Direct Laser Writing of Fluorescent Silver Nanoclusters

in Polyvinyl Alcohol films

Nazanin Karimi, Puskal Kunwar, Juha Toivonen

Department of PhysicsTampere University of Technology, Tampere, Finland

Frontiers in Optics, Oct 20, 2015 The Fairmont San Jose, San Jose, CA USA,

• Ultra-small nanomaterials with dimensions less than 2 nm

• Discrete energy levels similar to QDs • Unique properties such as strong broadband fluorescence emission.

http://physics.usc.edu/~kresin/

Metal nanoclusters (NCs)

L. Shang et al., Chem. Commun. (2008)

Excitation wavelength (nm)(a) 450(b) 480(c) 510(d) 540(e) 560(f) 580

http://www.eng.nus.edu.sg/EResnews/102014/rd6.html

Synthesis of NCs

• Reduction of metal ions in solutions in the presence of an encapsulating media such as DNA, polymers, and oligomers

• Femtosecond direct laser writing (DLW) in inorganic matrices such as glass, and zeolite.

• Photopolymerization using DLW to synthesize silver NCs in polymers

24.03.16 3

P. Kunwar et al., ACS Nano 8 (2014)

Synthesis of Ag NCs in Polyvinyl alcohol (PVA)

• Linear polymer consists of vinyl alcohol monomers

• Water-solubility, excellent film forming, stabilizing properties for colloids and NPs, biodegradability and biocompatibility

• Preparing Ag@PVA films by spin-coating of aqueous solutions of PVA and AgNO3 on glass substrates

4

Sample stage

 

 

 

         

 

 

 

 

 

 

  0   1

 

 

 

 

 

 

CW Laser(405 nm)

Polarizer

Lens(40 mm) Lens

(150 mm)

Pinhole(25 µm)

Shutter

Light source  

Objective(1.4 NA)

Piezo Contoroller

DAQ card

Camera

Lens(200 mm)

Filter

Dichroic mirror

5

Direct laser writing (DLW) to synthesize silver NCs in Ag@PVA films and stabilize them by photopolymerization

DLW

Fluorescence Imaging

Fluorescence Spectroscopy

   

  

LED light(470 nm)

     

Laser diode(473 nm)

6

• (a) Bright-field microscopy image of written structure in Ag@PVA

λdlw = 405 nm

• (b) Fluorescence microscopy image

λexc= 470 nm

• (c) Fluorescence intensity profile

Average line-breadth 400 nm

≅

5 μm

Flu

ores

cenc

e In

tens

ity(a

rb.

Uni

ts)

Distance (μm)0 2.5 5 7.5 10 12.5 15

5 μm

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Fluorescence emission spectra of laser-written silver NCs in Ag@PVA films

• The obtained emission spectrum is the characteristic of Ag NCs

• The fluorescence intensity increases with increasing the Ag concentration

• These results confirm the presence of Ag NCs in the structures

8

5 μm

• The depths of the grooves = 32 nm - 38 nm• Ag@PVA film thickness = 58 nm• DLW ablates a considerable amount of the

materials• However, the fluorescence signal was detected from the exposed areas• Structures are silver-rich

AFM images of the structures show the formation of grooves in Ag@PVA Idlw

(MW/cm2)

6

9

12

1 2 3 4 5 6 7 8 9

Writing Laser Intensity (MW/cm2)

Flu

ores

cenc

e In

tens

ity(a

rb. U

nits

)

Distance perpendicular to the grooves (μm)

Dis

tanc

e fr

om t

he f

ilm le

vel (

nm)

5 μm

(W/cm2)

60300

600

9

 λexc = 473 nm, Iexc = 60 W/cm2

τ1 (s) τ2 (s)

PVA 103 

990 

PMMA 5 350

Two-fold bleaching of NCs

• We demonstrated the synthesis of fluorescent and photostable Ag NCs for the first time in PVA

• Fluorescence emissiong of NCs is similar in character with NCs in solutions and laser-written NCs in PMAA

• NCs in PVA are more photostable than organic dyes and NCs stabilized in PMAA

• PVA is an excellent stabilizer for metal NCs

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Conclusions