side wall of t - te nanorods ( seeds ) ( fig . we must point out here that with the presence of sdbs surfactant , the growth of different planes of t - te nanorods seeds is largely confined . we speculate that the sidewalls are mostly passivated by sdbs while the axial growth planes ( direction ) are only partially passivated by sdbs . this has been confirmed by our experiment that the length of te nanorods changes greatly while only a small change was observed in the diameter ( fig . , the growth rate of the ( 001 ) plane is much faster than that of other planes of t - te nanorods ( seeds ) . at high activity of short t - te nanorods , the interactions between te nanorod tips and the lattice constant mismatch in se and te . the competition between se and te at the end of short te nanorods will result in the morphology change at the end of te nanorods . the new se / te single crystalline surface formed at the end or the side wall of te nanorods will act as new crystalline seeds and absorb the se and te dissolved from a - se , a - te colloids , and small t - te nanorods in the solution . finally , uniform v - shape or u - shape se / te nws in the final products will be obtained after refluxing for several minutes . in the case of long te nanorods , the frequency of proximal interactions between nanorod tips is low and we speculate that the concentration of tellurium is very low compared with selenium in the solution . the competition from se and te at the end of te nanorods ( seeds ) is very weak . no morphology changes occur in this case and only se / te nw with straight morphology will be obtained in the final products . schematic illustration of a plausible mechanism for the formation of v - shape se / te nw and straight se / te nw with sdbs as surfactant , aformation of a - se nanoparticles when adding selenious acid into the solution containing short te nanorods and nanoparticles , ba - se and a - te compete together and deposited at the end of te nanorods crystalline seeds , cv - shape structure formation and continue to grow from the seeds accompanied by the dissolution of a - se and a - te colloidal particles as relax energy of te nanorods at the end sides , dformation of a - se nanoparticles when adding selenious acid into the solution containing long te nanorods and few te nanoparticles , ea small amount of a - se and large quantity of a - te compete together and deposited at the end of te nanorods crystalline seeds , andfse / te nws with straight morphology formation and continue to grow from the seeds accompanied by the dissolution of a - te colloidal particles in conclusion , we have studied the nucleation and growth evolution of se / te nws prepared by chemical solution process . by varying key growth parameters such as using short te nanorods or long te nanorods as crystalline seeds , sequentially changing the se to te content , growth time , first time significant insights into the se / te nws with different morphology growth have been developed . the trigonal te nanorods were found to have the major role on the growth of single crystalline se / te nws , while the present of sdbs surfactant is necessary to restrain the grow direction of se / te nws . v - shape or u - shape se / te nws are most likely formed by the competition of selenium and tellurium at the end of te nanorods ( seeds ) and by the frequency of proximal interactions between nanorod tips . this conclusion is also supported by the analysis of se and te content in different part of se / te nw by edax . these findings are very useful for understanding and rational synthesis of se / te nws or other vi group compound semiconductor nws . this work is supported by nsfc project ( no 50703012 ) and the most project ( nos 2009cb930604 and 2009cb623602 ) .
we have studied the nucleation and growth of se  te nanowires ( nws ) , with different morphologies , grown by a chemical solution process . through systematic characterization of the se  te nw morphology as a function of the te nanocrystallines ( ncs ) precursor , the relative ratio between se and te , and the growth time , a number of significant insights into se  te nw growth by chemical solution processes have been developed . 
 specifically , we have found that : ( i ) the growth of se  te nws can be initiated from either long or short triganol te nanorods , ( ii ) the frequency of proximal interactions between nanorod tips and the competition between se and te at the end of short te nanorods results in v - shaped structures of se  te nws , the ratio between se and te having great effect on the morphology of se  te nws , ( iii ) by using long te nanorods as seeds , se  te nws with straight morphology were obtained . 
 many of these findings on se  te nw growth can be further generalized and provide very useful information for the rational synthesis of group vi based semiconductor nw compounds .