transparent conductive oxides ( tcos ) are impurity doped metal oxides , such as indium oxide ( in2o3 ) , zinc oxide ( zno ) , and tin oxide ( sno2 ) , which exhibit excellent properties of controllable low resistivity and high transparency in the visible region . tcos , generally fabricated as polycrystalline or amorphous thin films , are employed as transparent electrodes in a wide variety of optoelectronic devices , e.g. , flat panel displays , light emitting diodes , and photovoltaics [ 2 - 4 ] . currently , the tco thin films are deposited by sputtering , thermal deposition , chemical vapor deposition , or other gas phase deposition approaches , which generally require high temperature processing . nevertheless , high temperature treatments are disadvantageous to be applied on flexible or heat - sensitive substrates . the development of tco nanocrystals , together with printing techniques , may address the problem and lead to low cost printable transparent electrodes . recently , research into the production of tco nanocrystals , particularly indium - doped tin oxide ( ito ) nanocrystals , has steadily increased over the last few years [ 7 - 10 ] . for example , ito nanoparticles were achieved by an approach comprising microwave - assisted synthesis in ionic liquids . park et al . prepared colloidal ito nanoparticles by thermal decomposition of tin and indium precursors in oleylamine . in another study , non - agglomerated ito nanoparticles were obtained via a combination of nucleophilic attack and condensation  hydrolysis cascade reactions . however , the synthesis of tco nanocrystals , which corresponds to the growth of doped oxide nanocrystals , still faces significant challenges . a successful synthetic strategy to generate tco nanocrystals with high crystallinity , narrow size distribution , and homogeneous composition is yet to be achieved . this is due to lack of detailed understanding on the formation of doped nanocrystals , although attempts have been reported in literature . for instance ,  self purification  , a thermodynamical concept , was invoked to explain the low doping concentration in the case of mn - doped cdse nanocrystals . in contrast , erwin et al . show that for the mn - doped ii vi nanocrystals doping is controlled by impurity adsorption on the nanocrystal surfaces and the doping efficiency is mainly determined by three factors : surface morphology , nanocrystal shapes , and surfactants in the growth solution . in a recent publication , chen et al . suggest that the nanocrystal doping processes are strongly temperature - dependent . further research is essential to uncover the critical factors that govern the growth of doped nanocrystals . in this article , we describe a simple and facile approach to the generation of in - doped zno nanocrystals . we choose to study the in - doped zno nanocrystal system because in - doped zno thin films have been well - studied , demonstrating superior optoelectronic properties as tco thin films . in addition , both zinc oxide and indium oxide nanocrystals are readily synthesized by a one - step ester elimination reaction based on alcoholysis of metal carboxylate salts . the reactions can be monitored by fourier transform infrared spectroscopy ( ftir ) and uv  visible absorption spectra ( uv  vis ) , making them ideal for investigating the processes . the resulting nanocrystals from the reactions have been characterized by transmission electron microscopy ( tem ) , high - resolution transmission electron microscopy ( hrtem ) , energy - dispersive spectroscopy ( eds ) , inductively coupled plasma atomic emission spectrometer ( icp - aes ) , x - ray diffraction ( xrd ) , and uv  vis . indium acetate ( in(ac)3 ) and stearic acid ( sa ) was purchased from aldrich . zinc stearate ( zn(st)2 ) and 1-octadecanol ( oda ) was purchased from alfa aesar . indium stearate , which was used as indium precursor in the ester elimination reactions , was prepared as follows . in(ac)3 ( 20 mmol ) was mixed with sa ( 80 mmol ) and heated to 140 c under an argon atmosphere . the mixture was heated for 5 h , and then the product was isolated by addition of acetone . the resulting precipitate was filtered , vacuum - dried , and again treated with sa ( 80 mmol ) to ensure complete conversion of acetate to stearate . the final product ( in(st)3 ) was washed several times with acetone to remove excess sa . four typical reactions , namely pure zno reaction ( starting with 1 mmol of zn(st)2 ) , 2% in  zno reaction ( starting with 0.02 mmol of in(st)3 and 0.98 mmol of zn(st)2 ) , 5% in zno reaction ( starting with 0.05 mmol of in(st)3 and 0.95 mmol of zn(st)2 ) , and in2o3 reaction ( starting with 1 mmol of in(st)3 ) , were studied in this work . in a typical procedure , the metal carboxylate salts and 20 g of 1-octadecene ( ode ) the flask was purged with argon several times , and degassed under vacuum at 120 c for 20 min with vigorous magnetic stirring . 1-octadecanol ( 5 mmol ) dissolved in 5 g of ode at 150 c was quickly injected into the reaction solution . 50 ml of ethyl acetate were used to precipitate nanocrystals , and the nanocrystals were collected by centrifugation . the nanocrystals were then dispersed in toluene , and any insoluble residue was removed by centrifugation . transmission electron microscopy ( tem ) and high - resolution tem images were taken on a jeol jem 1230 microscope operated at 80 kv and tecnai g2 f20 transmission electron microscope at 200 kv equipped with an x - ray energy - dispersive spectroscopy ( eds ) system . specimens were prepared by depositing a drop of chcl3 solution containing the purified nanocrystals or the aliquots taken during the course of the reaction onto holey carbon - coated cu grids . x - ray diffraction patterns were obtained on a bede d1 system operating at 20 kv and 30 ma , using a cu k line (  = 1.5418  ) . the samples were prepared by directly spotting hot aliquots of a reaction mixture onto a kbr crystal . iris intrepid ii xsp inductively coupled plasma atomic emission spectrometer ( icp - aes ) were used for elemental analysis of doped nanocrystals . the optical properties of the colloidal in - doped zno nanocrystal solution were analyzed using unico uv 2102 ultraviolet the introduction of the indium - dopant precursor to the reagents dramatically affects the size of the resulting nanocrystals ( fig . 1 ) . pure zno nanocrystals have an average diameter of 16.1  1.8 nm . the 2% in  zno reaction and the 5% in zno reaction yield nanocrystals with sizes of 10.2  0.8 nm and 9.9  0.5 nm , respectively , suggesting that a relatively narrow size distribution . the reduction in the nanocrystal size implies that the indium species play an important role in the growth of the host nanocrystals . the growth rate of nanocrystals is significantly restrained by the adsorption of indium ions during the course of the reaction . such a size reduction of doped nanocrystals has also been observed in other doped nanocrystal systems . for instance , in the mn - doped indium oxide system , the dopant ions inhibit the growth of the nanocrystals and stabilize metastable hexagonal type of in2o3 nanocrystals with smaller dimensions . for the eu - doped zno nanocrystals , a sequential decrease of the crystallite size with the increasing eu content was observed . tem images ( left ) and size distribution histograms ( right ) of the nanocrystals from ( a ) the pure zno reaction , ( b ) the 2% in  zno reaction , ( c ) the 5% in the curves in histograms are the corresponding gaussian fits high - resolution transmission electron microscopy ( hrtem ) observations reveal the high crystalline nature of the nanocrystals produced by our method . as shown in fig . the crystal structure of the nanocrystals matches that of wurtzite type of zno , indicating that the indium atoms may incorporate into the host lattices and form well - crystallized doped nanocrystals . by carefully checking 162 hrtem images with clear lattice fringes of nanocrystals from one batch of the 5% in zno reaction , two nanocrystals have cubic crystal structure , identical to that of corundum type of in2o3 . zno reaction show that one nanocrystal has cubic crystal structure and all the other nanocrystals possess hexagonal crystal structure corresponding to that of wurtzite type of zno . the formation of nanocrystals with the crystal structure identical to that of corundum type of in2o3 , either pure indium oxide nanocrystals or zinc - doped indium oxide nanocrystals , shall be discussed later in the text . this agrees well with the icp - aes results that in:(zn + in ) molar ratio is 2.2 and 4.6% for the products from the 2% in a  d typical hrtem images of the nanocrystals from the 5% in - doped nanocrystals . e a typical eds spectrum of the products of the 5% in - doped nanocrystals the xrd profiles of the nanocrystals from the pure zno reaction , the 2% in  zno reaction and the 5% in  zno reaction ( fig . 3a ) match well with that of bulk wurtzite type of zno ( hexagonal structure ; jcpds : 36 - 1451 ) , consistent with the hrtem results . no additional peaks can be distinguished in the xrd pattern of the in - doped zno nanocrystals , excluding the existence of large quantity of nanocrystals with cubic structures . it is noteworthy that the reflections of corundum type of indium oxide are absent even when the reaction starts with 0.2 mmol of in(st)3 and 0.8 mmol of zn(st)2 ( data not shown ) . for comparison , in the case of eu - doped zno nanocrystals , the diffraction peaks from cubic eu2o3 are evident for the as - synthesized zno:10%eu nanocrystals . by carefully analyzing the xrd data , the full width at half maximum ( fwhm ) of ( 100 ) peak for the products from the pure zno reaction , the 2% in zno reaction are 0.9,1.4 , and 1.5 , respectively . according to the scherrer equation , the larger the fwhm is zno reaction shall be smaller than that of the nanocrystals from the pure zno reaction , which agrees with the tem observations . a xrd patterns of the products from the pure zno reaction , the 2% in the black lines are the patterns of bulk h - zno and c - in2o3 with major reflections assigned . b uv  vis spectra of the nanocrystals from the pure zno reactions , the 2% in - doped zno reactions , the 5% in - doped zno reactions , and the pure in2o3 reactions the uv  vis absorption spectra provide more information related to the doping of the nanocrystals . fitting the data to the sigmoidal formula gives an effective band gap of 3.35 , 3.43 , and 3.46 ev for the nanocrystals from the pure zno reaction , the 2% in thus , the obtained band gap agrees well with the reported bulk value ( 3.37 ev ) at room temperature . while the average diameter of the in - doped nanoparticles is ca . 10 nm , about 56 times of the exciton bohr radius of zno ( 1.8 nm ) , suggesting a likely presence of moderate quantum confinement effect ( qce ) . theoretical calculations predict a blueshift of ~45 mev for zno nanoparticles with diameters of ca . therefore , the observed blue - shift shall be due to both qce and the burstein  moss effect associated with heavy doping . accordingly , the contribution of indium doping to the blueshift of the optical band gap is deduced as ~35 and ~65 mev for the nanocrystals from the 2% in  zno reaction and the 5% in  zno reaction , respectively . such a value is smaller than that expected if we assume all the indium ions are on the substitutional sites . in other words , part of the dopant atoms may resident on the surface of nanocrystals , or congregate as nanocrystals with cubic crystal structures which are found in the products from the 5% in  zno reactions . we suggest that a higher reactivity of the indium precursor , in(st)3 , than that of the zinc precursor , zn(st)2 accounts for the formation of nanocrystals with cubic structures in the products from the 5% in  zno reactions . in the pure in2o3 reaction , the formation of in2o3 nanocrystals can be detected by uv  vis spectra only 30 s after the injection of alcohol ( fig . 4b ) . in contrast , the uv  vis signal of zno nanocrystals was detected when the reaction was lasted for 2 min ( fig . this indicates that zinc carboxylate salts are less reactive than the indium carboxylate salts , which can be confirmed by temporal evolution of ftir spectra for the pure zno reaction and the pure in2o3 reaction . the side product of the alcoholysis reactions to generate oxide nanocrystals is ester , which can be detected by ftir . therefore , the peak of the c = o stretch ( 1,740 cm ) from ester is an ideal indicator to monitor the reactions . as shown in fig . 5b , for the pure in2o3 reaction , the ester peak is evident 30 s after the injection of alcohol , confirming that nucleation of the in2o3 nanocrystals occurs in a timescale of < 30 s , much faster than that of the zno nanocrystals ( fig . 5c ) . note that in a study aiming to generate cuins2 nanocrystals , formation of cuxs nanophases was found to be inevitable when indium and copper carboxylates were used as the precursors . demonstrate that this problem can be solved by tuning the relative reactivity of cationic precursors . therefore , we suspect that by using indium precursors with lower reactivity , pure in - doped zno nanocrystals with wurtzite structures may be generated . a uv  vis spectra of the nanocrystals from aliquots taken from the reaction solutions at desirable time . for purification , 1 ml of hexane was added to the aliquots and the unreacted compounds and byproducts were removed by successive methanol extraction until the methanol phase was clear . b tem image of nanocrystals from aliquots taken from solution of the pure in2o3 reaction when reaction time is 30 s a temporal evolution of ftir spectra of the pure zno reaction . all spectra were normalized by the relatively intense c = c vibration band at 1,641 cm originating from ode ( the solvent ) as the standard . in - doped zno nanocrystals with high crystallinity and relatively narrow size distribution have been synthesized through a one - step reaction based on alcoholysis of metal carboxylate salts . tem analyses reveal a significant reduction in the sizes of the in - doped zno nanocrystals compared with that of the undoped zno nanocrystals . hrtem , eds , icp - aes , xrd investigations , together with uv  vis analyses indicate the incorporation of indium into the host zno lattices , leading to a blue - shift of the absorption peak . the study was financially supported by the zi jin program of zhejiang university , qian jiang foundation of zhejiang province under grant no . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
a method for the synthesis of high quality indium - doped zinc oxide ( in - doped zno ) nanocrystals was developed using a one - step ester elimination reaction based on alcoholysis of metal carboxylate salts . 
 the resulting nearly monodisperse nanocrystals are well - crystallized with typically crystal structure identical to that of wurtzite type of zno . 
 structural , optical , and elemental analyses on the products indicate the incorporation of indium into the host zno lattices . 
 the individual nanocrystals with cubic structures were observed in the 5% in 
 zno reaction , due to the relatively high reactivity of indium precursors . 
 our study would provide further insights for the growth of doped oxide nanocrystals , and deepen the understanding of doping process in colloidal nanocrystal syntheses .