Si-nanoparticles embedded in solid matrices for solar energy conversion: electronic and optical properties from first principles

Transcription

1 Si-nanoparticles embedded in solid matrices for solar energy conversion: electronic and optical properties from first principles S. Wippermann, M. Vörös, D. Rocca, T. Li, A. Gali, G. Zimanyi, F. Gygi, G. Galli

2 Multiple exciton generation in nanoparticles hν > 2Eg hot exciton Multiple exciton generation (MEG) rate greatly enhanced in colloidal nanoparticles (NPs) over bulk value due to quantum confinement and phonon bottleneck [1] For applications NPs are often embedded in a solid matrix for enhanced carrier mobilities, specific tailoring of electronic properties [2] & mechanical stability However: impact of NP-matrix interface on absorption of sunlight and MEG poorly understood; for applications NPs required with enhanced MEG rates on an absolute energy scale e - -phonon, Auger rec. MEG 1) Si-NPs embedded in a-zns, used as charge transport layer in recent experiments 2) high-pressure phase Si-NPs? single exciton multi-exciton [1] M. Beard, J. Phys. Chem. Lett. 2, 1282 (2011) [2] T. Li, F. Gygi, G. Galli, Phys. Rev. Lett. 107, (2011) Motivation Si-NPs in a-zns Si high-pressure phases Summary 01/10

3 Embedding Si-nanoparticles in a-zns Replace spherical region in 4x4x4 ZnS unit cell by Si (d = 1.1, 1.3, 1.6, 1.9 nm) & amorphize matrix by first principles molecular dynamics Si123 (1.6nm) DFT-LDA (QBox), EC = 80Ry, τ = 2fs, T=2400K, Si-atoms free to move for T<750K Different starting geometries, equilibration & cooling times lead to very similar structures Formation of sulphur-shell on Si-NP surface observed => Examine interface structure in detail Motivation Si-NPs in a-zns Si high-pressure phases Summary 02/10

4 Interface between Si-nanoparticle and a-zns Si atoms 4-fold coordinated Zn/S atoms in matrix mostly 4-fold coordinated S atoms at interface mostly 3-fold coordinated => S lone pairs on NP-surface Occasional ZnS dimer/chain formation on Si-NP surface bond configurations 1/2 3/2 1/2 3/2 1/2 2 3/2 3/2 S Zn 1/ Si NP-surface Si => Look at electronic properties Motivation Si-NPs in a-zns Si high-pressure phases Summary 03/10

5 Motivation Si-NPs in a-zns Si high-pressure phases Summary 04/10 Electronic & optical properties Absorption of composite structure redshifted compared to bulk a-zns Si-NP draws S from matrix => formation of small Zn-clusters Near-gap states at Zn-clusters HOMO/LUMO localized around strained Zn-Zn and Zn-Si bonds (near-)homo states of NP involve S lone pairs at NP-surface => strong gap reduction of NP => Extract NP from matrix to separate NP- and matrix-contributions

6 Motivation Si-NPs in a-zns Si high-pressure phases Summary 05/10 Extracted nanoparticles Compare NP extracted from MD with fully relaxed sulphurized NP of same size Very close agreement for spectra and gaps => electronic & optical prop. dominated by S-shell Si35 extracted from MD Si35 + S-shell Replace part of S-shell in fully relaxed model by Zn Duplicate interfacial features observed in MD (dimers, chains) Without strain hardly any difference in spectra Remaining difference between spectra of extracted & sulphurized NPs caused by strain

7 Phases of Si under pressure β-tin (Si-II) Si-I pressure (~12 GPa) R8 (Si-XII) gradual pressure release (~8 GPa) composite Si-nanoparticle structure has always larger gap than bulk-si smaller gaps required for efficient MEG experimental evidence for BC8 Si- NPs in a-si/sio2 layered structure, stable at ambient conditions [3] => investigate Si-NPs made from high pressure phases [3] T. Arguirov et al., Appl. Phys. Lett. 89, (2006) ambient pressure hex. diam. (Si-IV) Motivation Si-NPs in a-zns Si high-pressure phases Summary BC8 (Si-III) annealing (470 K) 06/10

8 Gap vs. nanoparticle-size dependence Known-to-exist BC8 nanoparticles feature significantly smaller gap than diamond-like Si NPs Support by GW calculations (indicated by crosses/stars) Si-I hex. dia. R8 BC8 Si35H36 Si36H40 Si34H36 Si34H38 Motivation Si-NPs in a-zns Si high-pressure phases Summary 07/10

9 Optical absorption spectra (TDDFPT) Known-to-exist BC8 nanoparticles feature significantly smaller gap than diamond-like Si NPs Support by GW calculations Optical spectra exhibit significantly lower absorption onset for BC8-NPs compared to Si-I NPs => What about MEG? Si-I hex. dia. R8 BC8 Si35H36 Si36H40 Si34H36 Si34H38 Motivation Si-NPs in a-zns Si high-pressure phases Summary 08/10

10 Impact Ionization rates Diamond-like Si-I nanoparticles feature highest impact ionization rates on a relative energy scale However: BC8 nanoparticles win significantly over Si-I NPs on an absolute energy scale, due to lower gap Γ (1/ps) BC8 76 R8 76 HXD 66 diamond 66 diamond 78 relative energy scale Γ (1/ps) BC8 76 R8 76 HXD 66 diamond 66 diamond 78 absolute energy scale Energy/E g Motivation Si-NPs in a-zns Si high-pressure phases Summary Energy (ev) 09/10

11 Summary Performed first principles molecular dynamics (MD) calculations for 1.1nm - 1.9nm sized Si-nanoparticles (NPs) embedded in a-zns Sulphur-shell formation on NP-surface with 3-fold coordination => lone pairs Optical & electronic properties of embedded NPs dominated by sulphur-shell and interfacial strain; implications for other S-based matrices and ligands (i. e. EDT) Investigated electronic and optical properties of Si-NPs made from different Si highpressure phases BC8-phase NPs feature smaller gap, lower optical absorption onset and higher impact ionization rates on an absolute scale than Si-I NPs Thanks to my coworkers and thank you for your attention! Poster: K , 2-5pm, Tuesday Feb. 28, Exhibit Hall C NSF/Solar DMR NISE-project Motivation Si-NPs in a-zns Si high-pressure phases Summary 10/10

12 Motivation Si-NPs in in a-zns Si high-pressure phases Summary 01/14 Isodensity plots LUMO+2 HOMO-2 HOMO-12 HOMO-1 LUMO+1 HOMO LUMO