semiconductor nanowires and their associated heterostructures are ideal candidates to achieve one - dimensional quantum confinement in materials , and thereby they are ideal candidates to explore the physical properties of materials in one - dimension . promising physical properties and wide variety of applications were demonstrated using these semiconductor nanostructures . many nanowire based devices have been demonstrated , including nanowire diodes , photodiodes , single - electron transistors , and field - effect transistors . various mechanisms have been used to synthesize these semiconductor nanowires , such as vapor  liquid nanowires growth via the vls mechanism   offers the opportunity to produce axial , radial , and branched   nanowire heterostructures with control over the nanowire size , shape , and location . as a consequence radial nanowire heterostructures which consist of core , shell , and multi - shell morphologies , offer the flexibility to tailor the bandgap structure of radial nanowire heterostructures . such flexibility can be used to tune the desired electrical and optical properties . many semiconductors of iii  v and ii  vi compounds can adopt the polytypism of zinc - blende / wurtzite crystal structures based on their growth conditions and difference in the internal energies of two crystal structures for a specific material . zinc - blende structure has  abcabc  stacking sequence along 111 directions , whereas wurtzite structure has  ababab  stacking sequence along 0001 directions . this polytypism gives rise to different band structures depending upon its crystal structure , which , in turn , allows the realization of polytype superlattice structures . polytypism is often observed when these semiconductors grown in the form of nanowires , especially using au nanoparticle catalysts . for example , inas nanowires grown via metal  organic chemical vapor deposition ( mocvd ) method generally show wurtzite structure , whereas , gaas nanowires retain its bulk zinc - blende crystal structure . since the nanowires can act as ideal one - dimensional materials , novel physical properties can be achieved when inas nanowires sheathed with gaas . the resultant gaas / inas core / shell nanowire structures can give interesting optical and electronic properties of interest to device applications . since both the semiconductor materials show different crystal structures when they grow axially , it will be scientifically important and technologically necessary to explore how they behave when they grow laterally . in this study , we grow inas / gaas core  shell structures using mocvd method . they are characterized by detailed transmission electron microscopy ( tem ) , in terms of their compositional and structural characteristics . inas / gaas core / shell nanowire heterostructures were grown in a horizontal flow mocvd reactor at 100 mbar with a growth temperature of 450 c . firstly , inas nanowires were grown for 30 min on a gaas   substrate using au catalysts with a nominal size of ~30 nm by flowing trimethylindium ( tmi ) and ash3 . gaas is then deposited on these nanowires for 30 min by switching off the tmi flow and switching on the trimethylgallium ( tmg ) flow . flow rates of tmi , tmg , and ash3are 1.2  10 , 1.2  10 , and 5.4  10 mol / min , respectively . the fabricated nanowire heterostructures were characterized by scanning electron microscopy ( sem , jeol 890 ) and tem [ tecnai f20 ] . tem specimens were prepared by ultrasonicating the nanowires in ethanol for 10 min followed by dispersal onto holey carbon films . figure 1 shows a typical sem image of gaas / inas nanowire heterostructures , in which almost all nanowires grew perpendicular to the substrate surface , i.e. , along the   direction . the nanowires are tapered , suggesting that lateral growth has taken place , particularly in the bottom region . the inset of fig . 1 is a tem image of the top portion of a typical gaas / inas nanowire and shows the detailed morphology of the gaas / inas nanowire with a au catalyst at the tip . since gaas was deposited for 30 min , it is anticipated that gaas lateral growth has taken place around the inas nanowires . to understand the lateral growth behavior of gaas around inas nanowires , figure 2a shows a tem image of gaas / inas radial nanowire heterostructure with a high - magnification image in fig . the moir fringes in the middle region and stain contrast in the outer region suggest the core / shell structure . to determine the chemical compositional characteristics , energy dispersive spectroscopy ( eds ) analysis was performed . figure 2c shows the qualitative analysis of eds line scan across the nanowire , clearly showing the inas / gaas core / shell nanowire heterostructure . we anticipate that such difference in the quantity is due to higher volume of gaas shell than the inas core . in order to determine the structural characteristics of these nanowire heterostructures , figure 2d shows a selective area electron diffraction pattern taken from one of these nanowires . two sets of wurtzite diffraction patterns ( along   zone axis ) with their diffraction spots accompanied by additional double diffraction spots can be seen . the lattice mismatch between two sets of diffraction spots can be measured to be ~6.5  0.5% in both the axial and lateral directions , suggesting that the misfit strain between the two materials is almost relaxed in both axial and lateral directions and there is no lattice distortion . to further determine the structural characteristics at atomic level , high - resolution tem ( hrtem ) was conducted . inas core region can be identified by the presence of the strain contrast , and both the inas core and the gaas shell have the wurtzite structure . sem image of gaas / inas nanowire heterostructures , where the substrate normal is tilted 10 away from the incident electron beam direction . inset shows a tem image of the top portion of a typical gaas / inas nanowire alow magnification tem images of a gaas / inas nanowire heterostructure.ba high - magnification tem image of the gaas / inas nanowire , and its corresponding eds line - scan spectrum across the nanowire is shown in ( c).dan electron diffraction pattern taken on the gaas / inas core / shell structure.ehigh resolution tem image showing gaas / inas interface region as mentioned earlier , inas nanowires grown along 111b directions using mocvd adopt the wurtzite structure . however , when gaas nanowires grow axially they preserve its bulk crystal structure , the zinc - blende structure . in fact , when inas and gaas were grown alternately along the 111 directions to form axial nanowire heterostructures , they were observed to have alternating between wurtzite and zinc - blende crystal structures , respectively . however , the results shown in this study and references   show that when inas is sheathed around gaas nanowires , the sheathed inas adopt the zinc - blende structure ; while when gaas is sheathed over the inas nanowires , the sheathed gaas adopt the wurtzite structure . in order to understand this structural difference between the axial and radial heterostructures , we verify the lattice registry between zinc - blende and wurtzite crystal structures by placing both the structures axially and laterally . figure 3a shows   projected zinc - blende structure of gaas placed above the   projected wurtzite structure of inas . figure 3b shows both the atomic structures when placed laterally to each other . as can be seen from fig . 3a , when these structures are placed one another along the 111 direction , they can have a lattice registry between them . in fact , both wurtzite and zinc - blende structures are stack of { 111 } or { 0001 } planes with a difference in the stacking sequence . since the atomic arrangements in both { 111 } and { 0001 } planes are identical , they can coexist when the nanowires grow in 111 or { 0001 } directions with a lattice registry between the two structures . such a possibility is believed to be the reason for the coexistence of both wurtzite and zinc - blende structures even in two - dimensions when they grow in 111 direction . aa schematic diagram showing   projected zinc - blende structure placed above the   projected wurtzite structure , andbshows both the structures but placed in the lateral direction in the case of lateral direction ( fig . 3b ) on the other hand , both crystal structures can not have a lattice registry except for each sixth layer , as shown by the arrows . in fact , lack of this lattice registry would cause high energy heterointerfaces , and it is observed that such energetic conditions would transform the wurtzite structure into zinc - blende structure when the wurtzite nanowires are sheathed with two - dimensional zinc - blende layers . similarly , in our current study , the normally zinc - blende gaas structure transforms into a wurtzite structure when brought into contact with the inas nw side walls , by nucleating epitaxially on the nanowire sidewalls . we have grown inas / gaas core / shell structures using mocvd method , and the transmission electron microscopy investigations show that both the core and shell contain wurtzite structure . in contrast , when inas / gaas heterostructures grow in 111 or 0001 axial directions , both the materials can have different crystal structures . this structural difference between both the axial and lateral direction is explained in terms of crystallography and interfacial structure . m. paladugu acknowledges the support of an international postgraduate research scholarship . the australian national fabrication facility established under the australian government s national collaborative research infrastructure strategy is gratefully acknowledged for access to the facilities used in this study .
gaas was radially deposited on inas nanowires by metal 
 organic chemical vapor deposition and resultant nanowire heterostructures were characterized by detailed electron microscopy investigations . 
 the gaas shells have been grown in wurtzite structure , epitaxially on the wurtzite structured inas nanowire cores . 
 the fundamental reason of structural evolution in terms of material nucleation and interfacial structure is given .