Semiconductor Nanocrystals from Nonthermal Plasmas Rebecca J. Anthony University of Minnesota 1
Nanocrystals in devices efficient light emitters and absorbers versatile deposition schemes possibility for flexible devices inexpensive to produce www.nanosolar.com Tan, et al.,journal of Applied Physics, 105, 2009 2
Outline Plasma synthesis and crystallization of silicon nanoparticles Control of surface properties SiNC-based LEDs electrode electron transport layer SiNCs hole transport layer hole injection layer transparent electrode 3
Plasma synthesis Argon + SiH 4 (5:95 in Helium) RF power: 13.56 MHz, 60-80W Pressure: ~ 1.4 Torr (~180 Pa) Gas flowrate: ~ 100 sccm 4
Surface-functionalizing SiNCs As-produced silicon nanoparticles Functionalization scheme Result R liquid-phase refluxing 215 C ligand: 1-dodecene = # photons emitted # photons absorbed L. Mangolini, D. Jurbergs, E. Rogojina, and U. Kortshagen, J. Lum., 2006. 5
Plasma power and nanoparticle crystallinity X-ray diffraction (XRD) 111 220 311 400 Raman spectroscopy plasma power Anthony and Kortshagen, Phys. Rev. B., 2009 6
Crystallinity and photoluminescence QY vs. plasma power PL comparison: amorphous and crystalline nanoparticles crystalline particles efficient PL amorphous particles virtually no PL Anthony and Kortshagen, Phys. Rev. B., 2009 7
Nanocrystal formation Nonthermal plasma p = 100-200 Pa T electrons = 50,000K = 300K T ion, gas H H particle temperature e- Ar + H 2 Ar nanoparticle gas temperature Particle temperature exceeds gas temperature L. Mangolini and U. Kortshagen, Physical Review E, 2009 8
Nanoparticle crystallization: plan 1)Synthesize amorphous NPs of different sizes in first plasma 2)Study conditions necessary to crystallize them in second plasma 3)Measure plasma parameters H density, ion density, electron temperature synthesis plasma low nominal power (15W) crystallization plasma variable power capacitive probe OES 9
Crystallization: 4 nm nanoparticles Crystallization Power: 30 W Amorphous nanoparticles (0 W 2 nd plasma) Nanocrystals (30 W 2 nd plasma) Nanocrystals (50 W 2 nd plasma) 10
Crystallization: 4 nm nanoparticles Crystallization Power: 30 W Raman spectra XRD patterns 40 W 30 W 25 W 20 W p o w er 10 W Next: what are n H, T e, and n i required? 11
n H and T e : Optical emission spectroscopy(oes) 750.4 intensity / a.u. H 2 696 810.4 811.5 840.8 656.8 600 650 700 750 800 850 900 wavelength / nm M. V. Malyshev and V. M. Donnelly, Phys. Rev. E. 1999 OES 12
OES analysis: 4 nm nanoparticles T e ~ 3.7 ev n H ~ 2 x 10 11 cm -3 13
Find n i : capacitive probe measurements Probe: driven by 1 MHz RF signal, modulated by square wave Capacitor: discharge rate to ion flux dv dt = ea p Γ i C p capacitive probe Braithwaite, N.J., Booth, J.P., Cunge, G., Plasma Sources Sci. Technol., 5, 677-684, 1996 14
Capacitive probe measurements p o w er dv dt = ea p Γ i C p k Γ i = n b T e i M Ar 1/2 capacitive probe 15
Probe results ion densities 16
Nanoparticle heating model G(T p, χ) L(T p, χ) = 0 Γ in H Γ out H = 0 Steady-state model of T p Energy balance: G (T p, χ : Heating terms: Ar + +e reactions and H-based reactions L (T p, χ : Cooling terms: conductive heat transfer to background gas and H desorption Hydrogen balance: Incoming H flux = outgoing H flux e- Ar + H H H 2 Ar nanoparticle Galli and Kortshagen, IEEE Transactions on Plasma Science, 38 (4) 2010 17
Nanoparticle heating model: results Nanoparticles only reaching 100-200 K above room T Crystallization cannot occur via thermal routes alone Next: examine possibility of H-induced crystallization 18
Optimizing the Nanocrystal Surface R. J. Anthony, D. J. Rowe, M. Stein, J. Yang, and U. R. Kortshagen, Advanced Functional Materials, 2011 19
Sidearm gas injection VS. L. Mangolini, E. Thimsen, and U. Kortshagen, Nano Letters, 2005 L. Mangolini, D. Jurbergs, E. Rogojina, and U. Kortshagen, J. Lum., 2006 20
What is on the silicon surface? Fourier-transform infrared spectroscopy (FTIR) Si-H x stretch mode Si-H x bend/wag mode 21
Injection gas experiment H 2 (100% Ar) injection gas inlet Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat., 2011 22
H 2 / Ar injection: QY and FTIR PL quantum yield Si-H 3 Si-H 2 Si-H Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat., 2011 23
H 2 / Ar injection: dangling bond density Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat., 2011 24
synthesis zone T SiNC ~ 100 s of K higher than T gas Hypotheses Hypothesis 1: Quench H-desorption Hypothesis 2: Improve H-coverage injection zone 25
PL spectra Gas injection: PL summary PL quantum yield Conclusion to hypothesis 1: quenching is important Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat., 2011 26
Gas injection FTIR: Si-H x stretch region Conclusion to hypothesis 2: H rxn is important Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat., 2011 27
Light-emitting Devices from Silicon Nanocrystals K.-Y. Cheng, R. J. Anthony, U. R. Kortshagen, and R. J. Holmes, Nano Letters, 2010 and 2011 28
Hybrid organic/inorganic light-emitting device E vacuum 2.3 ev LiF/Al 0.5nm/50nm Alq3: ETL SiNCs poly-tpd: HTL PEDOT:PSS-- HIL ITO on glass 20nm 53nm 25nm 30nm energy 4.7 ev ITO 5.2 ev PEDOT :PSS poly- TPD 5.1 ev 3.9 ev SiNCs 5.3 ev 3.1 ev Alq-3 5.8 ev 4.1 ev LiF/Al Cheng, Anthony, Kortshagen, and Holmes, Nano Letters, 2011 29
Electroluminescence near-infrared-emitting SiNCs red-emitting SiNCs Cheng, Anthony, Kortshagen, and Holmes., Nano Letters, 2011 30
Device characteristics peak efficiency: 8.6% External quantum efficiency = # photons emitted # charges injected = η injection x η internal x η extraction Cheng, Anthony, Kortshagen, and Holmes., Nano Letters, 2011 31
All-gas-phase LEDs Green device manufacturing R. J. Anthony, K.-Y. Cheng, Z. C. Holman, R. J. Holmes, and U. R. Kortshagen, Nano Letters, 2012 32
Solution-phase device processing Wet chemistry: Pros: highly successful well-studied flexible + R e.g., 1-dodecene liquid-phase refluxing 215 C Cons: can be time-intensive requires multiple distinct steps spin-casting causes material loss solvents can be environmentally harmful Can we develop a streamlined, green, gas-phase-only technique? 33
All-gas-phase scheme a) SiNC synthesis b) Gas-phase functionalization c) Film creation via impaction LiF/Al ~5Ǻ/50nm SiNCs ~150 nm SiNCs ~ 80nm ITO glass substrate Anthony, et al., Nano Letters, 2012 34
Gas-phase capping H 2 H 2 + alkene vapor 35
Surface modification: FTIR =CH 2 C-H x Si-H x C=C C-H x Si-H x functionalized unfunctionalized neat 1-dodecene Anthony, et al., Nano Letters, 2012 36
Impaction scheme 6.8mm 17.1mm 9.5mm x 1.5mm P up : 1.4 Torr (180 Pa) separation distance ~5mm P down : 0.23 Torr (30 Pa) to pumps Di Fonzo, et al. Applied Physics Letters, 2000 Holman and Kortshagen, Langmuir, 2010 Anthony, et al., Nano Letters, 2012 37
Film characteristics ρ SiNC film / ρ bulk Si density ratio 0.6 0.5 0.4 0.3 0.2 0.1 0 100 150 200 total flow rate (sccm) as measured using SEM + Rutherford backscattering SiNCs aluminum ITO-on-glass Anthony, et al., Nano Letters, 2012 38
Film characteristics XRD (d SiNC ~ 4.4nm) c Anthony, et al., Nano Letters, 2012 39
SiNC-only light-emitting device 0.5nm/50 nm LiF/Al ~5Ǻ/50nm SiNCs ~ ~150 80nm nm ITO glass substrate Anthony, et al., Nano Letters, 2012 40
Electroluminescence intensity (nw/cm 2 /nm) 0.30 0.25 0.20 0.15 0.10 0.05 c 0.00 500 550 600 650 700 750 800 850 900 950 wavelength (nm) Anthony, et al., Nano Letters, 2012 41
LED characteristics external quantum efficiency (%) 10-1 a 10-2 10-3 10-2 10-1 10 0 10 1 10 2 10 3 current density (ma/cm 2 ) First ever all-gas-phase NC device current density (ma/cm 2 ) peak efficiency > 0.02% Anthony, et al., Nano Letters, 2012 42
Summary Plasmas are effective for synthesis of high-quality nanoparticles Crystallization of nanoparticles does not occur through thermal routes alone Plasma effluent is highly reactive Gas-phase-only NC devices are viable 43
Acknowledgements Ph.D. Adviser: Prof. Uwe Kortshagen Postdoctoral Advisers: Profs. Eray Aydil, Steven Girshick, Uwe Kortshagen Collaborators: Dr. Kai-Yuan Cheng and Prof. R. J. Holmes (LEDs), David Rowe (H 2 Injection), Ryan Mello (EPR), Ryan Gresback (TEM), Dr. Zachary Holman (NC impaction) Funding: DOE Plasma Science Center and DOE grant DE/SC-0002391 NSF MRSEC Award Number DMR-0819885 44