vertically aligned zno nanorods ( nrs ) and nanowires ( nws ) are attracting much interest for several applications such as nanophotonics , dye - sensitized solar cells , electron field emitters , surround - gate field effect transistors , and nanopiezotronics .
a number of preparation methods by high temperature vapor transport   and low temperature chemical synthesis   were developed . for comparison
, the nr arrays can be classified from several aspects : physical and geometrical properties of the individual building blocks and their uniformity in length , in diameter , and in axis - to - substrate angle .
the nrs / nws can be distributed either randomly or in a well - defined way .
for instance , photonic crystals with well - defined defects are of importance in nanophotonics .
another demanding application is the construction of zno nw - based dc current generator , where the nws convert the mechanical energy of a vibrating pt - coated , zig - zag - shaped electrode to electric energy by exploiting the piezoelectric nature of zno . even for nanosensors , however , the generated power density ( ~80 nw / cm ) should be significantly increased . as liu et al
.   have pointed out the output voltage of the system , being now typically in the order of ~10 mv , can be drastically improved by increasing the number of the active nw - s , i.e. , the ones which are in continuously contact with the zigzag top electrode .
therefore , two approaches were proposed : improving the uniformity of the nws on one hand and patterning the array according to the dimension and shape of the top electrode .
vertical zno nanoarrays arranged in a designed pattern were recently produced by a few groups using different techniques , however , either the geometrical non - uniformity of the nws or the low density of the vertical microcrystals ( ~1 nr/m ) makes their use in nanogenerator application difficult .
solid ( vls ) method the metal catalyst droplet on the top of the nw can hinder the formation of the required schottky contact at the top electrode / nw interface . here , we demonstrate alternative fabrication routes which fulfill all the above crucial requirements by providing highly uniform , crystallographically oriented nrs in the 100-nm diameter range , in predefined , dense patterns .
our method benefits of the catalyst free , low temperature epitaxial growth , and the direct writing nanolithography .
we have tried two options for the formation of nr arrays . in the first ,
the desired nucleation pattern was drawn in a polymethyl - methacrylate ( pmma ) layer , which was subsequently removed resulting in an all - zno structure . in the second route ,
the nucleation pattern was realized in a hard metal coating ; therefore , the fabricated nrs were electrically contacted at the anchoring surface .
the process flow for the fabrication of all - zno nr arrays is shown in fig .
, the zn- and o - terminated single crystal zno wafers were washed ultrasonically in acetone , ethanol , and deionized water , which was followed by a thermal - annealing step in a quartz tube at 1,050 c for 8 h in oxygen atmosphere . in order to prevent the sublimation of zn
, the substrates were placed between yttrium stabilized zirconia ( ysz ) wafers before annealing .
the 250-nm - thick pmma resist layer was exposed by e - beam lithography in an elionix els-7500ex instrument ( fig .
circular spots of different ( 50100 nm ) diameters arranged in a triangular ( tri ) or honeycomb ( hc ) lattice were generated .
the aqueous bath contained the same ( 4 or 40 mm ) molar amount of zinc nitrate hexahydrate ( zn(no3)2  6h2o ) and hexamethylene tetramine ( ( ch2)6n4 ) . during the zno nanostructure growth
, the specimen was mounted upside - down on a polytetrafluoroethylene ( ptfe ) sample holder .
the nanocrystal growth was carried out  without an electric field applied  in a multipurpose oven for 13.5 h periods at a set temperature of 85 c . however , due to the high heat capacity of the glass container and the dry atmosphere , the warming up was relatively slow : the bath temperature reached 80 and 82 c after 2 and 3 h , respectively . following slow cooling , the sample was thoroughly washed in de - ionized water and purged in nitrogen . afterward , the pmma layer was removed in acetone .
this step also helps to lift - off the parasitic zno debris formed in the solution volume ( fig .
schematic process flow of all - zno ( a  d ) and anchored ( e  h ) nanorod arrays .
the processing steps for all - zno structure are : surface treatment of zno substrates ( a ) , pattern generation in pmma by e - beam lithography ( b ) , chemical nanowire growth ( c ) , and pmma removal ( d ) .
processing steps for the anchored zno array are : ru thin film deposition ( e ) , e - beam lithography ( f ) , arion milling ( g ) , and chemical nanorod growth after pmma removal ( h ) nanorods grown through a hard metal mask obtained by ar - ion milling are anchored in the single crystal substrate in the recessed dips etched during metal milling . thereby the fabrication of arrays of electrically contacted nrs
however , here the surface treatment process of zno substrate was followed by the deposition of a 30-nm - thick , high - quality ru layer by using ion - beam sputtering   ( fig .
1f ) and was transferred into the hard metal film by ar ion milling ( fig .
the same chemical growth method was used as for the all - zno arrays ( fig .
the preparation condition details for both all - zno and anchored arrays are summarized in table 1 .
summary of the growth parameters and the obtained nanorod dimensions the obtained nanostructures were visualized by a hitachi s4800 field emission scanning electron microscope ( fesem ) .
transmission electron microscope ( tem ) images were obtained by a 200 kv jeol jem-2010 instrument .
the electrical characterization of the individual nws was carried out in air by conductive afm technique by means of a sii nanotechnology inc . ,
the spring constant and resonant frequency of the used au - coated cantilever is 1.4 n / m and 26 khz , respectively .
the process flow for the fabrication of all - zno nr arrays is shown in fig .
, the zn- and o - terminated single crystal zno wafers were washed ultrasonically in acetone , ethanol , and deionized water , which was followed by a thermal - annealing step in a quartz tube at 1,050 c for 8 h in oxygen atmosphere . in order to prevent the sublimation of zn
, the substrates were placed between yttrium stabilized zirconia ( ysz ) wafers before annealing .
the 250-nm - thick pmma resist layer was exposed by e - beam lithography in an elionix els-7500ex instrument ( fig .
circular spots of different ( 50100 nm ) diameters arranged in a triangular ( tri ) or honeycomb ( hc ) lattice were generated .
the aqueous bath contained the same ( 4 or 40 mm ) molar amount of zinc nitrate hexahydrate ( zn(no3)2  6h2o ) and hexamethylene tetramine ( ( ch2)6n4 ) . during the zno nanostructure growth
, the specimen was mounted upside - down on a polytetrafluoroethylene ( ptfe ) sample holder .
the nanocrystal growth was carried out  without an electric field applied  in a multipurpose oven for 13.5 h periods at a set temperature of 85 c . however , due to the high heat capacity of the glass container and the dry atmosphere , the warming up was relatively slow : the bath temperature reached 80 and 82 c after 2 and 3 h , respectively . following slow cooling , the sample was thoroughly washed in de - ionized water and purged in nitrogen . afterward , the pmma layer was removed in acetone .
this step also helps to lift - off the parasitic zno debris formed in the solution volume ( fig .
schematic process flow of all - zno ( a  d ) and anchored ( e  h ) nanorod arrays .
the processing steps for all - zno structure are : surface treatment of zno substrates ( a ) , pattern generation in pmma by e - beam lithography ( b ) , chemical nanowire growth ( c ) , and pmma removal ( d ) .
processing steps for the anchored zno array are : ru thin film deposition ( e ) , e - beam lithography ( f ) , arion milling ( g ) , and chemical nanorod growth after pmma removal ( h )
nanorods grown through a hard metal mask obtained by ar - ion milling are anchored in the single crystal substrate in the recessed dips etched during metal milling .
thereby the fabrication of arrays of electrically contacted nrs is achieved . the process shown in fig .
however , here the surface treatment process of zno substrate was followed by the deposition of a 30-nm - thick , high - quality ru layer by using ion - beam sputtering   ( fig .
1f ) and was transferred into the hard metal film by ar ion milling ( fig .
the same chemical growth method was used as for the all - zno arrays ( fig .
the preparation condition details for both all - zno and anchored arrays are summarized in table 1 .
the obtained nanostructures were visualized by a hitachi s4800 field emission scanning electron microscope ( fesem ) .
transmission electron microscope ( tem ) images were obtained by a 200 kv jeol jem-2010 instrument .
the electrical characterization of the individual nws was carried out in air by conductive afm technique by means of a sii nanotechnology inc . ,
the spring constant and resonant frequency of the used au - coated cantilever is 1.4 n / m and 26 khz , respectively .
the sem images of the all - zno arrays fabricated at optimized conditions are shown in fig .
c . the c - axis - oriented nrs show hexagonal cross section , which are according to the crystal orientation of the substrate collectively aligned to each other .
the sidewalls of the prism - shaped rods correspond to the most stable non - polar   faces .
note the ~250 nm high bottleneck - shaped part at the bottom of the nanocrystals in fig .
we have found that by changing the template geometry , the diameter and the length of the nrs can be tuned in the range of 90170 nm and 0.52.3 m , respectively .
the perpendicularly standing nrs reflect excellent geometrical uniformity . according to the image analysis done on the fesem image ( pixel size of 1.4 nm ) shown in fig .
this is the diameter of a circle having the same area as the hexagonal cross section of the object .
we have tried the same growth conditions on zn- and o - polar zno surfaces , but no significant difference was found in the obtained arrays .
a typical example observed during the optimization of the growth parameters is inserted in fig .
2d . when the concentration in the growth solution is increased to 40 mm , the growing nrs coalesce at their non - polar sides to form a contiguous network .
fesem images on all - zno nanorod arrays prepared by soft - masking method in honeycomb ( a , d ) and on triangular ( b , c ) arrangements .
the single crystal nanorods have hexagonal cross - sections ; the uniformity of diameter can be < 2% ( b inset ) . when the concentration of the growth solution is increased to 40 mm ,
a coalescence of nanorods is observed ( d ) anchored , i.e. , metal back contacted arrays show similar geometrical features as the all - zno structures ( fig .
, we have also downscaled the pattern : the densest array had an rod - to - rod distance of 175 nm , which in hc lattice corresponds to a nr density of 25 nr/m
. however , in the case of high aspect ratio ( ~26:1 ) and short rod - to - rod distance , a self - attraction of nr tips occurs ( fig .
3c ) . perspective view ( a ) and top view ( b ) fesem images on bottom contacted , anchored zno nanorods prepared by hard - mask method .
when the aspect ratio is high , during the drying process ( c ) the nanorods attach to each other at their tips similar phenomenon was described by other groups , as well , albeit they used high temperature vapor transport methods .
wang et al .   explained the self - attraction by the accumulated coulomb charges at the nr / au catalyst droplet interface when charged by the primary electrons during sem observation .
have also observed self - attracted nws prepared by catalyst - free vapor  solid ( vs ) preparation method .
therefore , the charging can not be ascribed to the presence of catalysts . in our case
, the nr tip attachment can be attributed to surface tension of water during the drying process , as it was described by segawa et al .   for hybrid organic  inorganic nr .
we believe that further down - scaling is limited mainly by the resolution of our e - beam lithography facility rather than by growth kinetics . in fig .
4a , the cross - sectional fesem image of the so - called anchored - type nrs is provided .
the arrows on top and bottom mark the characteristic diameter of the rods being ca .
the development of this taper is the effect of the finite growth rate on the nonpolar faces of the sidewalls .
figure 4b reflects the anomaly encountered during ion - milling of the base - metal film ( ru ) through the pmma holes formed by e - beam lithography . after the removal of the pmma mask
, a cylindrical object is left surrounding the ion - milled hole in the metal .
this cylinder is composed of sputtered residues originating from the ru - film , mixed with zno from the underlying substrate and polymers formed from pmma components . a schematic cross section of the structure after ion - milling , but before pmma removal
the hexagonal faceting forms outside of this cylinder . that gives rise to the neck observed on the bottom of the nrs in fig .
fesem image of the cross section of anchored - type zno nanorods with indication of the size - distribution ( a ) , the parasitic ring remaining after removal of the pmma mask ( b ) , and from the ar - ion milled structure shown in the sketch in cross section ( c ) the tem observations revealed that both all - zno and anchored , contacted nrs are wurtzite type single crystals of high quality ( fig .
5 ) , where the rotational axis is parallel to the   direction . the tem observation reveals the single crystalline nature of the nanorods .
the fast growing   crystallographic direction is parallel to the longitudinal axis the electrical properties of the individual nrs in fig .
this can be ascribed to the effect of the condensates on the mantle surface of the nrs .
6 ) , which can be originated either from the contact between probe - tip and nr - tip or from the collar - shaped zno / ru interface at the bottom of the nr .
however , as it was shown earlier   and found here as well , the ru / single crystal zno interface - contact has ohmic character .
therefore , the au / zno nr contact is responsible for the observed rectifying behavior .
voltage characteristic recorded on an individual nanorod by conductive afm . in order to obtain reproducible results in air
increased contact force is required , which can be ascribed to the condensate formation on the nanorods ( inset ) in order to correctly describe the electrical behavior by an equivalent circuit and to separate the contributions of contact resistance , internal resistance of the nr , surface conductance , and piezoelectricity induced schottky barrier height change , a refinement of the measurement technique and further systematic investigation is required . still , in our work
the successful formation of a rectifying schottky contact between zno nr and the measuring tip could reproducibly be obtained .
this was pointed out by liu et al .   to be a necessary requirement for the operation of the dc nanogenerator with vibrating top contact .
we have demonstrated that by using homoepitaxial chemical growth method highly uniform , single crystalline nr arrays arranged in a predefined pattern can be prepared . by changing the growth parameters , diameter and length of the nrs
can be tuned in the range of 90170 nm and 500 nm2.3 m , respectively .
the monodispersity of the diameter of single crystalline nrs can be < 2% by maintaining an excellent uniformity in the longitudinal dimension .
we exploited two alternative synthesis routes using soft and hard under - layer to obtain all - zno and metal contacted , anchored nr arrays , respectively .
the former one can be a promising candidate for nanopillar - based photonic crystals , especially if a refractive index contrast between the nr and the zno substrate is realized .
on the other hand , anchored nr arrays contacted on the bottom are promising structures for nanopiezotronics .
the arrays show excellent uniformity in length and the dense pattern ( ~30 nr/m ) can be adjusted to the top vibrating electrode of the nanogenerator .
thereby a significant improvement in the output voltage , hence a more efficient energy harvesting can be predicted .
this work was supported by the  nanotechnology network project  of the ministry of education , culture , sports , science and technology ( mext ) in japan , and by the hungarian fundamental research found ( otka ) under contract pd 77578 .
 highly uniform and c - axis - aligned zno nanorod arrays were fabricated in predefined patterns by a low temperature homoepitaxial aqueous chemical method . 
 the nucleation seed patterns were realized in polymer and in metal thin films , resulting in , all - zno and bottom - contacted structures , respectively . 
 both of them show excellent geometrical uniformity : the cross - sectional uniformity according to the scanning electron micrographs across the array is lower than 2% . 
 the diameter of the hexagonal prism - shaped nanorods can be set in the range of 90170 nm while their typical length achievable is 0.52.3 m . the effect of the surface polarity was also examined , however , no significant difference was found between the arrays grown on zn - terminated and on o - terminated face of the zno single crystal . 
 the transmission electron microscopy observation revealed the single crystalline nature of the nanorods . 
 the current  voltage characteristics taken on an individual nanorod contacted by a au - coated atomic force microscope tip reflected schottky - type behavior . 
 the geometrical uniformity , the designable pattern , and the electrical properties make the presented nanorod arrays ideal candidates to be used in zno - based dc nanogenerator and in next - generation integrated piezoelectric nano - electromechanical systems ( nems ) . 