metal  organic frameworks ( mofs ) 
 are polymeric crystalline materials comprising of metal ions bridged 
 via co - ordination bonds by polydentate organic linkers .
mofs are typically formed by solvothermal reaction 
 of the metal salts with the ligand , and have attracted great interest 
 due to their potentially high internal surface area and porosity , 
 which have great technological potential for gas storage and capture , 
 sensing and catalysis .
a wide range of 
 organic molecules have been used as linkers for the construction of 
 mof materials , offering the potential for structures with highly tailored 
 properties and topologies through the systematic control of the geometry 
 and porosity as well as the integration of chemical functionality 
 into the structure of the organic linker .
it is also possible to grow mof materials on substrates to 
 form surface - mounted mofs , so - called surmofs .
surmofs have been grown 
 with thicknesses varying from a few monolayers up to several microns 
 and are of particular interest since they provide a route to the integration 
 of porous functional materials with thin film devices .
for example , surmof materials 
 are promising candidates for fabricating highly responsive gas sensors 
 since they strongly , and in some cases selectively , adsorb various 
 gases .
there have also been recent advances 
 in the fabrication of surmof prototype semiconductor devices , and 
 optical sensors based on interferometry .
several different approaches to the growth 
 of surmof thin films have been reported . in the simplest method ,
a substrate is introduced into the solvothermal 
 reaction solution leading , under suitable conditions , to the growth 
 of a thin film of crystallites .
it was 
 further demonstrated that the termination of noble metal and oxide 
 surfaces with , respectively , functionalized thiol and silane self - assembled 
 monolayers ( sams ) can promote the growth of surmof films .
in addition the crystallographic orientation of the film may be 
 controlled through the use of sams with a specific end - group such 
 as a cooh or oh termination .
functionalization of substrates with sams may be combined with lithographic 
 techniques to pattern the substrate surface , for example using microcontact 
 printing ( cp ) , to locally inhibit or promote surmof growth .
while representing significant progress , this 
 approach of direct solvothermal growth onto a substrate often results 
 in rough , polycrystalline films or , alternatively , an inhomogeneous 
 coverage of isolated crystals .
( lbl ) technique represents an alternative approach to the growth of surmofs 
 and has been investigated as a possible route to improving the morphology 
 of growth , and also the formation of surmof heterostructures . in the 
 lbl method
a substrate undergoes cyclic sequences of immersion in 
 a solution of the metal ion , followed by immersion into a solution 
 of the organic ligand ( or vice versa ) with potential 
 rinsing steps between immersion .
variations 
 on this implementation include exposure to metal / ligand solutions 
 in flow reactors , and through spray deposition .
importantly , the substrate is only exposed to one component ( metal 
 or ligand ) of the framework at each step in contrast to the solvothermal 
 method which exposes the surface to both simultaneously .
it has been 
 proposed that only one monolayer forms in each cycle of immersion 
 in framework component solutions and the surmof is thus grown in a 
 controllable  layer - by - layer  manner .
the lbl method can be combined with lithographic techniques 
 and surface chemical functionalization using sams to enhance further 
 the degree of control of the lateral , vertical and crystallographic 
 geometry of mof material .
extensions 
 to this idea include the growth of mof on mof structures by changing 
 the metal or ligand used during the growth process to create a layered 
 material , and post - synthetic modification of linkers to change 
 the chemical functionality of the linker molecules while preserving 
 the mof crystalline structure .
the lbl method 
 has been widely described as an epitaxial mode of growth and many 
 of the more exotic approaches to surmof growth such as 3-component 
 pillared structure mofs or heteroepitaxial 
 structures implicitly assume that the growth interface is , at least approximately , 
 parallel to the substrate , analogous to the frank  van der merwe 
 mode of thin film growth , and advances by a monolayer in each growth 
 cycle .
quartz crystal microbalance ( qcm ) measurements of the mass 
 uptake during each step offer some support for this idealized model 
 of growth , while images acquired using atomic force microscopy ( afm ) have confirmed 
 that the surmof thickness increases with the number of cycles , as 
 expected .
however , the growth 
 rate in some cases vastly exceeds that predicted by the ideal lbl 
 model , and there is no direct evidence 
 for the addition of precisely one monolayer in each growth cycle . 

indeed a recent scanning tunnelling microscopy ( stm ) study has shown 
 no clear evidence for epitaxial features during the growth of the 
 first few monolayers of mof growth .
overall the growth of surmofs using the lbl method appears to be 
 more complicated than that suggested by the idealized model .
this 
 has motivated the current study of the very early stages of the growth 
 of surmofs by sequential dipping .
in particular , we focus on the nucleation 
 and subsequent growth which occurs in the first 10 growth cycles and 
 use amplitude - modulated tapping ( ac ) mode afm to acquire images of 
 the surface after each of the first five cycles of growth , and then 
 again after the tenth cycle .
we use a variety of oriented and polycrystalline 
 au surfaces terminated , in some cases , by thiol layers . for most samples 
 it is possible to return to exactly the same position on the surface 
 allowing the repeated imaging of individual nanocrystals and subsequent 
 characterization of their dimensions and orientation relative to the 
 substrate at each stage of the growth process .
the use of oriented 
 gold surfaces complicates a direct comparison with other work although 
 it is clear that our results agree with some aspects of previous work , 
 notably the influence of thiol layers .
however , under the growth conditions 
 used here we do not observe the sequential addition of single mof 
 layers , and we find that the growing surface which is , in general , 
 not parallel to the substrate .
hkust-1 was chosen for this study since it has been studied extensively 
 both as a bulk material and as a surmof thin film grown on thiol or silane functionalized surfaces .
in addition due to the relative 
 lability of cu(ii ) centers , hkust-1 can be deposited under ambient 
 conditions at room temperature from dilute ethanolic solutions of 
 both metal salt and ligand using a sequential exposure technique .
surmofs 
 of hkust-1 were grown on au(111 ) ( 300 nm epitaxial au(111 ) layer on 
 mica , georg albert pvd , heidelberg , germany ) .
thiol sams were 
 deposited by immersion for 30 min in 0.2 mm ethanolic solutions of 
 either 16-mercaptanhexadecanoic acid ( mhda ) or 11-mercaptanundecanol 
 ( muda ) to terminate the surface with , cooh and oh 
 functional groups , respectively .
this was followed by rinsing in a 
 stream of ethanol for approximately 30 s and drying in a n2 stream .
once the substrates had been prepared , a registration mark 
 was formed in the au layer using a pair of tweezers ( cleaned by flame - annealing 
 in a butane flame for 30s and allowed to cool ) .
the substrate was 
 loaded into an asylum research cypher - s afm and the cantilever repeatedly 
 aligned to the registration mark using the inbuilt optics and the 
 coarse xy movement of the sample stage .
images of the substrate were 
 acquired in repulsive ac mode using olympus ac240ts silicon cantilevers 
 ( resonant frequency 70 khz , spring constant 2 n / m ) .
surmof was grown by immersion in separate 1 mm solutions of cu(o2cch3)2 and benzene-1,3,5-tricarboxylic 
 acid ( tma ) in ethanol ( fisher 99.95% ) ( figure 1 ) .
substrates were immersed for 1 min after 
 which the substrates were rinsed in ethanol , dried using n2 and subsequently immersed in the ligand solution for 1 min .
the 
 sample was again rinsed in ethanol to remove unreacted ligands and 
 dried with n2 .
after each cycle of immersion 
 in metal and ligand solutions the substrates were returned to the 
 afm for further imaging .
the optics of the afm were used to align 
 the cantilever with the registration mark to an accuracy in the range 
 1020 m .
the region of interest was found through further 
 alignment using the grain structure and terrace morphology on the 
 au(111 ) surface acquired in previous scans to provide registration . 

films of hkust-1 on polycrystalline au were prepared ( figure 1 ) using the same preparative 
 and sequential dipping processes on wafers of 300 nm au on si(100 ) 
 with a 50 nm ti bonding layer ( georg albert pvd ) . in the case of the 
 polycrystalline au ,
the lack of an easily identifiable grain structure 
 and the isotropy of the au surface structure prevented the repeated 
 imaging of the same area .
figure 2 shows afm images at different stages of the growth process 
 on various thiol - terminated au(111)/mica substrates .
each row shows 
 a sequence of images acquired for au(111 ) substrates prepared in different 
 ways : the top row ( figure 2a ) is a clean au(111 ) surface ; in the second row ( figure 2b ) the au(111 ) surface 
 is pretreated with mhda to give a cooh termination ; in the 
 bottom row ( figure 2c ) the au(111 ) surface has an oh termination through pretreatment 
 with muda . within each row afm images
are presented of the surface 
 prior to deposition of the mof in the left - hand column , followed by 
 images acquired after , running from left to right , 1 , 2 , 3 , 4 , 5 , 
 and 10 cycles of sequential dipping .
thus , a comparison of images 
 within a row provides a comparison of surfaces at different stages 
 of growth , while the columns provide a comparison of images acquired 
 after the same cycle of growth but with different surface functionalization .
afm images 
 of hkust-1 growth on ( a ) au(111 ) , ( b ) mhda on au(111 ) , and ( c ) muda 
 on au(111 ) at different stages during the growth process ; the number 
 of cycles of growth is shown along the horizontal axis and varies 
 from 0 ( i ) to 10 ( vii ) . the arrows in the images a 
c parts 
 i  vii indicate specific surface features to provide registry 
 between successive images .
all scale bars are 1 m . the use of registration marks to obtain images 
 of the same region of the surface is highlighted in , for example figure 2a(i , ii ) and figure 2c(i  iv ) . in 
 these images the terrace structure of the gold surface may be readily 
 resolved and , further , the detail of this structure is immediately 
 recognizable in successive images ( across the rows ) . as the mof layers 
 grow in the subsequent dipping cycles , the contrast in the images 
 is dominated by the mof crystallites when their height is greater 
 than the variation in height of the underlying gold surface ( typically 
 less than 2 nm over a 1 m area from afm data of 
 clean au(111 ) surfaces ) .
the terrace structure is not immediately 
 visible in such images ( see for example figure 2a(iii  vii ) ) , although it may be readily 
 discerned in processed images . to aid the identification of the registry 
 in figure 2 , particularly 
 for surfaces where there has been significant growth , we include on 
 each image an arrow identifying a specific surface feature as a reference . 

note that the loss of contrast of the gold terraces provides an approximate 
 indication of the point where there has been significant surmof growth . 

the images therefore highlight the differing growth rates due to surface 
 functionality with the muda surface having little growth until at 
 least five cycles , whereas in the case of the mhda terminated surface 
 the loss of contrast occurs after a single cycle indicating significant 
 growth has already occurred .
interestingly , we also reproducibly 
 observe hkust-1 growth on the bare au(111 ) metal surface as shown 
 in figure 2a .
there 
 have been several reports that mof growth on bare ( i.e. , in the absence 
 of functionalization by thiol sams ) gold does not occur , for example 
 in attempts to grow mof-5 under solvothermal 
 conditions , or hkust-1 using the lbl 
 technique on a polycrystalline au surface , but our results confirm that growth of hkust-1 is possible on au(111 ) . in figure 3
figure 3a shows the fraction 
 of the surface covered by the growing mof film with increasing cycles 
 of growth . for the cooh terminated surface there is already 
 > 5% coverage after 1 cycle and this value increases monotonically 
 over subsequent cycles , although the rate of increase drops between 
 5 and 10 cycles when the overall surface coverage is > 50% . on the 
 unfunctionalized au(111 ) surface
there is very little growth after 
 one cycle and the subsequent rate of increase of surface coverage 
 is also lower than for the cooh surface . for the oh 

terminated au(111 ) surface there is still a very low fraction of the 
 surface covered even after four growth cycles ; after which the coverage 
 increases very rapidly .
these data confirm that growth occurs immediately 
 for the cooh terminated surface , much more slowly for the 
 oh terminated surface , while the clean au(111 ) is an intermediate 
 case .
( a ) surface coverage , ( b ) island density , ( c ) average lateral island 
 size , and ( d ) volume of material per unit area ( or equivalently average 
 thickness ) deposited during the growth process plotted as a function 
 of the number of growth cycles . there are further differences in the morphology of the growing 
 surface , for example related to the number of nucleated islands of 
 mof crystallites . in figure 3b , we show the number of mof islands per unit area . for the 
 cooh terminated surface
there are 80 islands per square 
 micron after one cycle corresponding to an average center  center 
 island separation of 110 nm .
this increases to 130 
 islands m after 4 cycles , which then decreases 
 after further cycles indicating that islands have started to merge 
 at this point . the island density on the clean au(111 )
the growth on 
 the oh terminated surface differs markedly with very few islands 
 after three cycles followed by a sharp increase in areal density . from the data in figure 3 , parts a and b
, we derive the average lateral island size 
 ( figure 3c ) . from our 
 afm images
it is also possible to estimate the total volume of material 
 deposited and this is shown in figure 3d as a volume measured in nm adsorbed per 
 area ( in nm ) of substrate ( equivalent to the average height 
 of the film in nanometres ) . for the cooh terminated surface 

we observe a near - linear increase in both the island size and the 
 total volume of material deposited .
this implies that for this surface 
 a near constant amount of material is added in each growth cycle , 
 consistent with previous studies of polycrystalline gold terminated 
 by cooh . from the gradient of the data in figure 3d )
we estimate a growth rate ( averaged across 
 the sample ) of 1.7 nm per cycle , which is much greater than 
 the thickness of a single cu / tma layer ,  0.6 nm ( the lattice 
 constant of the unit cell of cubic hkust-1 , which contains four layers 
 of cu / tma is 2.6 nm ; note that in ref ( 28 ) a  cycle  
 is defined as two immersions in the metal and ligand , rather than 
 a single immersion in each solution as adopted here , so that in ref ( 28 ) the expected growth rate 
 within the layer - by - layer model is 1.3 nm / cycle for a  half - layer  
 comprising two layers of metal ions and two layers of molecules ) . 
 in previous work
a constant growth rate has been interpreted as supporting 
 evidence for layer - by - layer growth in the direction perpendicular 
 to the substrate , but it is clear that in this case a constant growth rate occurs 
 in combination with the locally accelerated growth of nanocrystallites , 
 rather than an extended even growth of homogeneous layers .
the 
 growth rate on the oh terminated surface is much lower compared 
 with the case of the cooh terminated substrate and , in agreement 
 with previous studies , more nonlinear .
this results in a more inhomogeneous distribution 
 of island sizes leading to , after 10 growth cycles , a combination 
 of a small number of large crystallites and a large number of very 
 small crystallites .
overall our results indicate that nucleation of mof on 
 these surfaces does not occur immediately but is likely preceded by 
 an accumulation of physisorbed material , possibly mediated by inhomogeneities 
 on the surface .
we therefore focus our discussion on the cooh 
 surface on which , as we show below , we can monitor the progressive 
 growth of individual crystallites .
this is also the most common choice 
 of termination for growth of hkust-1 on surfaces .
figure 4 shows images 
 ( figure4a  d ) 
 of a single 1 m region on mhda terminated au(111 ) 
 acquired over 4 stages of the growth process , together with selected 
 height profiles ( figure4e  h ) which are extracted from the same positions in successive 
 afm images ( shown as colorised lines in figure 4a  d ) .
these images confirm unequivocally 
 that we are able to image the same area on the substrate due to the 
 presence of identical , growing features , for example the two bright 
 ( topographically high ) features close to the top of each image .
afm images 
 of the same 1 m region on an mhda functionalized 
 au(111 ) region shown in figure 2 b at ( a ) 1 , ( b ) 3 , ( c ) 5 , and ( d ) 10 cycles of growth , respectively . 

the blue and white boxes in part d correspond to   and   oriented 
 crystallites respectively that can be traced throughout the entire 
 growth process .
( e  h ) line profiles across the features illustrated 
 in parts a  d , respectively , showing the growth of material 
 across this region .
we identify four distinct 
 types of surface feature ; the first of these are high features with 
 a near circular shape and no obvious faceting .
examples include the 
 islands mentioned above and also the feature through which a blue 
 line passes ; the blue height profiles are acquired along this trajectory 
 and show the height of these islands . surprisingly even after one 
 growth cycle the profile indicates a height , and apparent width , of 
 65 nm .
this feature grows so that after three cycles ( figure 4b , f ) the height is 
 110 nm ; the vertical size of this island is approximately 
 constant after this cycle .
it is possible that this feature corresponds 
 to a noncrystalline material , or a three - dimensional aggregate , since 
 there are no obvious facets as expected for a crystallite .
we have 
 investigated control samples which were exposed to only the metal , 
 or only the ligand ; from these controls it is clear that all the features 
 which we observe are formed only after immersion in both the metal 
 and the ligand , ruling out the possibility that any features are due 
 to one of these single components .
we also observe linear islands ; 
 see for example figure 4b where many linear features are observed .
a comparison with the 
 surface earlier ( figure 4a ) and later ( figure 4c ) in the growth process shows that these features grow rapidly in 
 one lateral direction in the first few growth cycles ( they are clearly 
 present , but shorter in figure 4a ) .
however , after an initial stage of growth the sizes of 
 these features saturate at a typical length of 150 nm and , 
 from the profiles , a width of 50 nm and a height of 30 
 nm ( see green and red profiles ) .
the other two types of surface 
 features are highly faceted with either a triangular or square base . 

these features are seen most clearly in figure 4d where they are identified with white ( triangular ) 
 or blue ( square ) outlines . unlike the linear and globular features , 

these crystallites become progressively larger through the cycles 
 of growth ; their size increases and does not saturate . for example , 
 the red profile in figure 4 extends over two triangular islands on either side of the 
 linear island on which the profile is centered ; these triangular islands 
 become higher and wider as the growth progresses . as we show 
 below
, the pyramidal features correspond to   oriented islands 
 terminated by { 111 } planes , while the triangles correspond to 
 oriented crystallites , also terminated by { 111 } surfaces .
it is possible 
 to identify all of these features at earlier stages of growth ( for 
 example the triangular features in figure 4c , and , more difficult to resolve on the 
 scale of these images , figure 4b ) . to highlight the  history  of a particular 
 crystallite we show in figure 5a the progressive growth of a pyramidal ( upper row ) and triangular 
 ( lower row ) crystallite .
the square and triangular bases of these 
 structures are very clearly resolved after 10 growth cycles and it 
 is possible to track these features back to earlier stages in their 
 growth ; the faceted shape is apparent after 3 cycles , but is poorly 
 resolved after 2 cycles .
after 1 cycle , we can resolve the presence 
 of a nucleated island but the shape can not be identified with confidence 
 at this stage .
( a ) afm images of two crystallites highlighted in figure 1d ) after successive 
 growth steps ; ( b ) plot of the height for each crystallite in part 
 a vs the number of growth cycles with a linear fit to the data measured 
 from line profiles across the crystallites relative to the surrounding 
 substrate .
these images allow the determination 
 of the growth rate of individual crystallites which complements the 
 average growth rates discussed above . in figure 6
, we show the variation of height with growth 
 cycle for the square and triangular islands highlighted in figure 4d .
although there 
 is some variation between individual crystallites the rate of increase 
 in height is 4.9 nm per cycle and 3.2 nm per cycle for the square 
 and triangular islands , respectively .
again , we emphasize that this 
 is not consistent with a model in which the growing interface advances 
 uniformly by one layer per cycle .
( a ) height as a function of growth cycles 
 for individual   oriented hkust-1 crystallites measured by afm 
 and ( b ) for   type crystallites ; ( c ) height vs width ( see inset 
 for location of measurement ) for   type crystallites and ( d ) height 
 vs width for   type crystallites .
we attribute the exposed facets for both island shapes to 
 { 111 } planes given their triangular symmetry ; in addition this is 
 consistent with the formation of free surfaces of bulk hkust-1 crystals 
 by { 111 } planes .
the geometry 
 of the square features is consistent with a crystallite growing with 
 the   direction oriented normal to the surface , leading to a pyramidal 
 structure ( a square base formed by a { 100 } plane and four faces formed 
 by { 111 } planes ; see for example ref ( 48 ) ) .
the ratio of height to the half - diagonal width 
 of the base is expected to be 1:1 for a face centered cubic crystallite 
 with this geometry .
this is consistent with observations shown in figure 6c , where this ratio 
 is plotted for the square islands highlighted in figure 4d at different stages of their 
 growth .
the presence of a   orientation is consistent with previous 
 data for hkust-1 grown on polycrystalline au substrates functionalized 
 with cooh end groups .
we assign the triangular 
 features to   oriented crystallites terminated by a { 111 } surface . 
 for these crystallites
the growing surface is near parallel to the 
 substrate , but we do not see a preferred in - plane orientation implying 
 that there is no epitaxial relationship with au(111 ) surface .
the 
 presence of these   crystallites is not expected from previous 
 studies of hkust-1 on mhda terminated au . however , in these previous 
 studies polycrystalline gold films were used as substrates , 
 rather than the oriented gold films used here . to check whether this 
 difference is significant
we have investigated a surface grown using 
 the protocols described above on mhda - terminated polycrystalline gold . 
 the results , shown in figure 7 , indicate the presence of many islands with typical dimensions 
 540 nm which have a faceted shape similar to that of the pyramidal 
  oriented islands discussed above , but the triangular islands 
 are not observed ; this observation is consistent with previous work showing the selective growth of   crystallites 
 on similar surfaces and other recent afm studies .
the pyramidal shape implies that these crystallites are 
 terminated by { 111 } planes .
afm image of 10 cycles of growth of hkust-1 
 on polycrystalline au on si coated with mhda .
the afm image shows 
 a predominantly square based pyramid nanocrystallite structure indicating 
 a preferential   orientation normal to the surface in agreement 
 with the literature .
the growth of hkust-1 on au(111)/mica islands 
 shows significant differences to previous work using polycrystalline 
 gold substrates .
first , we observe some growth even on clean au(111)/mica , 
 but the more interesting effects relate to growth on the mhda - terminated 
 surface which introduces a cooh functionality . here
we observe 
 the growth of four distinct types of island of which two may be readily 
 identified as   and   oriented crystallites .
using afm it 
 is possible to measure not only the average growth rate across the 
 surface , but also the local growth rate both horizontally and vertically 
 to the surface for individual crystallites .
both these rates are greater 
 than one layer / per cycle , while the local increase in height per cycle , 
 measured to be 4.9 and 3.2 nm for , respectively ,   and   type 
 crystallites , is significantly greater than the thickness of a single 
 cu / tma layer , 0.6 nm .
this observation demonstrates that in 
 local regions of the surface multiple ( 510 ) layers of mof 
 are grown in each cycle .
interestingly the largest crystalline 
 features which are identified in figure 5d may be traced all the way back to the first 
 deposition step in figure 4a .
this implies that the nucleation of all these islands has 
 already occurred after the first cycle of growth .
furthermore , we 
 can address the question of whether the growing crystallites are redissolved 
 or significantly redistributed when the substrates are reimmersed 
 in the relevant solutions .
the reproducible appearance of growing 
 islands and traceability of the location of structures on this highly 
 inhomogeneous surface implies that very little reorganization or solvent - assisted 
 recrystallization of material occurs under the conditions used here . for hkust-1
grown on polycrystalline gold , we observe significant 
 differences ; the lateral size of individual crystallites is considerably 
 smaller , and we do not observe triangular islands .
the larger crystallite 
 size on au(111)/mica may be due to the presence of larger , flatter 
 terraces , as compared to the rougher evaporated gold surfaces on which 
 we observe a higher density of nucleated islands , and thus smaller 
 crystallites .
this enhancement in nucleation density may be due to 
 inhomogeneities in the gold film , or in the mhda termination .
however , 
 our observation of oriented   growth is consistent with previous 
 studies of thicker film growth on this surface .
the termination 
 of the faceted crystallites by { 111 } planes of hkust-1 is observed 
 for both the   and   crystallites , and , in addition , for the 
 pyramidal shapes resolved on polycrystalline gold . as stated above , 

the free and therefore lowest energy , surfaces of bulk hkust-1 crystals 
 are formed by { 111 } planes , and the termination of the surmofs by 
 these same planes is therefore expected .
this implies that for growth 
 under near - equilibrium conditions , for which the lowest energy surface 
 will dominate , the growing interface is only parallel to the substrate 
 surface if the crystallographic orientation is normal to the lowest 
 energy surface plane .
this is observed for the triangular   oriented 
 islands , but not for the pyramidal   crystallites and implies 
 that the growing interface of   crystallites on both au(111 ) and 
 polycrystalline gold , can not be parallel to the substrate .
our afm 
 results also show that under these conditions the rate of propagation 
 of the growing interface is not homogeneous across a surface , and 
 that local rates vary significantly .
accordingly the incorporation 
 of single layers of mof can not be inferred from rates averaged across 
 a surface measured , for example , using a qcm to measure mass uptake .
the results presented here highlight the very significant contribution 
 that high resolution afm can contribute to the understanding of surmof 
 growth and in particular to monitor the local growth of individual 
 crystallites from the very early stages of nucleation .
it is clear 
 that this afm approach allows us to determine whether the layer - by - layer 
 method for surmof growth is valid under the conditions used here ; 
 we find that it is not and observe nanocrystalline growth .
we hope 
 that these results will motivate other working in the field to adopt 
 this technique to evaluate other surmofs to provide a rigorous test 
 of the assumptions of the layer - by - layer method .
our results suggest 
 that this approach to growth would be best suited to materials for 
 which it is possible to select , for example using self - assembled monolayers , 
 a crystallographic orientation that is normal to the lowest energy 
 surface of the mof . under these circumstances ,
 high resolution atomic force microscopy 
 ( afm ) is used to resolve the evolution of crystallites of a metal 
 organic framework ( hkust-1 ) grown on au(111 ) using a liquid - phase 
 layer - by - layer methodology . 
 the nucleation and faceting of individual 
 crystallites is followed by repeatedly imaging the same submicron 
 region after each cycle of growth and we find that the growing surface 
 is terminated by { 111 } facets leading to the formation of pyramidal 
 nanostructures for [ 100 ] oriented crystallites , and triangular [ 111 ] 
 islands with typical lateral dimensions of tens of nanometres . 
 afm 
 images reveal that crystallites can grow by 510 layers in 
 each cycle . 
 the growth rate depends on crystallographic orientation 
 and the morphology of the gold substrate , and we demonstrate that 
 under these conditions the growth is nanocrystalline with a morphology 
 determined by the minimum energy surface . 
Introduction
Methods
Results
and Discussion
Conclusions