Third-Order Optical Nonlinearity of Halogen-Bridged Nickel(III) Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 102

Fig. 7.10 (a) SEM image of nanocrystals of [Ni( L ) 2 Br]Br 2 .(b) Photograph of a [Ni( L ) 2 Br]Br 2 /
PMMA film. (c) Absorption spectra of a single crystal of [Ni(chxn) 2 Br]Br 2 ( upper panel ) and a [Ni
( L ) 2 Br]Br 2 /PMMA film ( lower panel ). Adapted from [ 5 ]

Ni-Cl-Cl and ~2.15 eV in Ni-Cl-NO 3 are the strong evidences for the higher-lying
odd-parity state |3 .

Since the higher-lying odd-parity state |3

>

> corresponds to the band-edge, the

energy difference between |3 is a crude measure

of the binding energy E b of the lowest excitonic state. In Ni-Br-Br, E b evaluated
from the difference is about 90 meV. The magnitude of E b increases as Ni-Br-Br
<

> and the lowest exciton state |1 >

< DE between

Ni-Cl-Cl Ni-Cl-NO 3 in Ni-X chain compounds. The splitting

> >

|1 also increases in this order. This tendency is also observed in the ER

studies as mentioned above. In Ni-Br-Br, the small excitonic effect makes two CT
excited states (|1

and |2

) almost degenerate, leading to the large transition dipole
moment between |1 > and |2 > and therefore to the large w

> and |2 >

(3) .

7.3.3 Z-Scan Measurements [ 5 ]

When we consider the application of the nonlinear optical materials to ultrafast
optical switching devices using, for example, optical waveguides, fabrication of a
thin film sample is necessary. Recently, the fabrication of high-quality thin films of
the Ni-X compound was successfully made, in which nanocrystals of a Ni-X
compound, [Ni( L ) 2 Br]Br 2 ( L :1,2-diaminohexadecane), having long alkyl chains
(C 14 H 29 ) shown in Fig. 7.1b were dispersed in an optical polymer (PMMA ¼

poly(methyl methacrylate)). Size of a nanocrystal is typically ~100 nm wide and
a few tens of nanometers thick as seen in the scanning electron microscopy (SEM)