Semiconductor Nanocrystals from Nonthermal Plasmas. Rebecca J. Anthony University of Minnesota

Transcription

1 Semiconductor Nanocrystals from Nonthermal Plasmas Rebecca J. Anthony University of Minnesota 1

2 Nanocrystals in devices efficient light emitters and absorbers versatile deposition schemes possibility for flexible devices inexpensive to produce Tan, et al.,journal of Applied Physics, 105,

3 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

4 Plasma synthesis Argon + SiH 4 (5:95 in Helium) RF power: MHz, 60-80W Pressure: ~ 1.4 Torr (~180 Pa) Gas flowrate: ~ 100 sccm 4

5 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.,

6 Plasma power and nanoparticle crystallinity X-ray diffraction (XRD) Raman spectroscopy plasma power Anthony and Kortshagen, Phys. Rev. B.,

7 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.,

8 Nanocrystal formation Nonthermal plasma p = 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,

9 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

10 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

11 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

12 n H and T e : Optical emission spectroscopy(oes) intensity / a.u. H wavelength / nm M. V. Malyshev and V. M. Donnelly, Phys. Rev. E OES 12

13 OES analysis: 4 nm nanoparticles T e ~ 3.7 ev n H ~ 2 x cm -3 13

14 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, ,

15 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

16 Probe results ion densities 16

17 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)

18 Nanoparticle heating model: results Nanoparticles only reaching K above room T Crystallization cannot occur via thermal routes alone Next: examine possibility of H-induced crystallization 18

19 Optimizing the Nanocrystal Surface R. J. Anthony, D. J. Rowe, M. Stein, J. Yang, and U. R. Kortshagen, Advanced Functional Materials,

20 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.,

21 What is on the silicon surface? Fourier-transform infrared spectroscopy (FTIR) Si-H x stretch mode Si-H x bend/wag mode 21

22 Injection gas experiment H 2 (100% Ar) injection gas inlet Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat.,

23 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.,

24 H 2 / Ar injection: dangling bond density Anthony, Rowe, Stein, Yang, and Kortshagen, Adv. Func. Mat.,

25 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

26 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.,

27 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.,

28 Light-emitting Devices from Silicon Nanocrystals K.-Y. Cheng, R. J. Anthony, U. R. Kortshagen, and R. J. Holmes, Nano Letters, 2010 and

29 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 ev 4.1 ev LiF/Al Cheng, Anthony, Kortshagen, and Holmes, Nano Letters,

30 Electroluminescence near-infrared-emitting SiNCs red-emitting SiNCs Cheng, Anthony, Kortshagen, and Holmes., Nano Letters,

31 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,

32 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,

33 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

34 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,

35 Gas-phase capping H 2 H 2 + alkene vapor 35

36 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,

37 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,

38 Film characteristics ρ SiNC film / ρ bulk Si density ratio total flow rate (sccm) as measured using SEM + Rutherford backscattering SiNCs aluminum ITO-on-glass Anthony, et al., Nano Letters,

39 Film characteristics XRD (d SiNC ~ 4.4nm) c Anthony, et al., Nano Letters,

40 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,

41 Electroluminescence intensity (nw/cm 2 /nm) c wavelength (nm) Anthony, et al., Nano Letters,

42 LED characteristics external quantum efficiency (%) 10-1 a 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,

43 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

44 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 NSF MRSEC Award Number DMR