PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2015
SNOM VISUALISATION OF LIGHT-TRIGGERED SWITCHING
IN PHOTOCHROMIC MATERIALS
D. S. FILIMONENKO, V. M. YASINSKII
Stepanov Institute of Physics, National Academy of Sciences of Belarus, Nezavisimosti
av., 68, Minsk, 220072, Belarus
G.T VASILYUK, S.A. MASKEVICH, A.E. GERMAN, V.F. OSKIRKO
Yanka Kupala State University, Ozheshko str., 22
Grodno,230023, Belarus
B.S. LUKYANOV, V.I. MINKIN
Institute of Physical and Organic Chemistry, Southern Federal University, Rostov-onDon, 344090, Russian Federation
Optical properties of hybrid nanostructured systems "metal nanoparticle-photochromic
molecule" were studied using Scanning near-field optical microscopy (SNOM). The
reversibility of photochromic reactions was demonstrated. From the result obtained it
may be inferred that SNOM is a useful tool in studying the local switching of
photochromic molecules, and it has much potential for nanomachining of nanoscale
structures and optical elements.
1. Introduction
The development of photochromic hybrid systems based on photochromic
organic molecules and nanoparticles of noble metals is a rapidly growing field of
nanotechnology [1]. Such systems can be used to create nanoscale photoswitches
for integrated optical circuits. In the presence of metal, these compounds express
important additional properties. When molecules are adsorbed on a metal surface
with nanoscale roughness, in parallel with the enhancement of Raman scattering
there also has been a rise in efficiency of photochemical processes, which is
attributed to an increased electromagnetic field near the metal surface [2, 3]. A
study of local dynamics of the switching from one conformational state of
photochromic material to another is of particular interest. Scanning near-field
optical microscopy (SNOM) provides ample opportunities for imaging of the
reversible light-triggered switching in photochromic materials. In addition, it is a
valuable and inexpensive tool for nanolithography with subwavelength
resolution. In this study, we investigated the optical response of spiropyran
molecules near nanostructured metal surface using the SNOM method.
1
2
2. Experimental
Hybrid nanostructured systems consisting of Ag nanoparticles with a shell
of spirocyclic photochromic molecules have been investigated [4].
Colloidal silver solution (diameter of silver nanoparticles is 8…40 nm) were
prepared by standard methods of citrate and borohydride reduction [5, 6].
Photochromic compounds were deposited on metal nanoparticles using
immobilization method. The glass substrates were immersed into a colloidal
solution then air dried.
3.
Results and discussion
UV irradiation (350 nm) of spiropyran films leads to the photochemically
reversible switching between colorless spiropyran (form I) and photocoloured
spiropyran (form II). Similar photochromic properties are also exhibited by
hybrid nanostructures "metal nanoparticle-photochromic molecule". Figure 1
shows the absorption difference spectrum of the samples, obtained by
subtracting the absorption coefficient spectrum before irradiation from that upon
irradiation at 350 nm for 120 seconds at room temperature. The form II spectrum
shows a more intense absorption peak at 630 nm. To observe the light-induced
switching with our home-build SNOM, we used a diode laser source at 650 nm,
which is close to the sample absorption peak.
Fig. 2 shows a schematic of the illumination-mode SNOM architecture used
for observation of the local reversible transmittance change. The beams of a
diode laser (650 nm) and a UV LED (350 nm) are combined using a dichroic
mirror (DM) and coupled into a multimode optical fiber (core diameter: 100 µm)
via a fiber coupler. The optical fiber with a tapered tip on another end (tip
diameter is approximately 200 nm) acts as a local light source to perform a lighttriggered switching. The light from the tip is transmitted through the sample and
detected by a photomultiplier tube. The fiber tip is held at a constant distance
from the sample by means of a shear-force feedback.
To induce photoswitching, the sample is UV-illuminated through the tip,
which is brought very close to the surface. The UV-illuminated area is then
scanned using 650 nm red laser diode, which falls in the absorption range of the
spiropyran form II.
Fig. 3 shows the SNOM images of the same sample area before (a) and after
UV irradiation (b). The local photochemical switching is clearly seen, as the
UV-illuminated area appears dark. The contrast is generated by the decrease in
transmittance of the spiropyran II as a consequence of the modification of the
chemical structure. After continuous illumination of the sample with white light,
3
the sample area returned to the initial transparent state (Figure 3 (c)). This
clearly demonstrates the reversibility of photochromic reactions.
0,15
1
0,1
0,05
0
350
400
450
500
550
600
650
700
nm
Figure 1. Absorption difference spectrum of the samples.
The experiments were performed with uncoated fiber tips, which means that
the illuminated area underneath the fiber is relatively large (several microns).
Using metal-coated tips, it is possible to create nanoscale structures with
dimensions confined only by subwavelength tip aperture.
650 nm
Figure 2. Schematic of the illumination-mode SNOM architecture.
4
a
b
c
Figure 3.SNOM images of the same sample area before UV irradiation (a), after UV irradiation (b),
and after continuous illumination with white light (c)
From the result obtained it may be inferred that SNOM is a useful tool in
studying the local switching of photochromic molecules, and it has much
potential for nanomachining of nanoscale structures and optical elements.
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