Systematic spatial and stoichiometric screening towards understanding the surface of ultrasmall oxygenated silicon nanocrystal

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Systematic

spatial

and

stoichiometric

screening

towards

understanding

the

surface

of

ultrasmall

oxygenated

silicon

nanocrystal

Shanawer

Niaz

a,b,∗

_,

_Aristides

_D.

_Zdetsis

b

_,

_Emmanuel

_N.

_Koukaras

b

_,

_Oˇguz

_Gülseren

a

_,

Imran

Sadiq

c

a_Department_of_Physics,_Bilkent_University,_Ankara_06800,_Turkey

b_Molecular_Engineering_Laboratory,_at_the_Department_of_Physics,_University_of_Patras,_Patras,_GR-26500,_Greece c_Centre_of_Excellence_in_Solid_State_Physics,_University_of_the_Punjab,_Lahore,_Pakistan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received15May2016

Receivedinrevisedform29June2016 Accepted30June2016

Availableonline2July2016 Keywords: Siliconnanocrystals Quantumdots Electronicproperties DFTcalculations Oxygenateddots Stoichiometry

a

b

s

t

r

a

c

t

Inmostoftherealisticabinitioandmodelcalculationswhichhaveappearedontheemissionoflight fromsiliconnanocrystals,theroleofsurfaceoxygenhasbeenusuallyignored,underestimatedor com-pletelyruledout.Weinvestigatetheoretically,bydensityfunctionaltheory(DFT/B3LYP)possiblemodes ofoxygenbondinginhydrogenterminatedsiliconquantumdotsusingasarepresentativecaseoftheSi29

nanocrystal.WehaveconsideredBridge-bondedoxygen(BBO),Doubly-bondedoxygen(DBO),hydroxyl (OH)andMixoftheseoxidizingagents.Duetostoichiometry,allcomparisonsperformedareunbiased withrespecttocompositionwhereasspatialdistributionofoxygenspeciespointedoutdrasticchange inelectronicandcohesivecharacteristicsofnanocrytals.Fromanoverallperspectiveofthisstudy,itis shownthatbridgebondedoxygenatedSinanocrystalsaccompaniedbyMixhavehigherbinding ener-giesandlargeelectronicgapcomparedtonanocrystalswithdoublybondedoxygenatoms.Inaddition, itisobservedthatthepresenceofOHalongwithBBO,DBOandmixedconﬁgurationsfurtherlowers electronicgapsandbindingenergiesbuttrendsinsamefashion.Itisalsodemonstratedthatwithin samecomposition,oxidizingconstituent,alongwiththeirspatialdistributionsubstantiallyalters bind-ingenergy,highestoccupiedmolecularorbital(HOMO)andlowestunoccupiedmolecularorbital(LUMO) gap(upto1.48eV)andlocalizationoffrontierorbitals.

1. Introduction

Siliconnanocrystals(SiNCs)areveryinteresting

nanomateri-als whose potential is still not discovered completely or even

understoodinmanyrespects.Comparedtobulksilicon,the

elec-tronic properties of SiNCs are signiﬁcantly dependent on their

size.In general,thesepropertiesare extremelysensitivetothe

surface conditions of nanocrystals such as passivation,

func-tionalization, spatial distribution of passivants, reconstruction

etc. Silicon nanocrystals (SiNCs) possess quantum conﬁnement

effect, large ratios of surface area to volume, nontoxicity and

biodegradability,leadingtotheuseofSiNCsinavarietyofﬁelds

∗ Correspondingauthorat:DepartmentofPhysics,BilkentUniversity,Ankara, 06800,Turkey.

E-mailaddresses:shanawersi@gmail.com,shanawersi@fen.bilkent.edu.tr

(S.Niaz).

suchas microelectronics,optoelectronics, photovoltaics, in-vivo

bioimaging,photosensitizing,drugdelivery,lab-on-chipsensing,

photocatalysis, phototherapeutics andmuch more [1–12].

Free-standingself-assembledSiNCsareoftensynthesizedwithsurface

hydrogenpassivationwhichcanbefurtheroxidized[8].Butthe

roleofsurfaceoxygenhasbeenusuallyignoredorunderestimated,

despitetheevidencegivenbyvariousexperiments[13–15].Many

techniqueshavebeenusedtoexploretheoxidationstateoftheSi

atomsinvolvedinbondingwithsurfaceoxygen[16–18].Inpast

years,alotofeffortshavebeencarriedoutinordertounderstand

surfacechemistryofsiliconNCsduetothepresenceofoxidizing

constituentsboth experimentally and theoretically [19–28].For

example,itwasreportedearlier[28]thatforBBOcontainingNCs

theredshiftofbandgapisfoundtobesmallercomparedtothe

DBOorcompletehydroxylation.Furthermore,forhydroxyl

passi-vation,energygapislargelydependentonamountofOHonsurface

andtheirspatialdistribution.Zdetsisetal.[25]demonstratedthat

BBO leadsto more stable nanocrystalwith largeHOMO-LUMO

http://dx.doi.org/10.1016/j.apsusc.2016.06.197

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772 S.Niazetal./AppliedSurfaceScience387(2016)771–778

gapandbindingenergiescomparedtoDBObondswithwhichwe

completelyagree.Xiaodongetal.[26]performedsimilaroxygen

treatmenttohydrosylilatedsiliconNCs,wheretheyconcludethat

BBOandOHhardlychangetheHOMO-LUMOgapatgroundstate

(whichisnottrueforhydrogenpassivatedNCs).Nazemietal.[27]

haveinvestigatedeffectofspatialpositionandspatialdistribution

ofBBOpassivantsonabsorptionspectraofhydrogenpassivated

NCs.Accordingtotheirﬁndings,spatialpositioncansigniﬁcantly

effectonHOMO-LUMOgap,opticalabsorptionandlocalization

cen-tersoffrontierorbitalswithwhichweagree.However,wehave

investigatedthatspatialdistributionofDBO,MixandOHalsoplay

vitalroleonsurfacechemistryinadditiontoBBO.

Inthis study,Si29nanocrystal(∼1nm)hasbeendeliberately

consideredforelectronicandcohesiveinvestigations,whichisnot

accidentalaswasdemonstratedinourpreviouswork[25].

There-fore,insteadofrandomselectionofoxygenbondformationand

selectivediscussionontheircharacteristics,wepresentrather

sys-tematic densityfunctional Theory(DFT) study. Henceoxidizing

constituentsofBridge-bondedoxygen(BBO),Doubly-bonded

oxy-gen(DBO),hydroxyl(OH)andmixedareexaminedwithrespect

totheircompositionstoichiometry(identicalisomers)andspatial

position(distribution).

Itisfoundthatelectronicandcohesivecharacteristics

signiﬁ-cantlychangewith(1)concentration,(2)spatialdistributionand

(3)typeofoxygenbondingonthesurfaceofsiliconnanocrystals.In

general,itisobservedthatbridgebondedoxygenatednanocrystals

alongwithMixhavehigherbindingenergiesandlargeelectronic

gapcomparedtonanocrystalswithdoublybondedoxygenatoms

whichisalsotrueifhydroxylgroupisalsopresent.Asfaras

con-cernedtothestabilityofNCs,weconﬁrmfromourresultsthatBBO

containingNCsaremorestablethanDBOwhereasMixbonding

showintermediatebehavior.

2. Modelandapproach

Initial structure of oxygen free hydrogen terminated Si29

nanocrystalisselectedfromourpreviousstudies[25,29].Inthis

study,oxygenatedSi29nanocrystalsareconstructedwhiletaking

specialcareofstoichiometryinordertopresentcomprehensiveand

unbiasedcomparison.Fig.1representssomefullyrelaxed

struc-tureswithrespecttovariousmodesofoxygenbondingandtheir

concentrations.Detailedinformation aboutoxygenbond

conﬁg-urationsalong withspatial positionsofthose stoichiometrically

identicalisomerscanalsobeseeninTable1.Asismentionedabove,

we have considered various types of oxygenbonding, namely,

Bridge-bondedoxygen(BBOor>O),Doubly-bondedoxygen(DBO

or O),hydroxyl(OH)andMix.Weputlargeemphasison

non-hydroxylatednanocrytalswhereasselectedcasesofhydroxylated

nanocrystalsarealsopartofthisresearch.

Forexample,incaseofnon-hydroxylatedSi29O6H24

nanocrys-tal,sixBBOatomsareintroducedinﬁrstsubgroupSi29(>O)6H24,

sixDBOatomsareintroducedinsecondsubgroupSi29( O)6H24

andinthirdsubgroupamixtureofthreeBBOandthreeDBOatoms

Si29(>O)3( O)3H24withexactly50%ratiowhichkeeps

composi-tionstoichiometry.Similartechniquehasbeenadoptedforother

non-hydroxylatednanocrystals where number of oxygen atom

variesfromO2 toO10 witheven distribution.For hydroxylated

nanocrystals,OHgroupisintroducedinadditiontothe

conﬁgu-rationadoptedabove.Forexample,inSi29O10H24nanocrystal,six

BBOatomsareintroducedinﬁrstsubgroupSi29(>O)6H20(OH)4,six

DBOatomsareintroducedinsecondsubgroupSi29( O)6H20(OH)4

and in third subgroup a mixture of three BBO and three DBO

atomsareintroducedSi29(>O)3( O)3H20(OH)4etc.Incontrastwith

previousexampleofnon-hydroxylatedNCs,fourhydrogenatoms

havebeenreplacedwithfourOHgroup.Forcomparisonwehave

alsoincludedoxygenfreehydrogenpassivatedSi29nanocrystali.e.

Si29H36(Table1).

Spatialpositionorspatialdistributionofoxidizingconstituents,

analogoustotheirstoichiometry,alterssurfacechemistrywhich

cannot beneglected.Hence,structures are furtherdividedinto

three categories dependingupon thepositionof oxygenbonds

whilekeepingtheircompositionsame.Thechoiceofspatial

posi-tionisstrongly dependentontheavailabilityof suitablesilicon

atomand theirvacantneighborhood whichcanbesaturateand

avoid dangling bond. In this process norepetition is involved,

hence,allpossiblecombinationsareincludedinthisstudy(Table1).

Duetothedifferentconﬁgurationsofnanocrystalsdependingon

thenumber ofoxygenatoms,theirorientationsand spatial

dis-tribution, symmetry of the structures differs compared to the

symmetryoforiginalstructurei.e.Td,whichisobvious.

Allcalculationsin thisworkarebased ondensityfunctional

theory(DFT)usingthehybridexchange-correlationfunctionalof

Becke,Lee,YangandParr(B3LYP)[30].Thisfunctionalhasbeen

showntoefﬁcientlyreproducethebandstructureofawide

vari-etyofmaterials,includingc-Si,withnoneedforfurthernumerical

adjustments[31].ConvergencecriteriafortheSCFenergiesandfor

theelectrondensity(rmsofthedensitymatrix),wereplacedat

10−7au,whereasfortheCartesiangradientstheconvergence

cri-terionwassetat10−4au.Thewholesystemwasrelaxedinthe

geometryoptimizationatthesametime bydemandingthatthe

totalforce(averageandmaximum)oneachatombepractically

zero,i.e.smallerthan10−4a.u.Ourcalculationswereperformed

withtheTURBOMOLE[32]suiteofprogramsusingGaussianatomic

orbitalbasissetsSVP[4s3p1d]forSiliconand[3s2p1d]forOxygen

[33].

3. Resultsanddiscussion

3.1. Cohesiveproperties

Wehavecalculatedbinding/atomizationenergyofoxygenated

siliconnanocrystalusingfollowingexpression[25]:

BENC=NSiE (Si) +NOE (O) +NHE (H) −ENC

SiNSiONOHNH

(1) whereENC

SiNSiONOHNH

istotalenergyofnanocrystal,E(Si),E(O)

andE(H)aretheenergiesofsilicon,oxygenandhydrogenatoms

(withrespecttovacuum)andNSi,NO,andNHarethenumberof

silicon,oxygenandhydrogenatoms.

Fig.2showsbindingenergyperheavyatomwithrespectto

thenumberofoxygenatomsonthesurfaceofsiliconnanocrystals.

Furtherdetailsaboutcomposition,bindingenergiesandelectronic

gapetc for all NCs can beseen in Table1. In general,

regard-lessofanyinterspeciescomparison,thebindingenergydecreases

withincreasingnumberofoxygenatomsonthesurfaceofsilicon

nanocrytals.AswecanseeinFig.2(a)thatBBOP1havehighest

bindingenergyvaluesandDBOP1(andDBOP2)havelowest

bind-ingenergyvalueswhereasallotherconﬁgurationsincludingmixed

oneshaveintermediatevalues.HencestructuralstabilityofBBOP1

isnotonlyhighercomparedwithotherspatialpositionsofBBObut

alsootherformofoxygenation(DBOandMix).Itisinterestingand

ratherclearfromFig.2thatspatialpositionofDBOdoesnoteffect

bindingenergy.However,incaseofthreedifferentspatialpositions

ofMixoxygenconﬁgurationswhichcontainbothBBOandDBO,

variationinbindingenergyisinﬂuencedmainlybythepresenceof

BBO.

Forclearer interpretationof bindingenergycomparision,we

sortedoutourresultsinfurthertwogroups.Hence,Fig.2(b)and(c)

shownanocrystalswithhighestandlowestbindingenergyvalues

respectively,extractedfromFig.2a.Thedifferenceinbinding

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Fig.1. Optimizedstructuresofnon-hydroxylatedSi29O8H20,Si29O10H16andhydroxylatedSi29O10H24,Si29O28H20nanocrystalswithBBO,DBO,OHandMixconﬁgurations.

Forexample,takingintoaccountfornanocrystalscontaining8

oxy-genatoms,bindingenergyperatomdeferenceis0.27eVwhereasin

caseof2(c)itis0.15eV.Tobemorespeciﬁc,anaverageinterspecies

bindingenergydifferenceisabout0.18eV/atom,whichcannotbe

neglected.Itisalsoevidentfromﬁgurethatthereisa

competi-tivedifferenceofbindingenergiesbetweenBBOandMixwhichis

againduetothepresenceofBBOinMixnanocrystals(asis

dis-cussedabove).ItisworthmentioningthatspatialpositionsofDBO

donotmuchalterenergiesasshowninbindingenergycomparison

ofFig.2.

Insetdiagram of Fig.2(c) shows bindingenergy analysisfor

hydroxylatedsiliconnanocrystalsincludingthepresenceofother

oxidizingagentsi.e.BBO,DBOandMix.Itisobservedthatthetrend

ofcohesivecharacteristicsdonotmuchchangebutenergy

low-erscomparedtonon-hydroxylatednanocrystals(seeFig.2aandc).

Numberofoxygenatomsshownininsetis10and28respectively

whichissumofoxygenatomsfromOHgroupandother

oxidiz-ingagentshencetheseenergiesmaycomparewiththeresultsof

non-hydroxylatedNCshaving6and8oxygenatomsrespectively.

3.2. Electronicproperties

Wenowfocusontheelectronicbehaviorofnanocrystalshence

Fig.3showsHOMO-LUMOgapaccordingtothenumberof

oxy-genatoms forallpossible combinationsofNCs withrespectto

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Table1

Detailsofstoichiometricallyidenticalisomers,oxidizingagents,bindingenergyperheavyatomandHOMO-LUMOgapforallconﬁgurationsunderstudy.

Cluster Formula ModesofBonds B.E.(eV/atom) H-LGap(eV) B.E.(eV/atom) H-LGap(eV) B.E.(eV/atom) H-LGap(eV)

OxygenFree

Si29H36 Si29H36 – 7.09 5.14 – – – –

Oxygenated(withoutOHgroup)

Position-1 Position-2 Position-3

Si29O2H32 Si29(>O)2H32 BBO 6.73 4.66 6.53 3.49 6.66 4.84 Si29( O)2H32 DBO 6.57 3.88 6.57 3.36 – – Si29(>O)1( O)1H32 Mix 6.65 3.95 6.65 3.85 6.65 3.85 Si29O4H28 Si29(>O)4H28 BBO 6.23 3.49 6.04 3.41 – – Si29( O)4H28 DBO 6.11 3.15 6.10 3.25 – – Si29(>O)2( O)2H28 Mix 6.26 3.65 6.08 3.03 6.19 3.42 Si29O6H24 Si29(>O)6H24 BBO 5.90 3.38 5.79 3.35 – – Si29( O)6H24 DBO 5.69 2.96 5.69 2.57 – – Si29(>O)3( O)3H24 Mix 5.91 3.36 5.82 3.28 5.86 2.90 Si29O8H20 Si29(>O)8H20 BBO 5.59 2.77 5.47 2.60 – – Si29( O)8H20 DBO 5.32 2.84 5.32 2.48 – – Si29(>O)4( O)4H20 Mix 5.46 2.94 5.44 2.85 5.46 2.91 Si29O10H16 Si29(>O)10H16 BBO 5.18 5.19 – – – – Si29( O)10H16 DBO 4.98 2.33 – – – – Si29(>O)5( O)5H16 Mix 5.04 2.60 – – – –

Oxygenated(withOHgroup)

Si29O10H24 Si29(>O)6H20(OH)4 BBO 5.87 3.09 – – – –

Si29( O)6H20(OH)4 DBO 5.67 2.56 – – – –

Si29(>O)3( O)3H20(OH)4 Mix 5.74 3.25 – – – –

Si29O28H20 Si29(>O)8(OH)20 BBO 5.60 2.26 – – – –

Si29( O)8(OH)20 DBO 5.45 2.11 – – – –

Si29(>O)4( O)4(OH)20 Mix 5.51 2.38 – – – –

Fig.2. (a)Bindingenergyperheavyatomwithrespecttonumberofoxygenatomsdiagramofallnanocrytalsconsideredwithpossiblemodesofoxygenbonding,spatial positionsandstoichiometriccompositions.(b)and(c)representcompositionswithhighestandlowestbindingenergiesvaluesrespectivelywhereasinsetof(c)shows bindingenergiesofselectedhydroxylatednanocrytals.Magentastarshowsbindingenergyofoxygenfreesiliconnanocrystal,Si29H36forcomparison.

ofoxygenatomsisincreasingthegapisdecreasingsigniﬁcantly.

Allcorrespondingconﬁgurationsof Fig.2areincludedin Fig.3

insamesequence. Unlikethebindingenergies shownin Fig.2,

HOMO-LUMOgapenergiesareratherdispersed(between2.33eV

and4.84eV)and showirregularbehavior asnumber ofoxygen

atomsisincreasingwhichisobviousduetothesensitivenatureof

siliconsurfacewhichcancausedrasticimpactonbandgap.Again,

onecanobserveacompetitionofgapenergiesbetweenBBOP1and

MixP1conﬁgurationswhichcontainlargegapvaluescomparedto

theDBOP1(andDBOP2)andrestoftheNCshaveintermediate

gapvalues.

Fig.3(b)and(c)correspondtonanocrystals withlargestand

smallestHOMO-LUMOgapvalues,respectively,reproducedfrom

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Fig.3. (a)HOMO-LUMOgapwithrespecttonumberofoxygenatomsofnanocrytalswithpossiblemodesofoxygenbonding,spatialpositionsandstoichiometriccompositions. (b)and(c)representnanocrystalswithonlylargestandsmallestHOMO-LUMOgapvaluesrespectively.MagentastarshowsHLgapofoxygenfreesiliconnanocrystal,Si29H36

forcomparison.

betweenBBOandDBOis0.96eVandinFig.3(c)thedifferenceis

just0.13eV.However,wemaydrawanimportantanalogybetween

numberofoxygenatomsandinterspeciesHOMO-LUMOgap

differ-enceinsuchawaythattheenergygapisuniformlydecreasingasfar

astheNCsarefullysaturatedwithoxygen.Inotherword,thegap

differenceis1.12eVincaseofO 2whichisdecreasingconsistently

to0.27eVforO 10case.

Oncemore,itisimportanttointroducecontributionofspatial

position(distribution)ofoxidizingagentswhichcanbeunderstood

whilelookingatbindingenergydifferenceandbandgapdifference

ofnanocrystalsespeciallyDBOcontainingisomers.Forexample,

thereisnodifferenceinbindingenergiesofDBOP1comparedwith

DBOP2(Fig.2a)whereasthegapdifferenceischanged,

dramati-cally,foridenticalcomposition(Fig.3a).HenceourresultsforBBO

supportthedemonstrationofRef.[27]butthisphenomenoncan

alsobeobservedclearly whenDBOorMix oxidizingagentsare

present.Likehydroxylatednanocrystalsalongwithotherformsof

oxygenation,non-hydroxylatednanocrystalsexhibitsimilartrends

(notshownhere).

Fig.4representsHOMOand LUMOenergieswithrespectto

increasingnumber ofoxygenatomsforBBO,DBOand Mix

con-ﬁgurations.HOMOandLUMOenergiesinFig.4acorrespondtothe

nanocrystalswithmaximumenergyvalueswithrespecttothe

spa-tialpositionofoxygenbonds.HOMOlevelisincreasingforBBOand

MixconﬁgurationsbutdecreasingincaseofDBOwhereasLUMO

levelisdecreasingforallconﬁgurationsinauniformwayasfar

asthenumberofoxygennumbersareincreasingwhichis

actu-allydiscriminationbetweenthenatureofDBOandotheroxidizing

constituents.The conﬁgurationsofnanocrystals withminimum

energieslevelsareshowninFig.4bwhichrepresentssimilar

ten-dencyasshowninFig.4aformaximumgapenergyvalues.

Inaddition,werepresentFig.5whichshowsthepartialdensity

ofstatesplotofselectednon-hydroxylated(Si29O8H20,Si29O10H16)

andhydroxylated(Si29O10H24,Si29O28H20)nanocrytalsalongwith

arbitraryposition of BBO, DBO and Mixed conﬁgurations.

Con-cerningBBOandMixedconﬁgurations,thecontributionofsilicon

atoms,near,HOMOisdominantcomparedtotheoxygenatoms

whereasincaseofDBObothsiliconandoxygencontributealmost

equally.Also,onecaneasilynoticefromthisﬁgurethattheLUMO

level rapidlyshifttowardHOMOlevel when BBOis introduced

whichisnotincaseofDBOandlesssigniﬁcantforMix.

Toexploretheeffectofincreasingcoverageonthepreferred

adsorption sites and the energy gaps we have considered a

representativeexampleof “oxygenation”in whichthehydroxyl

units have been added stepwise and we have monitored the

binding energyand HOMO-LUMO gap changesas thecoverage

increases. Namely we have considered two distinct cases (1)

Si29(>O)6H20(OH)4and(2)Si29(>O)6H16(OH)8.Thebindingenergy

andHOMO-LUMO gapfor(1)were5.87eV,and 3.09eV

respec-tively.Forcase(2)inwhich4moreOHwereaddedthebinding

energywaspracticallythesame,5.87eV,whereasthegap,

follow-ingthegeneraltrendwasreducedto2.92eV.Sowemayconclude

thattheincreasedcoveragewillnotaffectthebindingenergyand

thepreferredadsorptionsites,butitwillcontinuetodecreasethe

gap,aswasexpected.

Finally,Fig.6showsgraphicalrepresentationoffrontier

molec-ularorbitalsfortwoisomersofSi29O2H32nanocrystalwithrespect

tospatialpositionsofBBO,DBOandMixconﬁgurations.Withthe

helpofthisrepresentation,wedemonstratedthatspatialposition

signiﬁcantlyeffectlocalizationoffrontierorbitalswhichisfurther

investigatedforvariousoxidizingagents.HenceincaseofBBOof

position-1,HOMOislocalizedonlyonleftpartofNCincludingone

ofthetwooxygenatomswhichalterstheHOMO-1wherethe

distri-butionisonrightsideoftheNCwhereasLUMOislocalizedonentire

NCalongwithbothoxygenatomsandhasoverlapwithUMOs.For

DBOcase,chargedistributionisconsistentlyidenticalinall

consid-eredfrontiermolecularorbitalsandlocalizationcoversbothoxygen

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over-776 S.Niazetal./AppliedSurfaceScience387(2016)771–778

Fig.4.(a)and(b)showdiagramsofHOMOandLUMOenergiescorrespondtothegapsshowninFig.2bandcrespectively.

Fig.5. Partialdensityofstatesplotsofnon-hydroxylatedSi29O8H20,Si29O10H16andhydroxylatedSi29O10H24,Si29O28H20nanocrystalswithBBO,DBO,andMixconﬁgurations

(geometriesshowninFig.1).TheFermilevelhasbeensettozeroforclarity.

lapofHOMOandLUMOlocalizationwhichisdifferentcompared

withtherestofFMOsexceptHOMO-1.

Concerningposition-2,forBBOconﬁguration,HOMOandLUMO

localizedmainlyatrightendoftheNCincludingbothoxygenatoms.

Ontheotherhand,theOMOsandUMOs,localizationdiffersfrom

eachpairandasofHOMOandLUMOwheredistributiononboth

oxygenatomsisnotpresentatall.Likeposition-1,inDBO,the

local-izationofHOMOandLUMOoverlapwherebothdoublybonded

oxygenatomsareincludedandforotherfrontierorbitals

situa-tionisnotmuchdifferentwithvaryingconcentrationofthecharge

distributions.AsfarasconcerntotheMixconﬁguration,HOMO

andLUMOlocalizedstrongly onbottomoftheNCcomparedto

otheroxygenwhichhasverylesspercentageofcharge

distribu-tion.HOMO-1localizedmostlyontheleftincludingbothoxygen

atomsascanalsobeseenincaseofHOMO-2andUMOs.

4. Conclusions

We investigate theoretically, by density functional theory

(DFT/B3LYP)possiblemodesof oxygenbondingsuchas

Bridge-bonded oxygen (BBO), Doubly-bonded oxygen(DBO), hydroxyl

(OH)andMixof theseoxidizingagentsin hydrogenterminated

silicon quantumdotsusing asa representative case oftheSi29

nanocrystal.Duetostoichiometry,allcomparisonsperformedare

unbiasedwithrespecttocompositionwhereasspatialdistribution

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Fig.6. GraphicalrepresentationofvariousfrontiermolecularorbitalsofSi29O2H32isomers.

cohesivecharacteristicsofnanocrytals.Bridgebondedoxygenated

nanocrystalsaccompanied byMixhave higherbindingenergies

and largeelectronicgapcompared tonanocrystals withdoubly

bondedoxygenatoms.Inaddition,itisobservedthatthepresence

ofhydroxylgroupalongwithBBO,DBOandmixedconﬁgurations

furtherlowerselectronicgapsandbindingenergiesandtrends.Itis

alsodemonstratedthatoxidizingconstituentalongwiththeir

spa-tialdistributionsubstantiallyaltersbindingenergy,HOMO-LUMO

gap(upto1.48eV)andlocalizationoffrontierorbitalswithinsame

composition.Formanyscientiﬁcandindustrialapplications,a

suit-ablerangeofgapenergycanbeachievedbyappropriateselection

andimplementationofoxidizingagentand/ortheirspatial

distri-butionwhichmaybeasafealternativeofsizedependentbandgap

tunability.

Acknowledgments

ThisprojectissupportedbyTheScientiﬁcandTechnological

ResearchCouncilofTurkey(TÜB˙ITAK)under2216Research

Fel-lowshipProgrammeforInternationalResearchers.Computational

resourcesfromTurkishAcademicNetworkandInformation

Cen-ter(ULAKB˙IM)andNationalCentreforPhysicsPakistan(NCP)are

gratefullyacknowledged.

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