Raman spectroscopic characterization of crater walls formed upon single -shot high -energy femtosecond laser irradiation of dimethacrylate polymer doped with plasmonic gold nanorods

2025-04-29 0 0 518.93KB 17 页 10玖币

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Raman spectroscopic characterization of crater walls formed upon single-shot high-energy

femtosecond laser irradiation of dimethacrylate polymer doped with plasmonic gold

nanorods

István Rigó1, Judit Kámán1,Ágnes Nagyné Szokol1, Roman Holomb1, Attila Bonyár2, Melinda Szalóki3,

Alexandra Borók1,2, Shereen Zangana2, Péter Rácz1, Márk Aladi1, Miklós Ákos Kedves1, Gábor Galbács5,

László P. Csernai1,6,7, Tamás S. Biró1, Norbert Kroó1,8, Miklós Veres1, NAPLIFE Collaboration

1Wigner Research Centre for Physics, Budapest, Hungary

2Department of Electronics Technology, Faculty of Electrical Engineering and Informatics, Budapest University for

Economics and Informatics, Budapest, H1111, Hungary

3Department of Biomaterials and Prosthetic Dentistry, Faculty of Dentistry, University of Debrecen, Debrecen,

Hungary

4Centre for Energy Research, Institute of Technical Physics and Materials Science (MFA), H1121 Budapest, Hungary

5Department of Inorganic and Analytical Chemistry, University of Szeged, Szeged, H-6720 Hungary

6Department of Physics and Technology, University of Bergen, 5007 Bergen, Norway

7Frankfurt Institute for Advanced Studies,Frankfurt/Main, Germany

8Hungarian Academy of Sciences, 1051 Budapest, Hungary

Abstract

The bonding configuration of the crater walls formed in urethane dimethacrylate-based polymer

doped with plasmonic gold nanorods upon irradiation with a single-shot high-energy femtosecond

laser pulse has been studied by Raman spectroscopy. New Raman bands were detected in the 2000-

2500 cm-1 region of the Raman spectrum the intensities of which showed strong dependence on

the concentration of the plasmonic nanoparticles and the energy of the laser pulse. Based on model

calculations of the Raman frequencies of the polymer these peaks were attributed to carbon-

deuterium and nitrogen-deuterium vibrations. Their appearance might indicate the occurrence of

nuclear reactions in the polymer excited by the ultra-strong laser field amplified by the plasmonic

nanoparticles.

Keywords: plasmonic enhancement, femtosecond laser, nuclear reactions, Raman spectroscopy,

dimethacrylate polymer

1. Introduction

The use of high-energy short laser pulses is gaining significance in numerous applications.

Localized surface plasmon polaritons (LSPP) can, for example, be excited efficiently with these

pulses even up to very high laser intensities. One of the reasons to do so is that with the help of

plasmonic nanoparticles the electromagnetic field of the lasers can be amplified on the nanoscale

resulting in fields orders of magnitude higher, than those of the laser pulse. Our motivation to

explore this enhancement effect has been to use these high fields to realize tabletop plasmonic

nano-fusion processes [1]. Ti:Sa femtosecond laser pulses have been used to excite LSPPs in a

polymer sheet, containing resonant plasmonic gold nanorods, with laser intensities up to a few

times 1017 W/cm2. The first step in our studies has been the analysis of the craters formed by

individual laser shots in the material [2]. It was found, that the volume of these craters is always

significantly higher in the polymer containing the gold nanoparticles than in the same polymer

without them. The volume increase can be attributed only to nuclear processes, and it has been

found, that the crater volumes depend linearly on the energy of the laser pulses [2]. Since this

energy production has been found to be comparable, or even larger, than that of the energy of the

laser pulse, it has been decided to use vibrational spectroscopic method (Raman spectroscopy) to

characterize structural changes occurred in the polymeric structure at the crater walls formed upon

irradiation of the material with single-shot high-energy femtosecond laser pulse. The irradiation

with different pulse energies was performed on both a pure structure and also samples doped with

plasmonic gold nanorods (in two different concentrations) having plasmon resonance at the

wavelength of the femtosecond laser, and the Raman measurements were performed on both types

of samples.

Being sensitive to bonding configuration, Raman spectroscopy is widely used to characterize

different polymeric materials, including dimethacrylates. This method can be used to determine

the degree of conversion in these types of polymers, as well, through the changes and ratio of the

Raman peak of the C=C double bonds to that of a reference band belonging to a bond not affected

by the polymerization [3-5]. In addition to the bonds of the polymer frame it allows to study the

different C-H and N-H groups as well. The effect of the laser pulse on the polymer framework,

including the additional conversion upon irradiation in the presence of gold nanoparticles, has been

published earlier [6]. Here we report on the changes observable in the 2000-2600 cm-1 region,

being dependent on the presence of the nanoparticles and the pulse energy as well. In situ laser

induced breakdown spectroscopy (LIBS) measurements were also performed to validate the

Raman findings.

2. Methods

2.1.Materials and sample preparation

The photopolymerizable dimethacrylate resin mixture consists of urethane dimethacrylate

(UDMA) (Sigma Aldrich) and triethylene glycol dimethacrylate (TEGDMA) (Sigma Aldrich) in

3:1 mass ratio. Dodecanethiol-capped gold nanorods (Au-DDT) in size of 25 nm diameter and 75

nm length were purchased from Nanopartz Inc. (part no.: B12-25-700-1DDT-TOL-50-0.25, and

B12-25-750-1DDT-TOL-50-0.25).

The preparation method of pure (UDMA-X) and doped with gold nanorods (UDMA-Au)

polymer samples is described in detail elsewhere [6]. In short, the mixture of the UDMA and

TEGMA monomers and the photoinitiator (and Au-DDT) is placed on a glass slide in a template

allowing to obtain a thin layer of the resin. Then the polymerization mixture is irradiated with a

standard dental curing lamp emitting blue light with 3 min exposure time. The obtained round

shaped thin film samples are clear (UDMA-X non-doped polymer) or have pink color (UDMA-

Au doped samples). The gold nanorods were added to the monomer mixture in 2 different

concentrations of 0.124 m/m% and 0.182 m/m%, and the corresponding samples were marked as

UDMA-Au1 and UDMA-Au2 in this manuscript, respectively. In addition, on some figures, the

laser pulse energy in milliJoules is also added to the sample name, i.e. UDMA-X-25 corresponds

to the crater in the non-doped polymer irradiated with 25 mJ laser pulse.

2.2. Laser irradiation experiments

The irradiation of the samples was implemented by a Ti:Sapphire-based chirped-pulse two-

stage amplifier-laser system (Coherent Hydra) delivering pulses with 40 fs pulse length at 795 nm

central wavelength with 10 Hz repetition rate and 25 mJ maximum pulse energy. The beam was

focused with a lens having 50 cm focal length. The laser irradiation experiments were performed

under vacuum conditions to avoid nonlinear processes in air. The pressure in the vacuum chamber

was in the range of 10−6 mbar. The sample treatment was achieved by single pulses of different

energy, each illuminating a different region of the sample surface (this was achieved by shifting

the sample laterally after each pulse). Since the energy density is above the ablation threshold of

the material, crater formation was observed at the irradiation spot.

2.3.Raman spectroscopy

A Renishaw InVia micro-Raman spectrometer connected to a LeicaDM2700 microscope

was used for the Raman spectroscopic measurements. The Raman spectra were recorded on the

wall of the formed crater with 532 nm excitation in backscattering geometry, with the laser focused

into a spot of ~2 microns diameter on the sample surface by using a 50X/0.5 NA objective. The

laser power was ~6 mW which equals 5-10% of the maximum intensity of the laser source. The

spectra were recorded in the 1000 - 2650 cm-1 spectral region. Due to the low intensity of the

investigated Raman bands the accumulation time was set to 2 hours in each spot. Before the

measurements, calibration was done by using a silicon wafer and its characteristic peak at 520 cm-

1 Raman shift.

Since this study is about the evaluation and comparison of small-intensity Raman peaks

recorded on different craters formed upon single-shot laser irradiation, a special care was given to

the processing of the measured Raman spectra, that was done with the Origin 2019 software and

a custom Matlab code. The first step was the normalization of the spectra to the C-C peak at 1460

cm-1. Then, background subtraction was performed by using the Asymmetric Least Squares

method [7]. The integral intensity of the Raman peaks was determined by calculating the area

under the curve for the composite Raman band observed in the 2000-2600 cm-1 region.

2.4.Laser induced breakdown spectroscopy

The LIBS measurements were performed in situ, during the single-shot irradiation of the

polymer targets with the high-intensity laser pulses. The laser induced breakdown plasma emission

was collected at 45 degrees angle and collimated by placing a lens at a 4 cm distance from the

target inside the chamber. The collimated beam was then focused into an optical fiber with a core

diameter of 400 μm and transmitted into a high resolution double Echelle spectrometer (having a

4 pm resolution) equipped with an ICCD camera (Demon, LTB, Berlin). The spectrum recording

was done using a fixed gate width of 1 µs and a gate delay of 0.6 μs. The spectrometer was centered

around 656 nm and recorded the spectrum in a 3 nm spectral window.

2.5.Modeling and calculations of the selectively deuterized UDMA monomer

The chemical structure of urethane-dimethacrylate (UDMA) monomer is shown in

Figure 1. Part of this structure containing N-H and C-H2 groups was selected as a starting geometry

for further modeling and subsequent calculations. For better description of the chemical

environment, the dangling chemical bonds at the ends of the model were terminated with methyl

(CH3) and hydroxyl (OH) groups, as shown in Figure 2.

Figure 1. Chemical structure of UDMA monomer together with the part selected for further

modeling and calculations.

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摘要：

Ramanspectroscopiccharacterizationofcraterwallsformeduponsingle-shothigh-energyfemtosecondlaserirradiationofdimethacrylatepolymerdopedwithplasmonicgoldnanorodsIstvánRigó1,JuditKámán1,ÁgnesNagynéSzokol1,RomanHolomb1,AttilaBonyár2,MelindaSzalóki3,AlexandraBorók1,2,ShereenZangana2,PéterRácz1,MárkAladi1...

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Raman spectroscopic characterization of crater walls formed upon single -shot high -energy femtosecond laser irradiation of dimethacrylate polymer doped with plasmonic gold nanorods

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