Nanosecond and femtosecond pulsed laser ablation of metals: Plume characterisation and nanomaterial synthesis Tony Donnelly, Gearoid O’Connell and James G. Lunney Photonics Group, School of Physics and CRANN, Trinity College Dublin, Ireland.

-2 Pulsed laser ablation – pulsed laser irradiation of materials at FL ∼∼∼ 1 J cm . Direct route to functional nanomaterial 8 -2 12 -2 synthesis. Both nanosecond ( IL ∼∼∼ 10 W cm ) and femtosecond ( IL ∼∼∼ 10 W cm ) pulse duration lasers can be used. • Material independent. Metals, semiconductors, dielectrics, insulators can be ablated. • Capable of producing very small particles (1-100 nm) with narrow size distributions – Pulsed Laser Deposition (PLD). • Alternative to wet chemistry synthesis. Toxic process gases and chemical contamination are avoided.

Deposition apparatus and process monitoring Nanoparticle formation mechanisms

Deposition chamber Laser delivery optic(s) Substrate Pulsed laser a) ns PLD in vacuum Nanosecond (ns) pulsed lasers holder/manipulator nanoparticles are grown Langmuir probe • Excimer – 26 ns, 248 nm and thickness • Nd:YAG – 7 ns, 1064, 532, 355 nm on substrate surface [1,2] monitor unit • DPSS Nd:YAG – 40 ns, 355 nm from condensation of Focusing lens ablated species. Femtosecond (fs) pulsed laser • Ti:Sa – 130 fs, 800 nm ns PLD of Ag in vacuum - 7 nm equivalent thickness film. TEM of nanoparticle surface and corresponding nanoparticle iCCD 1 size distribution. Time and Space Nanosecond laser ablation for PLD Resolved iCCD Laser fluence, F = 1 – 5 J cm -2 Spectrocopy Imaging Laser intensity, I ~ 10 8 W cm -2 b) fs PLD in vacuum system L Laser spot ~ 0.04 cm -2 nanoparticles are produced Process chamber with Laser E ~100 mJ in and are ejected from the pumping system iCCD 2 Femtosecond laser ablation for PLD ablation target and deposit- and gas handling F = 0.1 – 1 J cm -2 ed on the substrate [3]. 1.6 nm UV-Vis 12 -2 Target holder IL ~ 10 W cm Spectrograph and rotator Laser spot ~ 5x10 -4 cm -2 Au (111) Laser E ~ 0.5 mJ fs PLD of Au in vacuum – HR-TEM imaging.

iCCD imaging Time & space resolved emission spectroscopy Langmuir ion probe 130 fs, 800 nm ablation of Au at 1.5 J cm -2 -2 Measured ionised component of ablation plume at 3 cm 130 fs, 800 nm ablation of Au, Ag at 1.5 J cm Ag and Au at 1.5 J cm -2

12 50 Ion yield 100 ns ∼∼∼100eV 5 deg plasma Anisimov fit, k=4.4 50 ns 100 ns plasma 200 ns 10 -10 deg

) 40 2 ) -15 deg 2 8 30

6 ions/cm

Au 10

nanoparticles 20 4

Ionsignal (mA/cm 2 10 Ion yield (10

0 0 0 5 10 15 -40 -30 -20 -10 0 10 20 30 40 nanoparticles Time (µs) Angle (deg) 5 15 ∼∼∼135eV 5 deg Ion yield Anisimov fit k=4.2 4 -10 deg )

-15 deg 2 ) 2 2 plume components for fs ablation [4,5] Ag 10 3 ions/cm

10

(i) atomic/ionic (ii) nanoparticles 2 5

Ag Stark broadening of 521 and Ion signal(mA/cm 1 50 ns 200 ns 5 μs Ion yield (10 546 nm lines for ne estimate. 0 0 0 5 10 15 -40 -30 -20 -10 0 10 20 30 40 Plasma plume analysis 10 ns Time (µs) Angle (deg) Spectral lineouts – taken at position of peak of plasma emission from iCCD images. Ion Time of Flight signals for Ag (top) and Au (bottom). Au Corresponding ion angular distributions are shown right. Au Ag Total ion yields: Ag – 1.2x10 12 , Au – 4x10 11 (< 1% total ionisation)

50 ns 100 ns 200 ns Ag

1200 l/mm, Voight profile fit. Instrument broadening dominant for t > 100 ns. Stark parameters Ion energy distributions. Average ion energy: Ag – 86 eV, Au – 83 eV 200 ns 300 ns taken from Djenize et al. [7] Average ion velocities: : Ag – 1.2x10 6 cm s -1, Au – 0.92x10 1.2x10 6 Au plasma T estimate – Boltzmann plots e Ag plasma conditions estimate – compare to 521nm line: cm s -1 PrismSPECT [6] simulation (in red above) 18 -3 10 ns – ne = 6.1x10 cm (2.2 nm) 17 -3 50 ns – ne = 9x10 cm (0.32 nm) Al Fit parameters: (integrated line intensities, not width) 546nm line: 10 ns – n = 7x10 18 cm -3 (2.75 nm) 15 -3 e Single wavelength 200 ns: Te=1.0 eV, ni=9.5x10 cm , L=0.72mm, Z=1 50 ns – n = 1x10 18 cm -3 (0.36 nm) 15 -3 e 300 ns: Te=0.75 eV, ni=3.5x10 cm , L=1.07mm, Z=0.97 PrismSPECT Stark broadening ? 521 nm line: Te = 1.1 eV at 100 ns, 1.0 eV at 200 ns laser absorption PrismSPECT calculation shows optically thin plasma 17 -3 10 ns – ne = 2.3x10 cm -1 τ < 0.0015 cm for 546 nm line under above conditions 50 ns – n = 1x10 16 cm -3 At late times nanoparticle plume self emission becomes weak and Nanoparticle plume analysis e > 1 order magnitude discrepancy difficult to detect. Can measure nanoparticle plume using a novel Spectral lineouts – integrated over continuum measured in iCCD images Sn single wavelength laser absorption technique [8]. Au at 1.5 J cm -2. 200 ns Continuum emission analysis – hot nanoparticles emitting blackbody-like spectrum. Emission of a single nanoparticle:

Different behaviour for different metals. Weak nanoparticle plume emission for Must include nanoparticle emissivity ε(λ,a) Setup for nanoparticle plume absorption. Ag, weak plasma plume emission for Sn. Combination of plume temperature, emissivity and iCCD camera sensitivity. 600 ns m = complex refractive index, a = NP radius. Spectral emission density per unit volume 10 -5 mbar 10 mbar 1 bar in NP plume, u( λ,T) Room temp emissivity for a=5 nm NPs of Au, Ag and Al (top). Au NP plasma 1 μs emissivity vs. temperature (bot) Time resolved extinction due to plume expansion and calculated

JNP (λ,T ), ρL and ρNP are the coeff. of density-length product using iCCD imaging analysis. emissivity, the material liquid density and mass density of the nanoparticle plume. plasma 4 -1 vslow =3.7x10 cm s NPs Spectral radiance, LNP (λ,T ) from a NP NPs NPs layer is given by 3 μs

Integrated spectral radiance Plume expansion in a background gas. Au at Fitting procedure can be combined with iCCD Temporal variation of Au NP plume Density length product -2 Nanoparticle plume front 1.5 J cm in Ar. 1 μs after laser irradiation. imaging to estimate plume mass. For Au at 1.5 J temperature extracted from NP optical position vs. time. calculated using a hydro- -2 13 cm , MNP ≈ 20 ng ( Na=6x10 ) emission fitting procedure. dynamic model [9].

References Thanks to: Future: [1] S. Dolbec, E. Irissou, M. Chaker, D. Guay, F. Rosei, and M. A. El Khakani, Phys. Rev. B, 70, 201406 (2004). - ns and fs ablation into high pressure gases. Hydrodynamic modelling of nanoparticle condensation. [2] T. Donnelly, B. Doggett, and J. G. Lunney, Applied Surface Science 252 (13), 4445 (2006). [3] S. Amoruso et al. , Appl. Phys. Lett., 84, 4502 (2004). [4] K. Oguri, Y. Okano, T. Nishikawa and H. Nakano, Phys. Rev. Lett. 99, 165003 (2007). - Time and space resolved absorption spectroscopy to track plasma and nanoparticle species in high pressure gases. [5] S. Amoruso, R. Bruzzese, X. Wang, N. N. Nedialkov and P. A. Atanasov, J. Phys. D: Appl. Phys., 40, 331-340 (2007). [6] http://www.prism-cs.com/Software/PrismSpect/PrismSPECT.htm - Pt and Pd as target materials – interest in Pt group metals for nanocatalysts. Atomic data generation and Stark [7] S. Djenize, A. Sreckovic and S. Bukvic, Spectrochim. Acta B, 60, 1552 – 1555 (2005). broadening parameters for these materials. Alloy materials. [8] G. O’Connell, T. Donnelly and J.G. Lunney, Appl. Phys. A. (2014) 117, 289–293. For more info: [9] S. I. Anisimov, D. Bäuerle, and B. S. Luk’yanchuk, Phys. Rev. B 48, 12076 (1993). Dr. Tony Donnelly: [email protected] - Line broadening and opacity issues in ns laser ablation plasmas in vacuum and gases.