Lub ntiaj teb nyob ib puag ncig peb yog qhov sib koom ua ke ntawm ntau qhov tshwm sim thiab cov txheej txheem los ntawm ntau yam kev tshawb fawb, nws yog qhov ua tsis tau zoo los ua ib qho tseem ceeb tshaj plaws. Txawm tias muaj qee qhov kev sib tw, ntau yam ntawm qee yam kev tshawb fawb muaj cov yam ntxwv zoo sib xws. Cia peb coj cov duab geometry ua piv txwv: txhua yam peb pom muaj qee yam zoo li, uas ib qho ntawm feem ntau nyob rau hauv qhov xwm txheej yog lub voj voog, lub voj voog, ib puag ncig, lub pob (qhov sib txawv ntawm lub ntsej muag). Lub siab xav ua spherical yog tshwm sim nyob rau hauv ob lub ntiaj teb thiab atomic pawg. Tab sis ib txwm muaj kev zam rau cov cai. Cov kws tshawb fawb los ntawm University of Leuven (Belgium) pom tias kub atoms tsis yog kheej kheej, tab sis cov pyramidal pawg. Dab tsi ua rau tus cwj pwm txawv txawv ntawm cov atoms kub, cov khoom zoo li cas cov pyramids muaj, thiab qhov kev tshawb pom no tuaj yeem siv rau hauv kev xyaum li cas? Peb kawm txog qhov no los ntawm kev tshaj tawm ntawm cov kws tshawb fawb. Mus.
Kev tshawb fawb hauv paus
Lub hav zoov ntawm txawv pawg ntawm kub atoms tau paub rau qee lub sijhawm. Cov qauv no muaj cov khoom siv tshuaj lom neeg thiab hluav taws xob txawv txawv, uas yog vim li cas kev txaus siab rau lawv tsuas yog nce ntau xyoo. Feem ntau cov kev tshawb fawb tau tsom mus rau txoj kev tshawb fawb ntawm qhov loj me, tab sis qhov kev kawm zoo li no yuav tsum muaj kev tswj xyuas kev sib txuas thiab kev ntsuas siab.
Lawm, muaj ntau hom pawg, tab sis qhov nrov tshaj plaws rau kev kawm yog Au20, uas yog, pawg ntawm 20 kub atoms. Nws cov koob meej yog vim nws cov symmetrical heev tetrahedral * qauv thiab kuj ceeb tias loj HOMO-LUMO (HL) by gap (gap)*.
Tetrahedron * - polyhedron nrog plaub daim duab peb sab li ntsej muag. Yog tias peb xav txog ib lub ntsej muag los ua lub hauv paus, ces lub tetrahedron tuaj yeem hu ua daim duab peb sab pyramid.
HOMO-LUMO gap (gap)* - HOMO thiab LUMO yog hom molecular orbitals (ib qho kev ua lej uas piav qhia txog tus cwj pwm yoj ntawm electrons hauv ib lub molecule). HOMO stands rau siab tshaj plaws nyob rau hauv molecular orbital, thiab LUMO stands rau qis tshaj unoccupied molecular orbital. Cov electrons ntawm ib lub molecule nyob rau hauv lub xeev av sau tag nrho cov orbitals nrog lub zog qis tshaj. Lub orbital uas muaj lub zog siab tshaj plaws ntawm cov ntim puv yog hu ua HOMO. Nyob rau hauv lem, LUMO yog lub zog qis tshaj orbital. Lub zog sib txawv ntawm ob hom orbitals no hu ua HOMO-LUMO qhov sib txawv.
Photoelectron spectroscopy ntawm Au20 qhia tias HOMO-LUMO qhov sib txawv yog 1.77 eV.
Sim ua raws li lub zog ua haujlwm ua haujlwm (ib txoj kev rau kev suav nrog lub zog hluav taws xob ntawm TD symmetry (tetranscedral symmetry), uas yog qhov ruaj khov kho geometry rau au20 pawg.
Cov kws tshawb fawb sau tseg tias kev tshawb fawb yav dhau los ntawm Au20 tau muab cov txiaj ntsig tsis raug vim qhov nyuaj ntawm cov txheej txheem. Yav dhau los, kev xa xov xaim hluav taws xob siv lub tshuab ntsuas hluav taws xob tau siv, lub zog siab ntawm lub nqaj cuam tshuam cov txiaj ntsig kev soj ntsuam: qhov hloov pauv tsis tu ncua ntawm Au20 tau pom ntawm cov qauv sib txawv. Hauv 5% ntawm cov duab tau txais, Au20 pawg yog tetrahedral, thiab nyob rau hauv tas li ntawd nws geometry yog kiag li tsis meej pem. Yog li ntawd, lub hav zoov ntawm tetrahedral Au20 qauv ntawm lub substrate ua los ntawm, piv txwv li, amorphous carbon yuav tsis tau hu ua XNUMX% pov thawj.
Hauv txoj kev tshawb no peb tab tom tshuaj xyuas niaj hnub no, cov kws tshawb fawb tau txiav txim siab siv txoj hauv kev zoo dua rau kev kawm Au20, uas yog scanning tunneling microscopy (STM) thiab scanning tunneling spectroscopy (STS). Cov khoom ntawm kev soj ntsuam yog Au20 pawg ntawm ultrathin NaCl films. STM tau tso cai rau peb kom paub meej tias daim duab peb sab symmetry ntawm cov qauv pyramidal, thiab STS cov ntaub ntawv ua rau nws muaj peev xwm los laij cov HOMO-LUMO qhov sib txawv, uas yog ntau npaum li 2.0 eV.
Kev npaj kawm
NaCl txheej tau loj hlob ntawm Au(111) substrate siv tshuaj vapor deposition ntawm 800 K nyob rau hauv ib tug STM chamber nyob rau hauv ultrahigh nqus.
Au20 pawg ions tau tsim los ntawm kev teeb tsa magnetron sputtering thiab xaiv qhov loj me uas siv lub lim dej quadrupole. Lub sputtering qhov chaw ua haujlwm nyob rau hauv kev nruam thiab tsim ib feem loj ntawm cov pawg them nyiaj, uas tom qab nkag mus rau lub lim dej quadrupole. Cov pawg xaiv tau muab tso rau hauv NaCl/Au(111) substrate. Rau qhov tsawg-density deposition, pawg flux yog 30 pA (picoamps) thiab lub sij hawm deposition yog 9 feeb; rau high-density deposition, nws yog 1 nA (nanoamps) thiab 15 feeb. Lub siab nyob rau hauv lub chamber yog 10-9 mbar.
Cov txiaj ntsig tshawb fawb
Cov pawg-xaiv anionic Au20 pawg uas muaj kev tiv thaiv tsawg heev tau muab tso rau ntawm chav tsev kub mus rau ultrathin NaCl Islands tuaj, suav nrog 2L, 3L, thiab 4L (atomic txheej).
Duab #1
rau 1A Nws tuaj yeem pom tau tias feem ntau ntawm NaCl loj hlob muaj peb txheej, thaj chaw uas muaj ob thiab plaub txheej nyob ib cheeb tsam me me, thiab 5L thaj chaw tau xyaum tsis tuaj.
Au20 pawg tau pom nyob rau hauv peb- thiab plaub-txheej cheeb tsam, tab sis tsis nyob hauv 2L. Qhov no tau piav qhia los ntawm qhov tseeb tias Au20 tuaj yeem dhau los ntawm 2L NaCl, tab sis nyob rau hauv rooj plaub ntawm 3L thiab 4L NaCl, nws tau khaws cia rau ntawm lawv qhov chaw. Thaum tsis tshua muaj txheej ntom ntom hauv thaj tsam ntawm 200 x 200 nm, los ntawm 0 mus rau 4 pawg tau pom tsis muaj cov cim qhia ntawm Au20 agglomeration (sau).
Vim tias qhov kev tiv thaiv siab dhau ntawm 4L NaCl thiab qhov tsis ruaj khov thaum luam ib zaug Au20 ntawm 4L NaCl, cov kws tshawb fawb tau tsom mus rau kev kawm pawg ntawm 3L NaCl.
Duab #2
Microscopy ntawm pawg hauv 3L NaCl pom tias lawv qhov siab yog 0.88 Β± 0.12 nm. Daim duab no yog nyob rau hauv kev pom zoo zoo nrog cov qauv tsim, uas kwv yees qhov siab ntawm 0.94 Β± 0.01 nm (2A). Microscopy kuj qhia tau hais tias qee pawg muaj ib daim duab peb sab nrog ib lub atom protruding nyob rau saum toj, uas nyob rau hauv kev xyaum qhia tau hais tias theoretical kev tshawb fawb txog lub pyramidal zoo ntawm lub Au20 qauv (2B).
Cov kws tshawb fawb sau tseg tias thaum pom cov khoom peb-seem me me, xws li Au20 pawg, nws nyuaj heev kom tsis txhob muaj qee qhov tsis raug. Txhawm rau kom tau txais cov duab tseeb tshaj plaws (ob qho tib si los ntawm atomic thiab geometric point of view), nws yuav tsum tau siv lub tswv yim atomically ntse Cl-functionalized microscope tip. Lub pyramidal duab tau txheeb xyuas hauv ob pawg (1B ΠΈ 1S), peb-dimensional dluab uas muaj nyob rau hauv 1D ΠΈ 1E, feem.
Txawm hais tias daim duab peb sab thiab qhov siab faib tawm qhia tau tias cov pawg tso nyiaj khaws cia cov duab pyramidal, STM duab (1B ΠΈ 1S) tsis qhia cov qauv tetrahedral zoo meej. Lub kaum sab xis loj tshaj plaws hauv daim duab 1B yog 78Β°. Thiab qhov no yog 30% ntau tshaj 60 Β° rau ib qho zoo tagnrho tetrahedron nrog Td symmetry.
Tej zaum yuav muaj ob qho laj thawj rau qhov no. Ua ntej, muaj qhov tsis raug ntawm qhov kev ntsuas nws tus kheej, tshwm sim los ntawm ob qho tib si los ntawm qhov nyuaj ntawm cov txheej txheem no thiab los ntawm qhov tseeb tias lub hau ntawm lub koob tsom iav tsis nruj, thiab qhov no tuaj yeem cuam tshuam cov duab. Qhov thib ob yog vim li cas vim qhov kev cuam tshuam sab hauv ntawm Au20 txhawb nqa. Thaum Au20 pawg nrog Td symmetry av ntawm ib square NaCl lattice, lub symmetry mismatch distorts lub zoo tagnrho tetrahedral qauv ntawm Au20.
Txhawm rau kom paub tias vim li cas rau qhov sib txawv ntawm cov duab, cov kws tshawb fawb tau txheeb xyuas cov ntaub ntawv ntawm symmetry ntawm peb qhov zoo tshaj plaws Au20 qauv ntawm NaCl. Raws li qhov tshwm sim, nws tau pom tias cov pawg tsuas yog me ntsis cuam tshuam los ntawm cov qauv tetrahedral zoo tagnrho nrog Td symmetry nrog qhov sib txawv siab tshaj plaws hauv atomic txoj haujlwm ntawm 0.45. Yog li ntawd, distortions nyob rau hauv cov duab yog tshwm sim los ntawm inaccuracies nyob rau hauv lub imaging txheej txheem nws tus kheej, thiab tsis yog ib tug deviations nyob rau hauv lub deposition ntawm pawg nyob rau hauv lub substrate thiab / los yog kev sib cuam tshuam ntawm lawv.
Tsis tsuas yog cov ntaub ntawv saum toj kawg nkaus yog cov cim qhia tseeb ntawm cov qauv pyramidal ntawm Au20 pawg, tab sis kuj muaj qhov sib txawv HL loj (li 1.8 eV) piv rau lwm yam Au20. isomers * nrog qis zog (hauv txoj kev xav hauv qab 0.5 eV).
Isomers * - cov qauv uas zoo ib yam nyob rau hauv atomic muaj pes tsawg leeg thiab molecular hnyav, tab sis txawv nyob rau hauv lawv cov qauv los yog kev npaj ntawm atoms.
Kev soj ntsuam ntawm cov khoom hluav taws xob ntawm pawg tso rau ntawm lub substrate siv scanning tunneling spectroscopy (1F) ua kom tau txais qhov sib txawv conductivity spectrum (dI/dV) ntawm Au20 pawg, uas qhia tau hais tias muaj qhov sib txawv loj (piv txwv li) sib npaug ntawm 3.1 eV.
Txij li cov pawg yog hluav taws xob sib cais los ntawm insulating NaCl films, ib tug ob-barrier tunnel junction (DBTJ) yog tsim, uas ua rau ib leeg-electron tunneling teebmeem. Yog li, qhov tsis sib xws hauv dI / dV spectrum yog qhov tshwm sim ntawm kev sib koom ua haujlwm ntawm quantum HL discontinuity (EHL) thiab classical Coulomb zog (Ec). Kev ntsuas ntawm kev tawg hauv cov spectrum pom los ntawm 2.4 txog 3.1 eV rau xya pawg (1F). Qhov pom kev tsis sib xws yog loj dua li HL discontinuities (1.8 eV) hauv Au20 roj theem.
Qhov sib txawv ntawm kev tawg hauv cov pawg sib txawv yog vim qhov kev ntsuas nws tus kheej (qhov chaw ntawm rab koob txheeb ze rau pawg). Qhov sib txawv loj tshaj plaws ntsuas hauv dI / dV spectra yog 3.1 eV. Nyob rau hauv cov ntaub ntawv no, lub ntsis yog nyob deb ntawm cov pawg, uas ua rau cov hluav taws xob capacitance ntawm lub ntsis thiab cov pawg tsawg tshaj li ntawm cov pawg thiab Au(111) substrate.
Tom ntej no, peb tau ua cov kev suav ntawm HL ruptures ntawm dawb Au20 pawg thiab cov nyob rau ntawm 3L NaCl.
Graph 2C qhia qhov sib txawv ntawm cov xeev nkhaus rau cov roj-theem Au20 tetrahedron uas nws qhov sib txawv HL yog 1.78 eV. Thaum cov pawg nyob rau ntawm 3L NaCl/Au(111), distortions nce thiab qhov sib txawv HL txo los ntawm 1.73 mus rau 1.51 eV, uas yog piv rau HL qhov sib txawv ntawm 2.0 eV tau txais thaum kev sim ntsuas.
Hauv kev tshawb fawb yav dhau los, nws tau pom tias Au20 isomers nrog Cs-symmetric qauv muaj qhov sib txawv HL ntawm 0.688 eV, thiab cov qauv nrog amorphous symmetry - 0.93 eV. Ua raws li cov kev soj ntsuam no thiab cov txiaj ntsig ntawm kev ntsuas, cov kws tshawb fawb tuaj txog qhov xaus tias qhov sib txawv loj loj tuaj yeem ua tau tsuas yog nyob rau hauv cov xwm txheej ntawm cov qauv tetrahedral pyramidal.
Cov theem tom ntej ntawm kev tshawb fawb yog kev tshawb fawb ntawm pawg-cluster kev sib cuam tshuam, uas ntau Au3 (nce qhov ntom) tau tso rau ntawm 111L NaCl / Au (20) substrate.
Duab #3
Ntawm daim duab 3A daim duab saum toj kawg nkaus STM ntawm cov pawg tso cia tau pom. Txog 100 pawg tau pom nyob rau hauv cheeb tsam scanning (100 nm x 30 nm). Qhov ntau thiab tsawg ntawm kev sib cuam tshuam pawg ntawm 3L NaCl yog ntau dua lossis sib npaug rau qhov ntau thiab tsawg ntawm cov uas tau kawm hauv kev sim nrog ib pawg. Qhov no tuaj yeem piav qhia los ntawm diffusion thiab agglomeration (clumping) ntawm qhov chaw ntawm NaCl ntawm chav tsev kub.
Kev sib sau thiab kev loj hlob ntawm pawg tuaj yeem piav qhia los ntawm ob lub tswv yim: Ostwald ripening (recondensation) thiab Smoluchowski ripening (loj ntawm cov Islands tuaj). Nyob rau hauv cov ntaub ntawv ntawm Ostwald ripening, pawg loj loj hlob ntawm tus nqi ntawm cov me me, thaum atoms tom kawg raug cais los ntawm lawv thiab diffuse mus rau cov neeg nyob sib ze. Thaum lub sij hawm Smoluchowski ripening, cov khoom loj yog tsim los ntawm kev tsiv teb tsaws thiab agglomeration ntawm tag nrho pawg. Ib hom ripening tuaj yeem txawv ntawm lwm qhov raws li hauv qab no: nrog Ostwald ripening, kev faib tawm ntawm pawg loj thiab txuas ntxiv, thiab nrog Smoluchowski ripening, qhov loj yog faib discretely.
Ntawm kab kos 3B ΠΈ 3S cov txiaj ntsig ntawm kev tsom xam ntawm ntau dua 300 pawg tau pom, piv txwv li. kev faib tawm loj. Qhov ntau ntawm kev soj ntsuam pawg qhov siab yog qhov dav, tab sis peb pawg ntawm cov feem ntau tuaj yeem paub qhov txawv (3S0.85, 1.10 thiab 1.33 nm.
Raws li tuaj yeem pom hauv daim duab 3B, muaj kev sib raug zoo ntawm tus nqi ntawm qhov siab thiab qhov dav ntawm pawg. Cov kab ke pom tau pom cov yam ntxwv ntawm Smoluchowski maturation.
Kuj tseem muaj kev sib raug zoo ntawm cov pawg hauv qhov kev sim siab siab thiab qis qis. Yog li, ib pawg ntawm pawg nrog qhov siab ntawm 0.85 nm yog zoo ib yam nrog ib tus neeg pawg nrog qhov siab ntawm 0.88 nm hauv kev sim nrog tsawg ntom. Yog li ntawd, pawg los ntawm thawj pab pawg tau muab tus nqi Au20, thiab pawg los ntawm qhov thib ob (1.10 nm) thiab thib peb (1.33 nm) tau muab cov txiaj ntsig Au40 thiab Au60, feem.
Duab #4
Daim duab 4A peb tuaj yeem pom qhov sib txawv ntawm peb pawg ntawm pawg, dI / dV spectra uas tau qhia hauv daim duab 4B.
Raws li Au20 pawg sib koom ua ke rau hauv qhov sib txawv ntawm lub zog loj dua hauv spectrum, dI / dV txo. Yog li, rau txhua pab pawg tau txais cov txiaj ntsig tsis sib xws hauv qab no: Au20β3.0 eV, Au40β2.0 eV, thiab Au60β1.2 eV. Ua tsaug rau cov ntaub ntawv no, nrog rau cov duab saum toj kawg nkaus ntawm pawg kawm, nws tuaj yeem sib cav tias cov geometry ntawm pawg agglomerates yog ze rau spherical lossis hemispherical.
Txhawm rau kwv yees tus naj npawb ntawm cov atoms hauv pawg kheej kheej thiab hemispherical, koj tuaj yeem siv Ns = [(h / 2) / r]3 thiab Nh = 1/2 (h / r) 3, qhov twg h ΠΈ r sawv cev rau pawg qhov siab thiab lub vojvoog ntawm ib qho Au atom. Noj mus rau hauv tus account lub Wigner-Seitz vojvoog rau lub kub atom (r = 0.159 nm), peb tuaj yeem xam lawv tus lej rau qhov kwv yees kwv yees: pawg thib ob (Au40) - 41 atoms, pawg thib peb (Au60) - 68 atoms. Hauv kev kwv yees hemispherical, kwv yees tus naj npawb ntawm atoms 166 thiab 273 yog qhov siab dua hauv Au40 thiab Au60 hauv qhov kwv yees kwv yees. Yog li, nws tuaj yeem xaus lus tias geometry ntawm Au40 thiab Au60 yog kheej kheej es tsis yog hemispherical.
Yog xav paub ntxiv txog qhov nuances ntawm txoj kev tshawb no, kuv pom zoo kom mus saib ΠΈ rau nws.
Epilogue
Nyob rau hauv txoj kev tshawb no, cov kws tshawb fawb tau ua ke scanning tunneling spectroscopy thiab microscopy, uas tso cai rau lawv kom tau txais cov ntaub ntawv tseeb dua hais txog cov geometry ntawm pawg kub atoms. Nws tau pom tias Au20 pawg tso rau ntawm 3L NaCl / Au (111) substrate khaws nws cov roj-theem pyramidal qauv nrog qhov sib txawv HL loj. Nws kuj tau pom tias lub ntsiab mechanism ntawm kev loj hlob thiab kev koom tes ntawm pawg rau hauv pawg yog Smoluchowski maturation.
Cov kws tshawb fawb hu ua ib qho ntawm cov txiaj ntsig tseem ceeb ntawm lawv txoj haujlwm tsis yog ntau qhov txiaj ntsig ntawm kev tshawb fawb ntawm atomic pawg, tab sis yog txoj hauv kev los ua qhov kev tshawb fawb no. Yav dhau los, kev xa xov hluav taws xob siv hluav taws xob tsom iav siv, uas, vim nws cov khoom, cuam tshuam cov txiaj ntsig ntawm kev soj ntsuam. Txawm li cas los xij, txoj kev tshiab tau piav qhia hauv txoj haujlwm no tso cai rau peb kom tau txais cov ntaub ntawv raug.
Ntawm lwm yam, kev kawm pawg qauv tso cai rau peb nkag siab txog lawv cov catalytic thiab optical zog, uas yog qhov tseem ceeb heev rau lawv siv hauv pawg catalysts thiab optical devices. Tam sim no, pawg twb tau siv rau hauv roj hlwb thiab ntes cov pa roj carbon. Txawm li cas los xij, raws li cov kws tshawb fawb lawv tus kheej, qhov no tsis yog qhov txwv.
Ua tsaug rau kev nyeem ntawv, nyob xav paub thiab muaj lub limtiam zoo. π
Ib co ads π
Ua tsaug uas koj tau nyob nrog peb. Koj puas nyiam peb cov ntawv? Xav pom cov ntsiab lus nthuav ntxiv? Txhawb nqa peb los ntawm kev tso ib qho kev txiav txim lossis qhia rau cov phooj ywg, , ib qho tshwj xeeb analogue ntawm nkag-theem servers, uas tau tsim los ntawm peb rau koj: (muaj nrog RAID1 thiab RAID10, mus txog 24 cores thiab mus txog 40GB DDR4).
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