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after a long arduous effort , photoluminescence from silicon has been achieved from its nanocrystalline form  @xcite . a critical debate , however , continues over the nature of the interface chemistry of silicon nanocrystals ( si - ncs ) embedded in amorphous silica which has direct implications on the optical activity of the interface  @xcite . _ reported that the oxidation of porous silicon quantum dots results in a red shift in the photoluminescence ( pl ) spectra which indicates the importance of oxygen - related interface bond toplogy  @xcite . along this line , puzder and co - workers compared pl calculations of nanoclusters with different passivants and surface configurations and proposed the main reason for the red shift to be double si = o bonds @xcite . countering this , luppi _ et al . _ reported excitonic luminescence features caused by si - o - si bridge bonds at the surface of silicon nanoclusters @xcite . as a supporting evidence for the latter , gatti _ et al . _ recently demonstrated that si - o - si is the most stable isomer configuration @xcite . to reconcile , vasiliev _ et al . _ claimed that bridge bonds and double bonds have similar effect on pl @xcite . all of the work cited above represent density functional theory ( dft)-based calculations with small si clusters of less than 100 atoms surrounded by either passivants like hydrogen @xcite or oxygen @xcite . but actual samples are profoundly different : the fabricated systems consist of si - nc with a diameter larger than @xmath0  nm , embedded in _ amorphous _ silica ( a - sio@xmath1 ) matrix . identifying this fact , tu and tersoff studied si / a - sio2 interface using a bond order dependent empirical potential and proposed that si - o - si bridges are the main blocks at the interface lowering the surface energy @xcite . using the same model potential hadjisavvas _ studied si - ncs embedded in a - sio@xmath1 and they also reported bridge bonds as the mechanism for lowering surface strain energy @xcite . in our previous work on the formation and structure of si ncs , we also observed that this model potential falls short to characterize the system accurately , especially the structure and chemistry at the interface @xcite . as a rigorous approach to interface structure and dynamics , pasquarello _ et al . _ used first - principles molecular dynamics ( md ) technique to investigate planar si / sio@xmath1 interface @xcite . they observed that the oxygen atoms momentarily are bonded to three silicon atoms during oxidation process . the net effect of these threefold coordinated oxygen atoms during oxidation process was to expel silicon atoms out of interface . this is interpreted as a balancing process to decrease the increased density of si / sio@xmath1 interface due to oxidation @xcite . however , no substantial breakthrough was made over the last decade on this issue . in this work , we employ the reactive force field ( reaxff ) model developed by van duin _ et al . _ which improves brenner s reactive bond order model @xcite to a level of accuracy and validity allowing molecular dynamics simulations of the full reaction pathways in bulk  @xcite . the parameters for this force field were obtained from fitting to the results of ab initio calculations on relevant species as well as periodic boundary condition dft - based calculations of various crystalline polymorphs of relevant materials . the reaxff calculates bond orders which is the measure of bond strength from local geometry . this allows realistic chemical environment such as over / under coordination and bond breaking / formation for large - scale ( about 5000 atoms ) md simulations .    to facilitate our discussion regarding the surface - bonded oxygen complexes , we distinguish among three different types of silicon atoms . we label those silicon atoms with all silicon neighbors each with zero oxidation state as _ c _ , to denote core silicon atom . among the _ remaining _ ( non-_c _ ) silicon atoms , those with at least one bond to _ c _ are labeled as _ s _ , denoting as a surface silicon atom . for further investigation of nc , we seperate core si atoms into two sub - categories as inner - core and outer - core atoms : among core si atoms which have at least one surface si neighbour categorized as outer - core si atoms and rest of core si atoms categorized as inner - core si atoms . finally , any other silicon atom is labeled as _ m _ , denoting matrix silicon atom . hence , a complex consisting of an oxygen atom bonded with two surface silicon atoms is labeled as _ ss_. the other oxygen complexes are _ sm _ , _ ssm _ , _ sss _ , _ mms _ as sketched in fig . [ fig : abbrevations ] where the last three correspond to three - coordinated oxygen ( 3co ) atoms . we use reaxff to represent the interactions in the model system . we start with a large simulation cell ( box length @xmath2   ) of silica glass formed through a melting and quenching process used by one of us @xcite earlier to study silica glasses earlier . next , similar to hadjisavvas _ et al . _ @xcite , we delete all atoms within a predetermined radius to insert crystalline silicon to form nc . in this way , we create ncs with radii ranging from @xmath3    to @xmath4    . for the largest nc we insert @xmath5 si atoms into a spherical hole with radius @xmath4    created in amorphous matrix . even for this case , the minimum distance between nc surface to simulation box face is about @xmath6    which can still accommodate the interface layer . we also pay special attention in the removal of spherical region so that the correct stoichiometry for the amorphous matrix is met . thus , our amorphous matrix has two o atoms for every si atom with a density of @xmath7  g/@xmath8 which is the density of glass at room temperature and atmospheric pressure . we set periodic boundary conditions in all directions and while keeping nc at 100  k , we employ simulated annealing process to sio@xmath1 region to end up with an amorphous matrix free of artificial strain around the nc . then we set whole systems temperature to room temperature ( 300  k ) and continue performing md simulation for 75  ps to have thermal equilibrium between the two regions . we set md simulation time step to 0.25  fs for all simulations . for every 62.5  fs time interval , we have record the configurations to analyze the transitions taking place between different bond topologies . the numbers attached to each arrow in fig .  [ fig : abbrevations ] indicate the total number of registered transitions during the simulation in that direction between the complexes for a representative nc of radius @xmath9   . almost balanced rates in opposite directions is an assurance of the attainment of the steady state in our simulation . note that we do not observe any direct transition other than the paths indicated in fig . [ fig : abbrevations ] . for instance , a direct transition of the complex _ ss _ to _ sm _ does not take place , but it is possible through an intermediate transition over the _ ssm _ which is a 3co . we should also remark that the balanced transitions continue to take place after the steady state is attained which indicates that the interface bond topology is dynamics , i.e. not frozen . . ]        to analyze si - nc / a - sio@xmath1 interface , we construct nc surface using delaunay triangulation scheme ( fig .  [ fig : rdfncsisi ] inset ) @xcite . in two dimensions a delaunay triangulation of a set of points corresponds to defining triangles such that no point in the set is inside the circumcircle of any triangle . in three dimensions triangles extend to tetrahedra and circumcircles become circumspheres . since our ncs are nearly spherical in shape , we triangulate projection points of surface si atoms onto the unit sphere . hence , we apply delaunay triangulation over the two dimensional @xmath10-@xmath11 plane . we can then create surfaces using this triangulation . this surface enables us to calculate every atoms distance to nc surface . by this means we plot various data such as charge , bond order etc . with respect to distance to the surface , to extract information about surface chemistry of si - nc embedded in amorphous matrix . for different nc radii we observe similar trends in bond order distribution , average charges etc . , therefore , we present only the figures of the system for a typical nc of radius @xmath9   . a useful data to elucidate the structure of these systems is the radial distribution function ( rdf ) . in fig . [ fig : rdfncsisi ] we present rdf of nc atoms only , where the first broad peak centered around 2.34    with a 0.3    full width at half maximum value ( fwhm ) , represents si - si bond length distribution in nc ( fig .  [ fig : rdfncsisi ] inset ) . the maximum extent of the nc can also be read from same plot at about 27   , where the rdf goes to zero . observation of a broad first peak at fig  [ fig : rdfncsisi ] demands further investigation of si - si rdf of nc atoms . for this purpose , in fig . [ fig : detailedrdf ] we present bond length probability distributions ( akin to rdf ) for three categories of si atoms : inner core ( with no bonds to surface atoms ) , outer core ( bonded to surface ) and surface nc atoms . we observe from fig . [ fig : detailedrdf ] that si - si bond lengths in the inner core are centered around the equilibrium value and have a narrow width due mainly to thermal vibrations , whereas the bond length distributions of outer core and surface atoms have increasing shift for the most probable bond length values and broader widths . these shifts and particularly the increase in distribution widths can not be attributed to thermal broadening . taken together these two observations is a clear indication of increasing strain as a function of distance from the center of the nc . to further investigate this deviation of si - si bonds from crystalline si , in fig . [ fig : strain ] we present bond length distribution with respect to distance to nc surface averaged over 2  ps of simulation time after the steady state is reached . this figure illustrates the gradual development of radial strain from the center to nc surface . these observations show clearly that oxidation at the surface of nc results in a tensile strain at si - si bonds which becomes significant only around the interface , while keeping the inner core almost unstrained . this tensile strain in the nc agrees with previous measurement of hofmeister _ _ @xcite . another consequence of this tensile strain is that the total bond orders of core - nc atoms are somewhat smaller than those of oxide - si s as seen in fig . [ fig : panel3 ] . in the same figure we also show the calculated net charges using electron equilibration method @xcite . nearly zero net charges of the core - si atoms reflects the covalent type of bonding well within the nc . the bonding becomes increasingly ionic away from the nc core as observed by the charges of si atoms which reach the value of 1.3@xmath12 at the oxide region ( fig . [ fig : panel3 ] ) . as a result , the positive charges of surface - si atoms form a shell at the surface of nc . this observation is similar with those obtained with dft calculations @xcite . on the other hand , negative charges of oxygen atoms bonded to surface form another shell that enclose nc and finally total average charges approach to zero within the oxide region . in fig .  [ fig : panel3 ] we also observe that the magnitude of average charges of oxygen atoms which are bonded to surface are greater than those in the matrix . but , the bond orders are nearly the same . this is due to existence of 3co atoms bonded to surface . note that the average bond order of oxygen atoms which are bonded to surface is about two ( cf . [ fig : panel3 ] ) . thus , those oxygen atoms form three partial bonds , two strong and one weak bond . finally , we would like to note that unlike many others  @xcite , we do not observe any double bonds . cccc ccc cc ccc cc cc c @xmath13 & @xmath14 & @xmath15&@xmath16 & & & & & & + ( )&&&&@xmath17 & charge&@xmath18&@xmath17 & charge & @xmath19 & charge&@xmath18&@xmath20 & charge&@xmath21 & charge & + 11.0 & 10 & 25 & 32 & 2 & -0.88 & 169.7 & 24 & -0.76 & 5 & -0.74 & 97.0 & 0 &  & 1 & -0.81 & + 15.4 & 42 & 42 & 59 & 5 & -0.77 & 136.1 & 51 & -0.74 & 1 & -0.73 & 82.7 & 0 &  & 2 & -0.77 & + 18.0 & 83 & 62 & 77 & 11 & -0.82 & 120.5 & 56 & -0.74 & 1 & -0.83 & 141.2 & 0 &  & 9 & -0.79 & + 19.8 & 114 & 76 & 82 & 14 & -0.82 & 139.3 & 49 & -0.73 & 4 & -0.81 & 123.1 & 1 & -0.85 & 14 & -0.79 & + 26.8 & 353 & 143 & 170 & 20 & -0.81 & 118.0 & 123 & -0.74 & 11 & -0.79 & 120.9 & 0 &  & 15 & -0.76 & + 30.8 & 558 & 203 & 243 & 35 & -0.83 & 123.9 & 159 & -0.74 & 10 & -0.80 & 115.9 & 2 & -0.80 & 34 & -0.79 & + 33.4 & 718 & 238 & 268 & 44 & -0.83 & 123.0 & 179 & -0.75 & 18 & -0.80 & 125.7 & 4 & -0.84 & 23 & -0.78 & +    [ table : statatom ]    the occurrence of 3co has been noted by a number of groups . pasquarello showed that the bistable @xmath22 defect of @xmath23-quartz structure may lead to 3co as a metastable state as well as si - si dimer bond and calculated the energy of the former to be higher than the latter . pasquarello proposed that 3co acts as an _ intermediate metastable state _ during structural relaxations at the interface @xcite . similarly boero _ et al . _ observed 3co atoms in their _ ab initio _ calculations @xcite and reported this feature as a metastable state . in table [ table : statatom ] we present the collected statistical data at the end of the simulation of 75  ps . for all oxygen complexes , the number of bridges , average charges of bridge oxygens , and the average bridge angles for _ s_-o-_s _ are tabulated . we observe in table  [ table : statatom ] that the number of _ sss _ complexes is very small due to narrow bond angle requirement of this configuration . for the remaining 3co complexes , _ ssm _ and _ mms _ , their percentages are seen to increase with curvature . this can explain the fact that other studies @xcite which have identified the 3co complexes as metastable were based on the _ planar _ si / sio@xmath1 interfaces . so , this is an indication of the importance of curvature in the stability of 3co complexes . hence , as one would expect , there is a linear relation between the total number of bridges with surface area as indicated in fig . [ fig : nobridgesr2 ] . this finding is supported by kroll _ et al . _ who reported 3 and 33 such bridges for si - nc with radii @xmath24   and @xmath25   , respectively  @xcite . in conclusion , the realistic chemical environment provided by reactive force field model enables us to understand the bond topology of si - nc / a - sio@xmath1 interface and its internal dynamics . particularly , it reveals that there are different types of oxygen complexes at the si - nc surface some of which contain 3co complexes whereas there are no double bonds . the curvature has a positive effect on the occurence of 3co . the relative abundance of different complexes and their charge and geometrical characteristics are extracted . the inner core is observed to be almost unstrained while the outer core and the interface region of the nc are under increasing strain up to about a few per cents . in general , our work clearly shows that the si nc - oxide interface is more complicated than the previously proposed schemes which were based on solely simple bridge and double bonds . the provided information here paves the way to construct realistic monte carlo moves for the simulation of large - scale silicon nanostructures embedded in oxide matrix . this work has been supported by the european fp6 project seminano with the contract no . nmp4 ct2004 505285 , by the turkish scientific and technical council tbitak with the project no . the visit of tahir ain to bilkent university was facilitated by the tbitak bideb-2221 programme . the computational resources are supplied in part by tbitak through tr - grid e - infrastructure project . 99 t. canham , appl . phys . lett . * 57 * , 1046 ( 1990 ) . m. v. wolkin , j. jorne , p. m. fauchet , g. allan , and c. delerue , phys . * 82 * , 197 ( 1999 ) . a. puzder , a.j . williamson , j. c. grossman , and g. galli , phys . lett . * 88 * , 097401 ( 2002 ) . e. luppi , f. iori , r. magri , o. pulci , s. ossicini , e. degoli , and v. olevano , phys . rev . b * 75 * , 033303 ( 2007 ) . m. gatti and g. onida , phys . b * 72 * , 045442 ( 2005 ) . i. vasiliev , j. r. chelikovsky and r. martin , phys . b * 65 * , 121302 ( 2002 ) . y. tu and j. tersoff , phys . rev . lett . * 89 * , 086102 ( 2002 ) . g. hadjissavvas and p. c. kelires , phys . rev . lett . * 93 * , 226104 ( 2004 ) . d. e. yilmaz , c. bulutay , t. cagin , j. nanosci . _ in press_. a. pasquarello , m. s. hybertsen , and r. car , nature ( london ) * 395 * , 58 ( 1998 ) . d. w. brenner , phys . b * 42 * , 9458 ( 1990 ) ; d. w. brenner , o. a. shenderova , j. a. harrison , s. j. stuart , b. ni , and s. b. sinnot , j. phys . condens . matter * 14 * , 783 ( 2002 ) . a. c. t. van duin , a. strachan , s. stewman , q. zhang , x. xu , and w. a. goddard , iii , j. phys . a * 107 * , 3803 ( 2003 ) . e. demiralp , t. ain , and w. a. goddard , phys . * 82 * , 1708 ( 1999 ) . n. t. huff , e. demiralp , t. ain , and w. a. goddard , j. noncrystal . solids , * 253 * , 133 ( 1999 ) . r. j. renka , acm trans . math . soft . 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the interface chemistry of silicon nanocrystals ( ncs ) embedded in amorphous oxide matrix is studied through molecular dynamics simulations with the chemical environment described by the reactive force field model . 
 our results indicate that the si nc - oxide interface is more involved than the previously proposed schemes which were based on solely simple bridge or double bonds . 
 we identify different types of three - coordinated oxygen complexes , previously not noted . the abundance and the charge distribution of each oxygen complex is determined as a function of the nc size as well as the transitions among them . 
 the oxidation at the surface of nc induces tensile strain to si - si bonds which become significant only around the interface , while the inner core remains unstrained . unlike many earlier reports on the interface structure 
 , we do not observe any double bonds . 
 furthermore , our simulations and analysis reveal that the interface bond topology evolves among different oxygen bridges through these three - coordinated oxygen complexes .