the synthesis of nanocrystals 
 has taken a central place in inorganic 
 chemistry for decades , but it is only in more recent years that the 
 synthetic control has reached such precision that advanced nanotechnological 
 applications have truly benefited .
these applications range from quantum 
 dots in conducting and optically active composite materials to mechanically reinforced scratch - resistant 
 polymer nanocomposites and inorganic nanoparticles 
 as markers and therapeutic tools in medicine and 
 cosmetics .
low polydispersity and precise 
 control over nanoparticle size and shape are required for these applications .
a decisive breakthrough in the synthesis of iron oxide nanoparticles 
 was the work of hyeon and co - workers , who 
 pioneered the synthesis of monodisperse , spherical iron oxide nanoparticles 
 ( nps ) .
several different routes building on thermal decomposition 
 of metallo - organic precursors have been demonstrated to lead to the 
 synthesis of nanoparticles with low polydispersity .
the highest level of control 
 demonstrated for this method has been in combination with ligand - assisted , 
 for example , oleic acid - assisted , control over the growth of the np 
 core after nucleation . as first described by lamer and later 
 by talapin et al .
, the key to low polydispersity 
 is to achieve effective separation of burst nucleation from particle 
 growth
. a uniform growth rate of the particles then results in a monodisperse 
 nanoparticle population with the size set mainly by the molar precursor 
 to ligand ratio .
exquisite control over the nanoparticle size 
 distribution has been 
 verified in many publications , but time - resolved investigations of such burst nucleation and growth 
 processes are almost absent .
studies have been limited to the extraction 
 of samples for ex situ transmission electron microscopy ( tem ) analysis at low time resolution and absorption spectroscopy 
 that can shed light on the chemical composition of the sample , also 
 performed at low time resolution .
there 
 is , however , a solid theoretical foundation describing the growth 
 of monodisperse nanoparticles , which has been used to interpret the 
 experimental work on reaction kinetics that has been performed .
this 
 and related experimental work , relevant for our study , will be briefly 
 recapitulated here .
monodisperse nanoparticle or nanocrystal 
 growth requires size focusing , 
 although classical theory for nanoparticle growth predicts defocusing 
 to occur over time through ripening .
already , the early lsw ( lifshitz 
 and slyozov and wagner ) theory predicted that a unique shape of the size distribution 
 for the late stages of growth can be obtained independent of the initial 
 conditions .
talapin et al . later demonstrated 
 with a more complete analysis that even if the nucleation and growth 
 phases are not completely separated , for example , by the hot injection 
 method , a much higher nucleation than growth rate will still lead 
 to a similar outcome . for diffusion - limited 
 particle growth
, a narrow distribution was found in simulations to 
 be increasingly skewed toward smaller size as a function of time . 

reaction - limited growth results in much faster broadening and skewing 
 of the distribution with a tail toward smaller size .
these results 
 lay the basis for extended lsw models used to explain synthesis of 
 monodisperse nanoparticles such as quantum dots and superparamagnetic 
 iron oxide nanoparticles ( spion ) .
several additional conclusions were 
 drawn such as that for transport - limited growth , a lower transport 
 coefficient ( diffusion coefficient ) leads to a lower limiting standard 
 deviation of the particle size distribution .
a higher 
 surface tension yields a lower standard deviation , but originally 
 a standard deviation of 1520% regardless of nanoparticle size 
 or initial size distribution was simulated . while the original lamer - type model is not strictly applicable to 
 growth of ligand - stabilized nanoparticles of size close to the nucleation 
 size , extended kinetic models
a slower 
 growth rate yields crystal growth with fewer defects , 
 that is , nanoparticles growing close to the critical size have the 
 lowest growth rate and should therefore have higher crystallinity . whether this should lead to more faceted or 
 smoother crystals
is debated , but the argument for the growth of stable 
 facets is strong . a high supersaturation ratio 
 leads to low critical size and thereby to more nuclei being above 
 the critical size with positive growth rates . in hot - injection synthesis , the achievable very high supersaturation 
 ratio yields a simple explanation for the separation of burst nucleation 
 and growth phases . the argument for achieving extreme supersaturation 
 is less clear for heat - up methods , but 
 the very strong temperature dependence of the nucleation rate can be used to make a similar argument also 
 for finite heating rates by the heat - up method .
however , heat - up methods 
 have also been suggested to proceed through an initial decomposition 
 of precursors to polyiron oxo clusters that serve as monomers for 
 the nucleation and growth of iron oxide nanoparticles .
confirmed that decomposition of 
 precursors in a heat - up process using iron oleate complex precursors 
 did not result in immediate nucleation and growth of nanoparticles 
 and therefore confirmed the existence of an intermediate monomer state 
 argued to be polyiron oxo clusters .
a 
 similar intermediate is expected to play an important role in all 
 metal - oleate heat - up methods .
size - exclusion chromatography , seconded 
 by tem , and crystallization yield was used to monitor the size growth 
 of nanoparticles after nucleation at minute time resolution .
the nucleation , 
 regarded as the conversion of monomers to nanocrystals , was followed 
 only via the crystallization yield through elemental analysis , which 
 only had 10 min resolution and therefore could not produce the detailed 
 kinetics of this step .
nucleation was completed within 20 min after 
 the critical temperature of 320 c was reached and maintained . 

tem images , however , indicated the presence of a bimodal distribution 
 of nuclei and small nanoparticles within the first few minutes after 
 the critical temperature was reached , where most nuclei kept a constant 
 diameter of 1.7 nm . after this time , the tem analysis indicated 
 that the nucleation was complete and growth ensued , which for the 
 first few minutes served to smoothen ( round ) the shape and grow the 
 nps uniformly .
it should be noted that the tem images and histograms 
 did not trace the shift of the predominant small nuclei to larger 
 size , but rather seemed to indicate a sudden growth within the first 
 12 min in the population of the larger , 56 nm in diameter , 
 nanoparticles , which then continuously grew to close to 10 nm in diameter . 

their shape reverted back to faceted , seemingly cubic , in correspondence 
 to the lowest surface energy facets of magnetite and maghemite .
the 
 final step also corresponds to a broadening of the size distribution . given the gaps in the understanding of the steps leading up to 
 nucleation and the lack of sufficient time resolution data for the 
 nucleation and growth , the aim of our work is to extract the reaction 
 kinetics and precursor states during the synthesis of size - focused , 
 extremely monodisperse iron oxide nanoparticles with unprecedented 
 time resolution and statistical significance of the measurements . 
 by focusing on in situ detection of the formation of sample inhomogeneity , 
 interpreted as micelles , clusters , nuclei , and particles ,
we link 
 synthesis parameters to size distributions via clearly time - resolved 
 kinetic intermediate states .
we investigate the synthesis of 
 superparamagnetic iron oxide nanoparticles 
 by thermal decomposition of iron(0)pentacarbonyl ( fe(co)5 ) in dioctyl ether in the presence of oleic acid at a defined ratio , 
 following the method introduced by hyeon and co - workers . by continuous online extraction of small fractions 
 of sample during a running reaction , we follow the nucleation and 
 growth using small - angle x - ray scattering ( saxs ) , wide - angle x - ray 
 scattering ( waxs ) , and tem practically in situ . while tem can be used 
 to determine the morphology of synthesized nanoparticles , its statistical 
 relevance is poor .
furthermore , small clusters can not be reliably 
 imaged and metallo - organic precursor states such as micelles that 
 are only stable in solution can not be imaged at all under vacuum conditions . 

saxs and waxs can in this respect provide an invaluable tool to estimate 
 the average size , crystallinity , amount of scattering material , internal 
 surface area , and polydispersity of micelles , clusters , and particles continuously throughout the synthesis .
these methods have previously 
 been used to elucidate the reaction kinetics for formation of , for 
 example , pt nanoparticles from intermediate precursor cluster states . using in situ saxs / waxs we could follow the 
 entire formation of iron oleate complexes , further decomposition to 
 polyiron oxo clusters serving as monomers for nanoparticle nucleation , 
 as well as the nucleation and growth of iron oxide nanoparticles at 
 a time resolution of seconds .
the different phases could be clearly 
 distinguished , which confirmed the modeling of nanoparticle synthesis 
 by the heat - up method using the separation of burst nucleation and 
 growth phases demonstrated for hot injection synthesis routes , but with added detail on crucial and largely 
 unmapped intermediate phases . by this approach , we further investigated 
 the influence of the heating rate and in particular the fe(co)5/oleic acid ratio on particle growth kinetics and morphology .
iron(0)pentacarbonyl 
 ( fe(co)5 , 
 99,99% trace metal basis ) , oleic acid ( oa ,  93% technical 
 grade ) , and dioctyl ether ( oct2o , > 99% ) were purchased 
 from sigma - aldrich ; ethanol ( etoh > 96% technical grade ) and toluene 
 ( 95% rotipuran ) were obtained from carl roth .
superparamagnetic oleic acid stabilized 
 iron oxide nanoparticles were synthesized by thermal decomposition 
 of fe(co)5 in the presence of oleic acid as capping agent 
 according to a slightly modified heat - up procedure previously described 
 by hyeon et al . to investigate the kinetics 
 of this synthesis route ,
the amounts of solvent ( 50 ml ) and precursor 
 ( 1 ml ) were kept constant and either the amount of oleic acid or the 
 heating rate was varied ( see table 1 for experimental details ) . typically , a mixture of 
 50 ml of oct2o and the respective amount of oleic acid 
 ( 7.06 ml of oa for  large  , 4.48 ml of oa for  medium  , 
 and 3.87 ml of oa for  small  cores )
was heated with 
 a ramp of 10 k / min to 100 c under a constant flow of n2 and held at 100 c for 10 min to remove h2o .
one 
 milliliter of fe(co)5 was injected rapidly , and the reaction 
 mixture was heated to reflux ( 290 c ) with a temperature ramp 
 of 3 k / min and held at this temperature for 1 h. samples were continuously 
 removed by a pump and autosampler and collected as 80 fractions per 
 synthesis .
the nps were precipitated twice with etoh and dispersed 
 in 200 l of toluene .
experimental 
 details for the 
 np synthesis experiments where the size was varied by fe(co)5/oa ratio and uniformity by heating rate .
the x - ray sample 
 cell consisted 
 of a kapton tube with an inner diameter ( i d ) of 1.64 mm and a wall 
 thickness of 0.03 mm ( detakta gmbh , norderstedt ) and connected to 
 the reaction flask and autosampler by teflon tubing ( bola ) with an 
 i d of 1 mm . schematic of the experimental setup .
the water - cooled reflux aperture 
 was heated by an electronically coupled heating mantle .
the sample 
 was continuously pumped from the reaction mixture by a peristaltic 
 pump through the x - ray transparent kapton sample chamber and sampled 
 by an autosampler .
a peristaltic pump , high 
 precision multichannel dispenser ( ism935 
 ipc idex ismatec ) , was used to pump the solution out of the reaction 
 vessel .
the tube for the peristaltic pump was made of tygon r367 ( idex 
 ismatec ) with an i d of 0.64 mm and wall thickness of 0.91 mm .
the 
 flow velocity of the pump was calibrated prior to each experimental 
 run and was kept constant at 100 l / min for heating rates of 
 3 and 9 k / min and at 54.9 l / min for the 1 k / min heating rate . 

a camera was installed to count droplets in 10 min intervals to monitor 
 the flux .
the outlet fractions were collected using a biorad 2110 
 hplc fraction collector autosampler with an integration time of 5 
 min for all experiments except for  large  cores ( 3 
 k / min ) where an integration time of 1.5 min was set .
the outlet to 
 the autosampler was made of a stainless steel tube with an i d of 1 
 mm . to match the temperature in the reaction vessel with the 
 saxs / waxs 
 signal and the fractions of the autosampler
, the time lag between 
 the extraction of a sample and the measurement at the x - ray window 
 was considered .
combined 
 in situ saxs / waxs 
 measurements were carried out at the saxs beamline ( 5.2l ) of the synchrotron 
 light source elettra , trieste , italy , operated at 2 gev and 300 ma 
 ring current . the experiment was carried 
 out at a photon energy of 8 kev , with simultaneous acquisition of 
 the saxs signal with a pilatus3 1 m detector and waxs signal with 
 a pilatus 100 k detector .
the saxs sample to detector distance of 
 1000 mm gave access to a q - range from 0.136.5 
 nm , and the waxs detector setting gave access 
 to scattering vectors from 16.526 nm . 

the two setups were calibrated with the standards silver behenate 
 and p - bromo benzoic acid , respectively . to 
 check for radiation damage ,
a test run with an intermittent pumping 
 scheme was carried out which aimed at replacing the complete volume 
 in the tube with fresh material from the reaction vessel and taking 
 consecutive frames with 0.5 s exposure time and 10 s total frame time 
 for 2 min in 29 pumping cycles . within this time span
, no degradation 
 of the particles could be detected , and the effect of radiation damage 
 on the sample could therefore be neglected for the continuously analyzed 
 samples . the evaluated size and polydispersity can be seen in figures s4 and s5 .
data were acquired with 
 a 0.5 s time increment and integrated azimuthally 
 using the fit2d data package .
the waxs data have been averaged over 
 10 frames , giving a time resolution of 5 s , and the saxs data were 
 processed with 0.5 s time resolution .
the reaction mixture at 100 
 c at the beginning of the experiment was used to record the 
 background that was subtracted from the saxs and waxs data .
the waxs data of the magnetite ( 311 ) 
 reflection were fitted with a gaussian function , and its full - width 
 at half - maximum ( fwhm ) was used to calculate the crystallite size dxrd according to the scherrer relationship:1where k is the scherrer prefactor 
 ( 0.9082 in case of the ( 311 ) reflection in a cubic crystal system ) ,  the wavelength of the incident x - rays , 
 (2 ) is the fwhm of the diffraction peak , and  
 is half the scattering angle .
the intensity of the saxs signal i(q ) can generally be described as2where f(q ) is the form factor of the scattering signal , s(q ) the structure factor of the particle 
the contribution 
 of the structure factor only influences the detected signal if the 
 concentration of the particles is high enough to create short - range 
 order .
this contribution was only detected when particles accumulated 
 on the capillary wall at the end of the synthesis and was hence disregarded .
the form factor was described by the scattering of a sphere with 
 radius r as3to account for 
 the polydispersity of the system , 
 a schultz distribution of the radii of the cores was introduced .
could only be fitted to the data after nucleation , after which it 
 was used to determine growth and polydispersity of the particle size .
the invariant and the porod constant p can also 
 be used to describe the scattering signal .
the invariant describes the integral scattering intensity of the 
 system and shows the total amount of scattering material , regardless 
 of the structure of the scatterer .
it depends on the volume fraction 
 of the different phases and the scattering length density contrast , and it can be written as4at large q , the 
 scattering 
 curve can be described as5 in case of particles or structures with a 
 smooth surface .
the invariant and the porod constant 
 were calculated throughout the core synthesis , as they do not require 
 fitting of a model to features in the scattering curve .
tem studies 
 were performed on a fei tecnai g2 20 transmission electron microscope 
 operating at 120 kv or 200 kv for high resolution imaging .
samples 
 were prepared by dropping dispersions of nps in toluene onto a 300-mesh 
 carbon - coated copper grid and subsequently evaporating the solvent 
 in air .
size distributions were evaluated using pebbles software package with a local intensity fitting 
 algorithm .
we followed the evolution 
 of the size and polydispersity of the 
 nps with subsecond time resolution during synthesis using saxs / waxs .
figure 2 shows the temporal 
 evolution of the saxs signal during the synthesis of the  large  

the shoulder and the minima shift toward 
 smaller q with time , which is expected for nps that 
 increase in size .
the raw saxs signals also show increasing definition 
 of multiple minima , which strongly indicates the increasing monodispersity 
 of the system with time . toward the end of the measurement ,
sharp 
 peaks appear at q = 0.51 nm and q = 0.60 nm . these peaks correspond 
 to a face - centered - cubic superlattice of particles that forms due 
 to the high monodispersity and concentration of nps on the capillary 
 wall in the measurement zone .
temporal evolution 
 of the saxs signals recorded continuously on the sample mixture for 
 the synthesis of large nps for a q - range from 0.36.5 
 nm . during the heat up phase , the temperature 
 is continuously increased and kept constant from t = 0 .
the increasingly defined minima and final appearance of structure 
 peaks corresponding to hexagonally ordered particles show the growth 
 of the increasingly monodisperse nanoparticles .
figure 3 shows 
 tem 
 micrographs taken at selected time intervals from the synthesis of 
 the large nps . at early times after nucleation , the particles appear 
 smaller , less uniform , and more polydisperse than toward the end of 
 the synthesis .
the narrowing of the size distribution becomes evident 
 from the assembly of regular superlattices of nps in hexagonal order 
 as the growth progresses .
the electron 
 diffraction pattern at t = 0 min is blurry , and no 
 sign of crystallinity is observed . at t
= 12.4 min , 
 a diffraction pattern typical for maghemite or magnetite starts to 
 form and becomes more pronounced as the nps grow .
the ratio of d - spacings of the obtained pattern is in good agreement 
 with the jcpds database numbers for magnetite or maghemite .
tem micrographs and electron diffraction patterns of selected 
 fractions 
 for the synthesis of  large  cores . the nanoparticles 
 grow homogeneously , and superlattices are formed when the polydispersity 
 of the nanoparticles approaches 5% .
the 
 diffraction patterns show the increase in crystallinity with the reaction 
 time and thus particle size .
the saxs data were fitted with a form factor describing polydisperse 
 spheres to extract the time evolution of the average radius and width 
 of the particle size distribution during synthesis .
nucleation is expected to start when the reflux temperature of 290 
 c is reached ( t = 0 ) in figure 2 .
figure 4 shows that  large  nanoparticles grew 
 quickly in size 0.5 min after the final temperature was reached ( red 
 crosses ) and proceeded until saturation , in the case of  large  

as the particles start to grow , it is evident 
 that they are highly polydisperse .
after peaking , as soon as the nps have reached a size that can be 
 determined by saxs , the polydispersity quickly decreases to a very 
 low value of 5% indicative of size focusing .
time - evolution of radius 
 from saxs and waxs data of  large  
 core synthesis at 3 k / min . extracted data for the nanoparticle radius 
 ( red crosses ) , polydispersity of the saxs radius ( blue triangles ) 
 and the crystallite radius determined as half the scherrer width ( black 
 squares ) as a function of time .
the growth of the iron oxide crystal size in the sample could 
 be 
 determined using the diffraction signal recorded by waxs .
half the 
 scherrer width calculated from the waxs signal is plotted in figure 4 and compared to 
 the particle radius fitted to the saxs data .
the 
 formation of crystalline particles is seen 13.3 min after the final 
 temperature is reached ( t = 13.3 , figure 4 , black squares ) , which is 
 in good agreement with the electron diffraction data ( insets figure 3 ) .
the waxs data 
 reveal a growth of the crystal size that is initiated after the start 
 of growth of the iron oxide nanoparticles but that is faster than 
 the particle growth . at the end of the particle growth ,
the crystal 
 radius estimated from the scherrer width is similar to the particle 
 size .
the observed small difference is likely related to the estimate 
 of the scherrer prefactor or to the different assumptions inherently 
 made for polydispersity in the two respective models ; the scherrer 
 width is only sensitive to the coherently scattering domains in the 
 sample and does not take polydispersity into account , while the fit 
 of the spherical form factor to the saxs data explicitly allows for 
 the polydispersity to be fitted .
figure 5 shows the 
 invariant and the porod constant for the data analyzed in figure 4 , corresponding to 
 the synthesis of  large  cores at 3 k / min heating rate . 

this analysis provides valuable insights into the kinetics of precursor 
 decomposition to form iron oleate complexes during the heat - up phase 
 of the synthesis , followed by the formation of polyiron oxo clusters 
 that act as precursors for the nanoparticle growth .
time - evolution of porod 
 constant and invariant for  large  
 np synthesis .
the porod constant ( black crosses ) and invariant ( red 
 triangles ) of the saxs data recorded for 
the concerted increase during the heat - up 
 phase corresponds to the formation of the iron oleate complex followed 
 by burst nucleation of polyiron oxo clusters shortly after the final 
 temperature of 290 c is reached .
these clusters are consumed 
 as monomers for the core growth , which yields a decrease in porod 
 constant ( internal area ) , while the invariant increases during nanoparticle 
 growth due to an increase in scattering contrast .
the porod constant ( black crosses ) correlates with the inner 
 surface 
 of the system .
it starts to increase at about 140 c , which we 
 interpret as the onset of formation of the iron oleate complex since 
 the temperature is too low for oxide particle growth and the saxs 
 data does not reveal the formation of particles at this stage . although 
 the formation of particles is ruled out by the saxs data ( figure 4 ) , it is possible 
 that a different precursor is already formed during this early part 
 of the heat - up phase .
have for example suggested that 
 polyiron oxo clusters are formed from the oleate complex as the actual 
 precursor for nanoparticle nucleation and growth .
the concentration 
 of this precursor reaches a stable plateau at about 240 c in figure 5 .
the peak 
 starting at t = 0.8 min with a maximum 
 at t = 6.5 min , shortly after the reflux temperature 
 of 290 c has been reached , indicates the burst nucleation .
this 
 is in good agreement with the onset of np growth seen in figure 4 .
the decay of inner 
 surface thereafter shows the growth of the particles by consuming 
 the precursor . toward the end of the particle growth , the internal 
 area as estimated by the porod
constant remains stable as the nanoparticles 
 have reached their final size and the precursor has been depleted .
the invariant ( figure 5 , red triangles ) is proportional to the total amount of scattering 
 material irrespective of its size , but it is sensitive to the scattering 
 contrast and concentration .
the invariant mimics the growth , plateau , 
 and burst nucleation indicated by the porod constant .
after nucleation , 
 however , the invariant continuously increases , which indicates a further 
 increase in the total amount of scattering material or in the contrast 
 between the scattering centers and the complex organic solvent phase . the invariant would not increase if the particles only grow by 
 consuming precursors formed during the heat - up phase since the total 
 amount of scattering material would be constant .
thus , the increase 
 in the invariant should be attributed either to the creation of more 
 scattering material from the oa - containing medium or to an increase 
 in the scattering contrast between the scattering centers and the 
 solvent .
the latter is possible in our system since the interpretation 
 of the invariant as being directly proportional to the amount of scattering 
 material in the solution is strictly true only for a two - phase system ; 
 for a complex system as the one under study with at least three phases , 
 the interface between the high electron density phase ( iron complexes / clusters ) 
 and the low density organic solvent can change in density , extension , 
 and composition . that the iron oleate complexes and precursor states 
 are detectable through both the invariant and the porod constant implies 
 that the complex fluid has structure .
otherwise , the fluid will not 
 contain a phase boundary with sufficient scattering contrast .
most 
 likely , a micellar structure of complexes or precursors with an oleate / oleic 
 acid shell is present in the oct2o with additional dissolved 
 oa .
it is this oleate / oleic acid shell that can potentially change 
 in density , composition , and ordering such that it contributes more 
 or less to the scattering signal .
the fe(co)5/oa ratio is known to control 
 the size of the final nanoparticles . 

for the same concentration of fe(co)5 , a decrease in the 
 oa concentration is expected to yield smaller nps.figure 6 shows the evolution of size and polydispersity fitted from the saxs 
 data for synthesis of  small  ,  medium  , 
 and  large  iron oxide nanoparticles by varying the 
 fe(co)5/oa concentration according to table 1 .
the heating rate was in all 
 cases kept at 3 k / min , stopping at 290 c , and holding the final 
 temperature for 66 min ( small and medium nps ) or 83 min ( large nps ) . 

a sharp increase in radius can be observed for all np sizes , whereas 
 the temperature for onset of this rapid growth decreases as the oa 
 concentration decreases . for large particles ( black circles ) ,
the 
 growth starts at about 0.5 min after the final temperature is reached 
 ( cf .
table s1 in the supporting information for additional numerical parameters extracted from all reactions ) . 

this onset is lowered to 260 c for midsized ( red circles ) 
 and to 240250 c for small nps ( blue circles ) . 

the final size of nps is reached faster for small nps ( 6.5 min after 
 reaching the final temperature ) than for  medium 
nps 
 ( 9.6 min after reaching 290 c ) and  large  nps 
 ( 60 + minutes after reaching 290 c ) .
in contrast to the other 
 sizes , the polydispersity of  small  nanoparticles increases 
 continuously during the growth phase and is also significantly higher 
 than for the  medium  and  large  particles . 

it should however be noted that for such small particles very small 
 differences in the number of atoms per particle lead to high polydispersity . 

only one side maximum can be seen in the q - range 
 of the experimental setup for  small  particles , making 
 the fit less robust and precise .
the data gaps at 250270 c 
 and at 15 min are due to an interruption of the experiment 
 during which the beam had to be turned off .
time - evolution of radii 
 and polydispersities for synthesis of nps 
 of different size .
the size growth kinetics ( circles ) and polydispersity 
 ( pd , crosses ) as a function of the fe(co)5/oa ratio for 
 samples heated at 3 k / min .
 small  ( blue ) ,  medium  
 ( red ) , and  large  ( black ) particles correspond to synthesis 
 with fe(co)5/oa ratios in table 1 . the kinetics and temperature dependence of the porod constant 
 and 
 invariant as a function of the fe(co)5/oa ratio for samples 
 heated at 3 k / min are shown in figure 7 .
the invariant and porod constant both increase during 
 the beginning of the heat - up phase irrespective of changes in the 
 fe(co)5/oa ratio .
the rate of formation of the iron oleate 
 complex therefore seems to be independent of the precursor ratio and 
 the oleic acid concentration .
thus , when the temperature has reached 
 210220 c , the fe(co)5 seems to have been 
 fully decomposed to form the iron oleate complex .
they spectroscopically measured the signal of the released 
 co - ligand from fe(co)5 and found that the decomposition 
 of fe(co)5 was completed at 210 c . a second 
 phase that follows directly after the smooth increase in the invariant 
 and porod constant observed up to 210
c was observed but was 
 difficult to distinguish from the fe(co)5 decomposition 
 phase . this continued increase in the invariant ( signifying an increase 
 in scattering material and contrast in the sample ) above 210
for higher 
 fe(co)5/oa ratio ( decreasing oa concentration ) , both the 
 invariant and the porod constant show a steep increase to higher saturation 
 values ; conversely , saturation is observed at low values for the synthesis 
 of  large  particles with low fe(co)5/oa 
 ratio .
time - evolution of porod constant and invariant for the synthesis 
 of nps of different size .
the kinetics and temperature dependence 
 of the porod constant ( circles ) and invariant ( crosses ) for different 
 fe(co)5/oa ratios in samples heated at 3 k / min .
 small  
 ( blue ) ,  medium  ( red ) , and  large  ( black ) 
 particles correspond to synthesis with fe(co)5/oa ratios 
 in table 1 .
thus , this difference in porod and invariant is not due to the different 
 concentrations of iron oleate complex since all fe(co)5 is consumed in the reaction ; it must be traced back to a difference 
 in structure and contrast variation within the sample .
a first interpretation 
 is that there are more clusters / particles ( larger internal area ) in 
 this phase for a higher fe(co)5/oa ratio , leading to  small  
 particles at the end of the synthesis .
the reaction leading to this 
 difference is extended over a longer time and therefore slower for 
  small  particles compared to for  large  
 particles .
the initial phases encompassing reactions of the 
 precursors are 
 followed by a plateau in both the invariant and the porod constant . 
 the length of the plateau decreases with increased fe(co)5/oa ratio .
thus , it seems that the supersaturation condition ( i.e. , 
 the end of the plateau ) occurs at lower temperature for higher fe(co)5/oa ratio , although the iron oleate concentration is the same . 

this correlates with both a higher concentration of precursors ( height 
 of the plateau ) and a lower excess of oa . at the end of the 
 plateau ,
burst nucleation is observed , at which 
 point the invariant decreases for all samples .
data are missing during 
 the nucleation and initial growth of  small  particles 
 due to an air - bubble forming in the system , which makes it less evident 
 in this case .
it is difficult to compare if there is a difference 
 in the increase in invariant during the growth phase .
the invariant 
 and the core growth start at different values , and it is difficult 
 to extract data during the nucleation and initial growth phase for 
 both the  small  and the  medium  data 
 sets .
the final value of the invariant is , however , higher for higher 
 fe(co)5/oa ratios ( smaller nps ) .
the invariant thus scales 
 inversely with particle size at a constant concentration of fe(co)5 precursor and presumably constant inorganic particle mass 
 fraction .
as expected , the internal area , as described by the 
 porod constant , 
 dramatically decreases for all ratios shortly after nucleation .
the decrease in area and increase in scattering correlate well 
 during the growth phase .
the growth therefore clearly consumes precursor 
 left after the nucleation step to bring down the concentration from 
 supersaturation , and in this process the scattering contrast in the 
 sample increases . in summary ,
rather unexpectedly , the scattering 
 signal of the precursor 
 formed from the iron oleate complex is higher at higher fe(co)5/oa ratio ( smaller nps ) ; this correlates with nucleation occurring 
 at a lower temperature .
tentatively , a similar increase in scattering 
 contrast occurs during growth for all particle sizes , but a larger 
 reduction in internal surface area is indicated for  small  
 particles than for  large  .
figure 8a shows 
 the kinetics of the porod constant 
 and the invariant for the np formation process as a function of the 
 heating rate . at higher heating rates ,
there is a slight lag in the 
 onset of the heat - up phase with respect to temperature . the plateau 
 in the porod constant and invariant after formation of the complexes 
 starts and ends with nucleation at a lower temperature for a lower 
 heating rate and reaches higher values for both 1 and 9 k / min compared 
 to for the 3 k / min heating rate .
synthesis of  large  particles 
 investigated as a 
 function of heating rate .
( b ) growth kinetics for radius 
 and polydispersity of  large  nanoparticles after nucleation . 

( c ) transmission electron micrographs of the particles after synthesis 
 and the respective size distributions from image analysis for 1 k / min , 
 3 k / min , and 9 k / min heating rates . the burst nucleation and particle growth ( indicated by arrows 
 in figure 8b )
start 
 before the 
 final temperature ( t = 276 c ) is reached at 
 a heating rate of 1 k / min but occur with a time lag at higher heating 
 rates , that is , 0.5 min after the final temperature has been reached 
 at a heating rate of 3 k / min and 6.7 min after the final temperature 
 has been reached for 9 k / min .
although the kinetics do not change 
 much with heating rate , its 
 influence on the particle morphology is significant .
figure 8c shows representative transmission 
 electron micrographs of the nps synthesized at different heating rates 
 and their respective size histograms .
it is obvious that the nps synthesized 
 at 1 and 9 k / min show a broader size distribution and that there are 
 frequent occurrences of nps with irregular shape .
the formation of 
 superlattices during assembly of oa - coated nps on the tem grid is 
 a qualitative measure of low polydispersity and high sphericity .
this is only observed for samples synthesized 
 at 3 k / min heating rate .
nps synthesized with 3 and 9 k / min heating 
 rates have similar final radii of 4.4 nm measured by saxs , 
 and in agreement with a statistical analysis of the tem data ( 4.3 
 and 4.5 nm respectively , figure 8c ) .
this size is slightly larger than the 3.9 
 nm average radius observed for the 1 k / min heating rate .
the polydispersity 
 of the 3 k / min synthesis seems lower immediately after nucleation 
 and decreases much faster than for the other heating rates .
a characteristic 
 dip in polydispersity within the first 30 min was also observed at 
 this heating rate , which led to an observed higher final polydispersity 
 than at the minimum .
this dip was not observed for the other heating 
 rates , which demonstrated slower , monotonous decreases in polydispersity , 
 eventually reaching a similar final value for all heating rates .
however , 
 although the final saxs polydispersities are similar , it becomes clear 
 from the tem in figure 8c that particles synthesized at 3 k / min are significantly more spherical 
 and uniform , to the level of showing self - crystallization , while the 
 other heating rates produce particles with plenty of outlier particle 
 sizes and inhomogeneous shapes .
our 
 high time resolution saxs data and corresponding tem images 
 of the nanoparticle synthesis confirm the previous model for the core 
 synthesis .
however , they also provide crucial 
 additional detail to the process , most importantly concerning the 
 formation of precursors and the nucleation step . by studying the porod 
 constant and invariant changes over time ,
we clearly identify six 
 phases of the synthesis that are mapped and illustrated in figure 9 : ( i ) heat - up lag 
 phase ; ( ii ) decomposition of the precursors to form iron oleate ; ( iii ) 
 formation of clusters or prenuclei ( precursors ) ; ( iv ) a second heating 
 lag phase ; ( v ) burst nucleation ; and ( vi ) core growth with narrowing 
 of the size distribution .
the lengths of the different phases , in 
 particular the second lag phase ( iv ) and the growth phase ( vi ) , vary 
 with the synthesis conditions .
distinct phases identified in the synthesis of iron oxide 
 nps via 
 heat - up thermal decomposition .
six growth phases are identified , exemplified 
 in the data plot of the porod and invariant for the synthesis of  large  
 particles .
the fact that the iron oleate complexes 
 and precursor states are detectable through both the invariant and 
 the porod constant implies that the complex fluid has structure , since 
 otherwise the fluid will not contain any phase boundaries with sufficient 
 scattering contrast .
we emphasize that the saxs signal during the 
 heat - up phase is devoid of a distinct particle signal and waxs is 
 similarly indicating the absence of crystalline material in the sample . 

this suggests that the most likely origin of the increased scattering 
 lies in micellar structures of complexes or precursor with an oleate / oleic 
 acid shell .
these are formed in the dioctyl ether solvent with additional 
 dissolved oa in the continuous phase .
the most pronounced difference 
 between samples was observed when the fe(co)5/oa ratio 
 was varied with constant concentration of fe(co)5 .
the 
 difference between the samples is evident already during the heat - up 
 phase . during this phase
the different samples start to deviate in 
 the scattering signal at temperatures slightly above 210 c . 
 up to this temperature ,
the different samples show very similar rates 
 of increase in invariant and porod constant .
this temperature range 
 agrees well with the temperature range within which in situ infrared 
 spectroscopy has revealed decomposition of fe(co)5 through 
 the release of co and a strong reduction of the signal of the carboxyl 
 group of free oa , indicating that the formation of the iron oleate 
 complex is completed at that temperature .
the invariant and the internal area drastically increase above 
 210 c for  small  particles , while they increase 
 at a slower rate for  medium  and  large  
 cores .
this strongly indicates that above 210 c the 
 iron oleate complex undergoes a reaction to a different precursor 
 state for the nanoparticle formation .
have also suggested 
 that such a conversion occurs , hypothetically to polyiron oxo clusters . 
 in their in situ infrared spectroscopy study ,
the onset of the signal 
 of the metal binding carboxyl groups is at 210 c , and 
 they suggest that an intermediate species is formed between the decomposition 
 of iron oleate and the reaching of the final temperature .
the presence of this intermediate species was 
 additionally supported in follow - up work by the same group , in which 
 maldi - tof - ms and tem were used to resolve small particles in the sample 
 before nucleation occurred .
these particles 
 possessed neither the same magnetic properties nor crystallinity as 
 superparamagnetic iron oxide nanoparticles after nucleation .
our size - sensitive 
 and highly time - resolved study using saxs further strongly supports 
 the formation of dense clusters as intermediates since the scattering 
 signal continues to increase after the formation of iron oleate . furthermore
, 
 we observe that the evolution of the signal differs as a function 
 of the fe(co)5/oa ratio .
the kinetics of formation of scattering 
 centers suggests that the fe(co)5 is fully decomposed and 
 the pool of iron oleate fully formed already at 210 c . 

the scattering signal is consistent with that these complexes form 
 small but diffuse ( presumably micellar ) aggregates from which precursor 
 clusters can form .
furthermore , the formation of the precursor clusters 
 proceeds over a short time - window at temperatures slightly above 210 
 c .
the formation of clusters already at this low temperature 
 agrees with the results of bronstein et al . and park et al . who showed that the 
 dissociation of the first oleate group from the iron oleate complex 
 upon heating starts at about 200 c
the decomposition of the 
 iron oleate complexes in aggregates to form more strongly scattering 
 clusters is in all cases complete before 250 c , or 15 
 min after starting , for a heating rate of 3 k / min , indicated by the 
 onset of the pronounced plateau in the plot of the porod constant 
 and invariant .
this is in disagreement with the findings of bronstein 
 et al . and park et al .
they suggest the dissociation of the two remaining coordinated 
 oleate groups at a temperature of 300 c .
however , one should 
 keep in mind that their iron oleate complexes were synthesized ex 
 situ .
it has been shown that oleate complexes show very different 
 stabilities depending on their synthesis parameters and handling , presumably leading to different levels of purity
. it also seems a higher concentration of precursor clusters is formed 
 at higher fe(co)5/oa ratios , that is , at lower excess of 
 oa and constant concentration of fe(co)5 .
oleate coordinated 
 to the cluster surface will act as a shell protecting the clusters 
 from interaction , but they simultaneously can contribute to the total 
 scattering intensity since the organized molecules in the shell produce 
 a small but significant scattering contrast to the surrounding solution . 

the latter can explain that the  small  cores not only 
 have a higher internal area , but also have a highly increased overall 
 scattering signal ( invariant ) ; the number of clusters formed is higher , 
 which increases the total amount of dense , structured oa around clusters 
 in the sample .
our data thus show that the cluster composition varies 
 with the synthesis conditions ( fe(co)5/oa ratio ) and that 
 the oleate packing density on the cluster surface is not constant 
 as suggested from a fit to particle growth for a single set of conditions .
the fact that the stoichiometric ratio of precursors 
 can influence the ligation of primary particles consumed during nucleation 
 and thereby affect diffusion limited nanoparticle growth is not unprecedented ; 
 it has been demonstrated by angelopoulus and co - workers experimentally 
 and theoretically for diffusion limited growth of pt nanoparticles .
importantly , the clusters are amorphous ( no 
 crystallinity is detected ) and too small or polydisperse to be called 
 particles since a form factor can not be seen in the scattering signal 
 when only this precursor state is present .
the heating rate 
 affects the cluster formation step , with both 
 high ( 9 k / min ) and low ( 1 k / min ) heating rates leading to a higher 
 invariant plateau proportional to a higher internal area and somewhat 
 higher porod constant proportional to the scattering contrast in the 
 sample ( figure 8a ) . 
 for the slow 1 k
/ min heating rate , this again seems to correspond 
 to a larger number of clusters being formed .
the biggest difference 
 is observed for the high 9 k / min heating rate .
it is unclear if this 
 also is due to the formation of a larger number of smaller clusters 
 since the correlation of this increase in signal with final nanoparticle 
 size is different than for the other samples , as discussed below .
classical nucleation theory describes the crystal nucleation rate per unit volume as6where the 
 activation energy for homogeneous 
 nucleation is described by7and bn is a pre - exponential 
 factor depending on many factors regarding the precursor species , 
  is the surface tension , and vm is the molar volume of the solid .
 is the difference 
 in chemical potential between the solid particle and the free monomer 
 states , which for oversaturation can be approximated as8where the oversaturation s is the ratio between the current and the equilibrium concentration 
 of monomer
. a higher number of clusters ( precursors ) therefore 
 contributes to lower the barrier and strongly contributes to increase 
 the rate of nucleation , which can now occur at a lower temperature 
 as shown by our data .
a higher fe(co)5/oa ratio corresponds 
 to a higher concentration of clusters , a higher supersaturation ratio , 
 and therefore synthesis of  small  nps .
we also note 
 that the high fe(co)5/oa ratio with more and smaller clusters 
 suggests a thinner shell around clusters for  small  
 core synthesis and an increase in  , which also contributes 
 to a lowering of the temperature at which supersaturation leads to 
 burst nucleation . for  large  cores , the lower concentration 
 of precursor clusters to begin with and the presumably denser shell 
 leading to better stabilization of larger clusters translates into 
 a long lag phase during further heating ; no structural change occurs 
 in the sample due to the larger energetic barrier to reaction of the 
 dense shell .
it is clear from eq 8 that the nucleation rate is very sensitive to both temperature 
 and difference in chemical potential ; this enables the separation 
 of nucleation and growth as well as to control the nucleation condition .
it previously has been suggested that decomposition of fe(co)5 proceeds over a large temperature range during the heat - up 
 phase .
our results indicate that this 
 depends on the fe(co)5/oa ratio since only at lower fe(co)5/oa ratios structural change is a observed in the temperature 
 range between formation of the iron oleate complex and reflux .
furthermore , 
 the structural change observed in this temperature range for the synthesis 
 of  small  nanoparticles seems to correspond directly 
 to the burst nucleation and growth occurring at lower temperature . 

cores by this route there can be , and is , a significant separation 
 ( in temperature and time ) between formation of precursor clusters 
 and nucleation .
nucleation will occur as soon as the temperature 
 is high enough 
 to overcome the energy barrier of homogeneous nucleation from the 
 precursor .
burst nucleation is achieved 
 if a supersaturated solution of precursor is suddenly heated above 
 the nucleation temperature .
the peak in the porod constant ( and to 
 a smaller extent the invariant ) found in phase ( v ) , which lasts for 
 only a few minutes , is evidence that the burst nucleation occurs . 

the maximum inner surface area is obtained during the nucleation step . 
 the fact that the porod constant of the sample containing nuclei is 
 higher than that containing only precursor clusters is likely due 
 to the diffuse and unstructured interface expected for the clusters ; 
 the magnitude of the porod constant depends on the contrast at this 
 interface in addition to on the total inner surface area for a multiphase 
 system .
the early burst nucleation at temperatures below the 
 reflux temperature 
 for  small  cores can be problematic for synthesis of 
 monodisperse nanoparticles .
if precursors are still formed as nucleation 
 takes place , there might be continuous nucleation , leading to the 
 nucleation and growth phases that are not separated .
this could partly 
 explain why  small  cores not only have a much higher 
 relative polydispersity than  large 
particles , but 
 also a less regular spherical morphology , as evident from tem data 
 ( figure s1 ) .
a possible suggestion based 
 on this could be to use ligands with even higher density and surface 
 affinity for the synthesis of small cores .
the heating rate 
 seems to affect the nucleation step as well as 
 the formation of clusters ( cf .
the nucleation takes place at a delay after the 
 reflux temperature has been reached when the heating rate is high 
 ( 9 k / min ) .
while the direct cause for this lag is not clear , this 
 could , for example , relate to heating gradients in the sample as the 
 temperature is raised fast at the periphery of the reaction vessel , 
 or a lower accumulated probability that nucleation has initiated after 
 the shorter total time spent at sufficiently high temperature .
such 
 temperature lags or gradients imply that a more heterogeneous nucleation 
 is likely at lower heating rates .
this hypothesis is also supported 
 by the tem of final nanoparticle distributions ; most of the 9 k / min 
 sample has a size and size distribution similar to the 3 k / min ( optimal ) 
 heating rate , but there is also a significant fraction of larger and 
 nonspherical particles present . it is likely that this difference 
 can be traced back to nonuniform nucleation . despite that the cluster 
 formation step and nucleation step
however , we note that 
 there is a large difference also in the preceding cluster formation 
 step , where a much larger internal area is evident for the 9 k / min 
 compared to the 3 k / min sample , but uniquely in this case it is obviously 
 not correlated to a larger number of small cores after completed growth , 
 but to higher final polydispersity and higher number of irregular 
 cores .
the most interesting effect is observed for the lowest 
 1 k / min 
 heating rate . for this heating rate , nucleation starts at a temperature 
 significantly below the reflux temperature .
again , the effect seems 
 to be a more heterogeneous nucleation phase , where cores nucleate 
 and start growing at different temperatures before stable conditions 
 are reached .
presumably , nucleation is spread out in time and temperature 
 due to a finite nucleation probability also at lower temperature given 
 sufficient time .
nucleation is followed by what 
 is presumed to be diffusion - controlled growth ( phase ( vi ) ) .
a hallmark 
 of diffusion controlled growth is that it allows size focusing to 
 occur , that is , the polydispersity can narrow to below the typical 
 10% observed for separated burst nucleation and growth .
our data in figure 6 suggest that size focusing due to diffusion 
 controlled core growth is evident for both larger particle sizes . 

the synthesis of these larger cores is accomplished by choosing a 
 low fe(co)5/oa ratio .
the polydispersity shown in figure 6 decreases very rapidly 
 after the burst nucleation . for large cores ( low fe(co)5/oa ratio ) , size focusing was observed to occur after nucleation 
 as a decrease in variance of the fitted particle distribution .
in contrast , for the  small  cores ( high fe(co)5/oa ratio ) , a continuous increase in polydispersity was observed 
 from the beginning .
this can however be an artifact due to that the 
 nucleation and initial part of the growth is missing in the data due 
 to an interruption of the experiment by clogging . the origin 
 of the small increase in polydispersity after the initial size focusing 
 at 40 min reaction time for  large  cores is unclear 
 since ostwald ripening should not occur for diffusion controlled growth . 

although this increase is marginal at only 2% , it could suggest 
 that quenching the reaction at an optimal time during growth could 
 further reduce the polydispersity . the continuous increase in polydispersity 
 of  small  cores during the growth phase
could be related 
 to the more inhomogeneous nucleation for  small  cores . 
 as described above , for these nanoparticles ,
the precursor formation 
 and nucleation phases have merged and might cause heterogeneity also 
 in the growth phase since the nucleation phase is short .
an alternative 
 interpretation can be based on that our data show a higher concentration 
 of not only nuclei , but also precursor clusters for  small  
 cores . for mass conservation reasons , these must be smaller as well 
 as more reactive , which is supported by the kinetic data . the larger 
 amount and
higher reactivity of precursor clusters could presumably 
 lead to less controlled nucleation and growth and affect both polydispersity 
 and nonuniform morphology .
as seen 
 in figure 8b , the core 
 growth is slower , and polydispersity decreases slower and more continuously 
 for the 1 k / min heating rate than for the 3 k / min sample . at 3 k / min 
 heating rate , very strong size focusing
the final average core size is smaller at the slow heating 
 rate than at the two higher rates .
although the polydispersity estimated 
 from the fits is not much different to that of the optimal 3 k / min 
 heating rate sample , tem inspection of the 1 k / min sample clearly 
 demonstrates less uniform cores in terms of size ( figure 8c ) .
a slower growth rate , starting 
 at a lower temperature with nucleation less focused in time compared 
 to the optimal 3 k / min heating rate , thus seems to be the consequence 
 of a ( too ) slow heating rate ; this produces more faceted and therefore 
 not only potentially more crystalline , but also more heterogeneous 
 cores .
the average polydispersity after growth is not much higher 
 for the 9 k / min sample , and it is still converging at the point of 
 finishing the experiment . the cores are then still growing , albeit 
 slowly .
tem investigations , however , show that larger cores , potentially 
 growing for longer time , are present in the 9 k / min sample but not 
 in the 3 k / min sample ( figure 8c ) .
these cores will never converge to the high uniformity 
 observed at the intermediate heating rate even if the growth phase 
 had been prolonged .
we have demonstrated 
 a setup suitable for following the growth 
 of monodisperse nanoparticles using the heat - up and thermal decomposition 
 method at the highest time resolution to date using in situ saxs and 
 waxs .
this approach could in the future be combined to include , for 
 example , uv  vis / ir spectroscopies that were previously used 
 to provide chemical information on the growth stages during nanoparticle 
 synthesis .
while the time evolution of geometrical core size 
 growth and polydispersity 
 could be fitted with second resolution after nanoparticle nucleation , 
 the most important new insights were obtained by analyzing the time 
 evolution of the porod constant and the invariant before nucleation 
 occurred .
these parameters yield insight into changes in inhomogeneity 
 and the creation of strongly scattering material within the sample 
 without resorting to a detailed a priori assumed model for fitting 
 of data . using this analysis
, we found that optimal synthesis of monodisperse 
 iron oxide nanoparticles is composed of six well - defined phases in 
 agreement with , but providing increased detail to , current models 
 used to describe heat - up thermal decomposition synthesis of ( iron 
 oxide ) nanoparticles .
these phases are mostly well separated but include 
 intermeshed phases for iron oleate complex formation and precursor 
 ( polyiron oxo ) cluster formation .
strikingly , a clear separation of 
 the phases of cluster formation and burst nucleation at supersaturation 
 by an additional heat - up lag phase was found for the synthesis of 
 the most monodisperse and uniform nanoparticles . another novel insight 
 of this analysis of separated phases was the correlation of a difference 
 in the number , size , and density of precursor clusters formed during 
 heat - up with the number of nuclei formed during the burst nucleation 
 phase that controls the final particle size .
this could be exquisitely 
 controlled by the choice of fe(co)5/oa ratio , but it was 
 also affected by the heating rate .
the highest morphological 
 ( spherical ) uniformity and lowest relative 
 polydispersity was achieved for synthesis of large nanoparticles ( targeted 
 by a low fe(co)5/oa ratio ) at a heating rate of 3 k / min . 

these conditions seemed to achieve a clear separation of the kinetic 
 steps and a sufficiently slow growth to form spherical particles with 
 the lowest polydispersity .
the assumed optimal conditions for producing 
 small nanoparticles , however , seemed to lead to a fast growth rate 
 and smaller than expected crystal size , which indicated that further 
 optimization by tuning precursor ratios and heating rate could be 
 beneficial for performance and that quenching of the synthesis at 
 an optimal time could further reduce polydispersity .
 the 
 synthesis of iron oxide nanoparticles ( nps ) by thermal decomposition 
 of iron precursors using oleic acid as surfactant has evolved to a 
 state - of - the - art method to produce monodisperse , spherical nps . 
 the 
 principles behind such monodisperse syntheses are well - known : the 
 key is a separation between burst nucleation and growth phase , whereas 
 the size of the population is set by the precursor - to - surfactant ratio . 
 
 here we follow the thermal decomposition of iron pentacarbonyl in 
 the presence of oleic acid via in situ x - ray scattering . 
 this method 
 allows reaction kinetics and precursor states to be followed with 
 high time resolution and statistical significance . 
 our investigation 
 demonstrates that the final particle size is directly related to a 
 phase of inorganic cluster formation that takes place between precursor 
 decomposition and particle nucleation . the size and concentration 
 of clusters 
 were shown to be dependent on precursor - to - surfactant 
 ratio and heating rate , which in turn led to differences in the onset 
 of nucleation and concentration of nuclei after the burst nucleation 
 phase . 
 this first direct observation of prenucleation formation of 
 inorganic and micellar structures in iron oxide nanoparticle synthesis 
 by thermal decomposition likely has implications for synthesis of 
 other nps by similar routes . 
Introduction
Materials and Methods
Results
Discussion
Conclusions