Molecular dynamics simulation studies of material modification by charged particle irradiation

2004·4.8 MB·English

by  Jang, Inkook

MOLECULARDYNAMICSSIMULATIONSTUDIESOFMATERIAL
MODIFICATIONBYCHARGEDPARTICLEIRRADIATION
By
INKOOKJANG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2004
Copyright2004
by
InkookJang
ACKNOWLEDGMENTS
First,Iwouldliketoshowmysinceregratitudetomyadvisor,Dr.SusanB.Sinnott,
forherconstantguidanceandencouragement.Hersupportandinstanthelpthroughout
mygraduatecareerlaidasmoothwayformystudiesandresearchwork,andenabledme
toachievemygoal.IamalsogreatlythankfultoallthemembersinDr.Sinnott’s
researchgroupforacademicdiscussionandcheerfultalks.
Finally,Iwishtoexpressmygreatappreciationtomybelovedparents.
hi
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
iii
LISTOFTABLES vi
LISTOFFIGURES vii
ABSTRACT
ix
CHAPTER
GENERALINTRODUCTION
1 1
1.1MolecularSimulations 1
1.2MolecularDynamicsMethod 5
1.3PlasmaProcessing 7
1.3.1PlasmaModificationofPolymer 8
1.3.2FluorocarbonPlasma 8
1.3.3PolyatomicIonDeposition 9
1.4CarbonNanotubesJunctions 10
1.4.1CarbonNanotubes 10
1.4.2JoiningofCarbonNanotube 14
2 SIMULATIONMETHODS 16
2.1TheReactiveEmpiricalBondOrderPotential 16
2.2C-H-FPotential 19
2.3TheLennard-JonesPotential 23
2.4PeriodicBoundaryConditions 24
2.5Predictor-CorrectorAlgorithms 25
2.6TemperatureControlMethods 26
2.7TightBindingMethods 28
3 POLYATOMICIONBEAMDEPOSITIONONPOLYMERSURFACES 29
3.1SimulationDetails 31
3.1.1ContinuousFluorocarbonIonDepositiononPSSurface 32
3.1.2HydrocarbonIonDepositiononPSandPEsurfaces 34
3.2ResultsandDiscussion 34
IV
3.2.1ContinuousFluorocarbonIonDepositiononPS 34
3.3 3.2.2AngularInfluenceofCbHs^IonDepositiononPSSurface 52
3.2.3Polystyrenevs.Polyethylene 59
Conclusions 67
3.3.1ContinuousFluorocarbonIonDepositiononPS 67
3.3.2AngularInfluenceofCsHj^IonDepositiononPSSurface 68
3.3.3Polystyrenevs.Polyethylene 68
4 CARBONNANOTUBEWELDINGUNDERELECTRONBEAMIRRADIATION
70
4.1SimulationDetails 70
4.2ResultsandDiscussion 75
4.3Conclusions 86
5 GENERALCONCLUSIONS 88
LISTOFREFERENCES 91
BIOGRAPHICALSKETCH 101
V
LISTOFTABLES
Table page
22--1. Two-bodyparametersusedinequation(2-2),(2-3),(2-4),and(2-6) 20
32--2. ValuesofPfromequation(2-13)attheintegerpointsusedforcubicspline
interpolation 21
2-3. AtomizationenergiesofvariousmoleculescalculatedbynewC-H-Fpotentialand
semi-empirical(PM3)method 22
4. Lennard-Jonesparameters 23
1. Surfacedensityofindicatedspeciesthatremainbondedtocarbonatomsin,orare
embeddedwithin,thePSsurfacesafterCsFj^deposition 38
3-2. Surfacedensityofindicatedspeciesthatremainbondedtocarbonatomsin,orare
3- embeddedwithin,thePSsurfacesafterCFa"^deposition 38
43--3. YieldofscatteringspeciesforCFa"^deposition 41
3-4. YieldofscatteringspeciesforCaFs"^deposition 41
3-5. Surfacedensityofindicatedspeciesthatremainbondedtocarbonatomsin,orare
embeddedwithin,thePSsurfacesafter105CsFs"^depositionat100eV/ion 51
3-6. Surfacedensityofindicatedspeciesthatremainbondedtocarbonatomsin,orare
embeddedwithin,thePSsurfacesafter105CsFs"^depositionat50eV/ion 51
3-7. Percentageofoccurrenceoftheindicatedeventsaveragedover80trajectoriesat
differentincidentangles 53
8. PercentageofoccurrenceoftheindicatedeventsintheMDsimulationsof
depositionaveragedover80trajectories 60
1. ThedimensionsofeachCNTpairandirradiationarea 74
4-2. SummaryofbondingatthejunctionsformedbetweenthefollowingpairsofCNTs
byelectronbeamirradiationasdescribedinthetext 79
VI
1- LISTOFFIGURES
2-
Figure page
2-
31--1. TheschemetodetermineBravaisvector 11
2. Structuremodelsofcarbonnanotubes 12
1. Schematicrepresentationofperiodicboundarycondition 24
2. Schematicdiagramoftypicalheatbathapplication 27
1. Modelsof(a)PSand(b)PEchains 31
3-2. SchematicrepresentationofinitialPSsubstrate 33
3-3. StructuremodelsofPSsurfacesanddepthprofileofFdensityafterionbeam
depositionat50eV/ion 35
3-4. SnapshotformtheMDsimulationsthatshowsaCF2fragmentlinkingtwoPS
chains 40
3-5. FuptakeandtotaldepositionyieldofFasfunctionoffluenceduringdeposition..42
3-6. DegreeofPSsurfaceetchingduringionbeamdeposition 44
3-7. AveragepenetrationdepthsofindicatedspeciesinthePSsurfaces 46
3-8. AportionofthePSstructureafterdepositionof240CaFs"^ions 47
3-9. StructuremodelsofpartofthePSsurfacesafter105CsFs"^iondepositionat50
eV/ionand100eV/ion 48
3-10.StructuremodelsofPSsurfacesanddepthprofilesofFdensityafter105CsFs”^ion
deposition 49
3-11.Percentagesoftheindicatedproductspeciesthatremainbondedtocarbonatomsin
thePSsurfaceorareembeddedinthePSsurface 55
3-12.SnapshotfromtheMDsimulationsshowingtheproductsofthedissociationofthe
ionintoCH2fragmentsthatbondtothePSsurface 57
vii
3-13.AveragenumberofbrokenbondspertrajectoryinthePSsurfaceasaresultof
depositionattheindicatedanglesoverthe80trajectories 58
3-14.Averagepenetrationdepthsofmajorspecies(C3H5,CaHn,CH2)fromtheincident
ionsasafunctionofangle 59
33--15.Percentagesoftheindicatedproductspeciesthatremainbondedtocarbonatomsin,
orareembeddedwithin,thePSandPEsurfacesat50and25eV 63
4-
3-16.AveragenumberofbrokenbondsinthePSandPEsurfacesperMDtrajectoryasa
resultofCaffs'^depositionat50and25eV 65
17.Averagepenetrationdepthsofmajorspecies(C3H5,C2Hn,CH2)fromtheincident
ions 67
1. Initialconfigurationofcrossednanotubesforelectronbeamirradiation 72
4-2. FinalstructureofeachCNTpairafterelectronbeamirradiationandannealing 76
4-3. Structuremodelsof(a)ideal(5,5)-(5,5)CNTjunctionand(b)hydrogenterminated
(5,5)-(5,5)CNTjunction 80
4-4. ThechangeinenergyasafunctionofstrainduringthedeformationofweldedCNT
pairsafterannealing 81
4-5. DOS(fullcurve)andeigenvalueparticipationratio(histogram)aroundtheFermi
level(shiftedat£=0)forideal(5,5)-(5,5)CNTjunction 83
4-6. DOSandeigenvalueparticipationratioaroundtheFermilevelfor(5,5)-(5,5)CNT
junctionproducedbyelectronbeamirradiation 84
4-7. Plotofthelongitudinaldistributionoftheelectronprobabilitydensityforthefirst
feweigenvalues(«=0,...8)abovetheFermilevel(«=0)ofthe(5,5)-(5,5)CNT
junctionproducedbyelectronbeamirradiation 85
viii
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
MOLECULARDYNAMICSSIMULATIONSTUDIESOFMATERIAL
MODIFICATIONBYCHARGEDPARTICLEIRRADIATION
By
InkookJang
May2004
Chair: SusanB.Sinnott
MajorDepartment: MaterialsScienceandEngineering
Chargedparticleirradiationprocesses,suchasionbeamdepositionandelectron
beamirradiationareoftenusedtomodifythepropertiesofvariousmaterials.However,
detailedreactionmechanismsthatleadtostructuralchangesinthesubstratesaredifficult
todetermineexperimentally,asprimarilyindirectinformationisobtainedusing
experimentalmethods.Computersimulationmethodsarecomplementarytoexperimental
approachesandareabletoprovideinsightintothewayinwhichthesystemreachesits
finalstate.
Inthesestudies,moleculardynamicssimulationsareusedtoinvestigatepolyatomic
ionbeamdepositiononpolymersurfacesandcarbonnanotubeweldingunderelectron
beamirradiation.Theinteratomicforcesinthesimulationsaredeterminedusingthe
reactiveempiricalbondorderpotentialforshort-rangedcovalentinteractionsandthe
Lennard-Jonespotentialforlong-rangevanderWaalsinteractions.Forhydrocarbonion
deposition,someinclinedanglesarefoundtobemoreefficientforthedepositionofthe
IX
precursorsnecessaryforpolymerthin-filmgrowththannormalincidencedeposition.
Also,incidentenergiesof50eV/ionarepredictedtobemoreefficientthan25eV/ion,
andpolystyrene(PS)substratesfacilitatethegrowthofhydrocarbonthinfilmsmorethan
doespolyethylene.
Instudiesoffluorocarboniondeposition,thesimulationsshowhowandwhy
and playdifferentrolesinlow-energyplasmadepositionprocessesonPS.CFs"^
ionsdissociatemorereadilyonimpactandthelargenumberofFatomsthatareproduced
formcovalentbondswiththePSchains,whichbothefficientlyfluorinatesthePSand
slowsdownthinfilmgrowth.Incontrast,CsFs^ionsproducemorefluorocarbonpolymer
filmprecursorsthatenhancethinfilmgrowth,whileatthesametimefluorinatingthe
surfaceataslowerrate.
Inthestudyofcarbonnanotubewelding,thestructureofthegeneratedjunctionsis
characterizedbyalargedegreeofbondingdisorderthatleadstolowerstrengthand
higherductilitythanthosecharacterizing“ideal”junctions.Thegeneratedjunctionsalso
operateasquantumdots,incontrasttotheohmicbehaviorofidealjunctions.
X