An In Intro troduction to to Computational Nan anoscience Lin - PowerPoint PPT Presentation

An In Intro troduction to to Computational Nan anoscience Lin in-Wang Wang ng Material Sc Science Div ivision Lawrence Berk rkeley Natio tional Lab US S Department t of of Ene nerg rgy Offic ffice of of Sc Scie ience • What can n we do o ? • How ow do o we do o it it ? • Examples Contact: linwang wang, lwwang@lbl.gov

Making ne new soli olid sta tate te mate terials A 2 B • New cry rystal com ompounds 2 • All lloys A  x B 1 x • Im Impurity and nd dop oping • Modify fying th the siz ize and nd shape of of th the mate terial Contact: linwang wang, lwwang@lbl.gov

Nanostructure as a ne new mate terial Defi finiti tion: Nanostr tructure is is an n assembly of of nanometer scale “building blocks”. Why na nano nometer scale: This is is is th the scale le when th the properties of these “building blocks” become different fro from bulk lk. siz ize Ele lectron Wavefuncti tion Nanostructure Bot oth are re in in na nano nometers Contact: linwang wang, lwwang@lbl.gov

Com omputati tional challenge ato toms na nanostr tructures bul ulk mol olecules In Infi finite 1-100 100 1000-10^6 1000 10^6 (1-10 atom (1 toms siz ize ato toms ato toms in a unit in nit cell ll) • Ab ini initio meth thod Challenge for for Ab ini initio method • Effe ffective mass com omputati tional method meth thod na nanoscience. 3 ( ) O N Ab ini initio New meth thodology Even lar larger ele lements ts and nd algo lgorithm Su Supercomputer and nd re reli liability (E (ES!) Contact: linwang wang, lwwang@lbl.gov

Computational methods: accuracy versus size GW/BSE, Coupled Cluster Accuracy Time dependent DFT Direct LDA Non-selfconsistent LDA Empirical Pseudopotential 10 1 10 2 10 3 10 4 10 5 10 6 Number of atoms Contact: linwang wang, lwwang@lbl.gov

Ab ini initio dens nsity fun functional calc lculations 1       2 { ( )} ( ) ( ) V r r E r i i i 2 Selfconsistency  { }  1 ,.., i i N N ele lectron N wave fun functions N     2 Se ( ) | ( ) | r r i i Density Functional ( r ) V Contact: linwang wang, lwwang@lbl.gov

Two tas tasks for for a hybrid na nano no com omputation meth thod nsity  (1) (1) To o ge get t th the pote otential V(r (r) [or [o r th the charge dens (r)] (r) so o we will ill have th the Hamiltonian. (We want ab ini initi tio re reli liability, but t no not t a full full ab ini initi tio calculation) (2) To (2) o solv olve th the sing ingle part rticle Hamiltonian (Schroedinger’s equation), to get the physical properties. 1       2 { ( )} ( ) ( ) V r r E r i i i 2 (Not the usual PDE, many eigen states, don’t want and ne need to to solv olve all ll of of th them) Contact: linwang wang, lwwang@lbl.gov

Charge patc tching meth thod Non on-selfconsistent LDA Se Selfconsistent LDA quality pote otential for for ca calculation of of a a singl ingle nanotube na graphite sheet gra Get t info information fro from small syste tem ab in initi itio calc., th then n ge gene nerate th the charge dens nsities for for lar large syste tems Contact: linwang wang, lwwang@lbl.gov

Charge patc tching: fre free stan tanding quantu tum dots ots In In 675 675 P 652 LDA quality calc lculations (e (eig igen ene nergy err rror ~ 20 meV) 64 pro rocessors (IB (IBM SP SP3) for for ~ 1 hou our r Total charge density CBM VBM motifs Contact: linwang wang, lwwang@lbl.gov

Pla lanewave expansion of of th the wavefunction 1       2 { ( )} ( ) ( ) V r r E r i i i 2    iqr ( ) ( ) r C q e q Fast t Fou ourier Tra ransformation betw tween real space  (r) re (r) an and Fou ourier space C(q (q). Contact: linwang wang, lwwang@lbl.gov

Fol olded Sp Spectr trum Meth thod 1       2 { ( )} ( ) ( ) V r r E r i i i 2             2 2 ( ) ( ) H H ref i i ref i i i i N Contact: linwang wang, lwwang@lbl.gov

NERSC NERSC: Nati ational Ene nerg rgy Research Sci Scientific Com omputing Cent nter Hop opper, C Cray XE6 machine, 150,0 ,000 com omputing cor ores, 1.3 .3 Peta tafl flops Contact: linwang wang, lwwang@lbl.gov

Examples of of ne new pro roperties • Ban and gap gap inc increase CdSe quantum dot ot • Si Single ele lectr tron effe ffects ts on on tra trans nsport t (C (Cou oulomb blo lockade). • Mechanical pro roperties, surface effe ffects and nd no no dis islocations Contact: linwang wang, lwwang@lbl.gov

Fre ree stan tanding quantum dots ots CdSe quantum dot ot TEM im image • Chemically synth nthesised • In Inte terior atom toms are re in in bulk lk cry rystal stru tructure • Su Surface atom toms are re passivated • Dia iamete ter ~ 20-100 A • A fe few th thou ousand atom toms, beyond ab ini initi tio meth thod Contact: linwang wang, lwwang@lbl.gov

CdSe qua uantum dots ots as as biol iological tags tags • Optic tically mor ore stab table th than dye mole olecules • Can n have multi ltiple colo olors Contact: linwang wang, lwwang@lbl.gov

Qua uantum dot ot wav avefunctions Cro ross secti tion ele lectron wavefunctions Contact: linwang wang, lwwang@lbl.gov

CdSe quantum dot ot re results Contact: linwang wang, lwwang@lbl.gov

CdTe na nano nowire Exp: Calc: Contact: linwang wang, lwwang@lbl.gov

Quantum dot and wire calculations for semiconductor materials IV-IV: Si III-V: GaAs, InAs, InP, GaN, AlN, InN II-VI: CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, ZnO Contact: linwang wang, lwwang@lbl.gov

Pola olarization of of CdSe quantum ro rods Cd CdSe qua quantum rod rods The elec lectron wav avefunctio ions of f a a qua quantum ro rods Contact: linwang wang, lwwang@lbl.gov

Pola olarization of of quantum ro rods (co (continued) (meV) Calc. Expt. 100 40 1.30 t (m 30 80 1.25 shift Stock shif 20 60 1.20 10 40 Stoc 1.15 Energy (eV) 0 2 4 6 8 10 1.2 1.6 2.0 2.4 2.8 Aspect As t ratio tio of of the the qu quantum rod ods 1.10 -1.10 -1.15 -1.20 0.6 -1.25 Polarization 0.4 -1.30 Calc: Expt: -1.35 0.2 -1.40 0.0 -1.45 1.0 1.2 1.4 1.6 1.8 2.0 0 2 4 6 8 10 Aspect Ratio Aspect ratio Contact: linwang wang, lwwang@lbl.gov

Quantum wir ire ele lectronic state tates (c) CBM [111] (a) CBM (xz-plane) x y (d) VBM (b) VBM (xz-plane) d=5.18 nm Contact: linwang wang, lwwang@lbl.gov

CdSe quantum dot: arrow shape (1) CB 1 (2) CB 2 (3) CB 3 L=9.9nm D=2nm (5) VB 2 (4) VB 1 (6) VB 3 Contact: linwang wang, lwwang@lbl.gov

Diff ifferent Bloc loch state tate characte ters for for th the VB state tates VB-1 VB VB VB-2 VB-4 VB VB-3 VB Contact: linwang wang, lwwang@lbl.gov

CdSe tetrapod electronic states Contact: linwang wang, lwwang@lbl.gov

Cor ore/shell quantum dots ots CdSe CdSe/CdS CdSe/CdTe CBM VBM Contact: linwang wang, lwwang@lbl.gov

So Solar cell ll usin ing stab table, abundant, and nd env nv. beni nign mat ZnO/ZnS cor Zn ore/shell wir ire The absorption length is similar to bulk Si, Band gap lowers down further from superlattices. thus similar among f material for solar cell. 23% th theoretical effic fficiency for for sola olar cell ll Contact: linwang wang, lwwang@lbl.gov

CdSe cor ore ins inside CdS S na nano norode Hydrodynamic stra train pro rofi file (re (relaxed using ing VFF) 0.02 .02 0.01 .01 0.00 .00 -0.01 -0.02 Contact: linwang wang, lwwang@lbl.gov

The effe ffect t of of th the CdSe Se cor ore to to ele lectr tron and nd hole ole Wit ithout cor ore Wit ith cor ore Gre reen: ele lectr tron, re red: hole ole Contact: linwang wang, lwwang@lbl.gov

Effe ffect of of surfa urface dip ipole mom oment Cd te term rminated surfa rface Cd and nd Se Se te term rminate ted surf rface Contact: linwang wang, lwwang@lbl.gov

The shift ift of of CdSe cor ore Contact: linwang wang, lwwang@lbl.gov

How to calculate an exciton in nanosystem? Mat aterial A Mat aterial B el elect ctron CBM VBM ho hole What t is is th the bind inding ene nergy of of th the inte interface exciton ? The approach: G GW+BSE Approximation: GW  LDA+C (for (for bulk lk shor ort t ra rang nge effe ffect) P(r) (r) (su (surface imag age po potentia ial, for for lon long ran range effe ffe SE  CI BSE I calculation Contact: linwang wang, lwwang@lbl.gov