Nonlinear optical spectroscopy on one-dimensional excitons in silicon polymer, polysilane

Abstract

The modulus and phase of the complex third-order nonlinear optical (NLO) susceptibility X 3 ( -3to\co,co,co) have been measured on an all-trans silicon polymer, poly(dihexylsilane), over a wide photon energy range. Several distinct structures observed in the f (3) spectra are in excellent agreement with the theoretical results of a one-dimensional (ID) exciton model. Important aspects of the ID Wannier-like exciton structure in polysilanes are elucidated in relation to the NLO processes.PACS numbers: 73.20.Dx, 42.70.Nq, 78.65.He Much interest has been focused on the quasi-onedimensional (ID) semiconductors. In a-bonded inorganic semiconductors, nanoscopic structures called quantum wires are fabricated by elaborate techniques of crystal growth and microlithography [1]. Meanwhile, various organic polymers are synthesized to utilize the delocalized ^•-conjugated electronic states on the linear polymer chains [2]. In such quasi-lD inorganic as well as organic semiconductors, the lowest dipole-allowed exciton is predicted to have a very large binding energy and an unusually strong oscillator strength [3][4][5], due to the singular character of the ID Coulomb potential for excitons [6]. This problem has attracted considerable interest from the viewpoint of solid-state physics, but the details remain unexplored as of yet. In this paper, we report the thirdorder nonlinear optical (NLO) spectra of a silicon polymer, polysilane, demonstrating that the characteristic features of quasi-ID (Wannier-like) excitons can be fully revealed by nonlinear optical spectroscopy.Among the various kinds of polysilanes (TSi/?/?T, R and R' being organic sidegroups) with different conformations [7], we have chosen poly(dihexylsilane) (PDHS; R,R' =C 6 H 13) with an all-trans planar structure of obonded Si chains (with a djh symmetry), which may be regarded as a model system of the 1 D analog (or "an ultimate quantum wire") of a 3D silicon crystal. This polymer is predicted to have a direct gap between the obond-like valence band and the a* -bond-like conduction band at the T point [8]. The sharp absorption peak at 3.30 eV is assigned to the lowest allowed X B U exciton, while the two-photon absorption peak at 4.19 eV [9-1 1] is assigned to the second forbidden ] A g exciton. However, the whole profile of the ID excitonic structure has not yet been disclosed.To elucidate the quasi-lD excitonic structure in PDHS, we have undertaken extensive NLO spectroscopy over a wide photon energy range, as recently attempted on various ^--conjugated polymers [12]. In the sample preparation, PDHS powders with molecular weight of about 2.44xl0 5 were dissolved in heptane, and the solution was spin casted onto synthetic fused silica plates to form unoriented film samples. The films were dried at 380 K for 15 min and allowed to stand at room temperature overnight, before optical measurements.In the measurements of x 3 (~~3(o,co,co,a)) three kinds of tunable laser systems were used: a dye laser driven by a 15-ns-wide Xe-Cl excimer laser with use of thirteen ...

Fluorescence detection system based on silicon quantum dots–polysilane nanocomposites

“…This extended conjugation also makes polysilanes sensitive towards light, but this disadvantage could be turned into an advantage in applications such as micro-and nanolithography [22][23][24]. In addition, polysilanes have interesting non-linear optical (NLO) properties, and they possess intrinsic semiconductivity [25][26][27][28][29][30][31]. Polyhydrosilanes are a distinct class of polyorganosilane polymers containing hydrosilyl moieties [32].…”

Exciton-exciton scattering in disordered linear chains of poly(di-n-hexylsilane)

“…Levels C and D may not be exciton ͑bound electron-hole pair͒ states but may be charge carrier ͑free electron-hole pair͒ states, considering that the band gap energy is estimated to be about 4.5 eV. 2,3 In any case, however, most of the excitons which are once scattered to levels C or D continue to be excitons, even if excitons are transiently resolved into charge carriers, because the efficiency for charge carrier generation in x-x annihilation is actually negligible. 3 These excitons will subsequently relax to the level A by some times of phonon scattering and will take part in the competing processes of radiative decay and x-x annihilation.…”

“…7 Although the Frenkel exciton model can also explain the nonlinear spectroscopies, 8 the intermediate exciton model 9 between the Frenkel exciton and the Wannier exciton seems to be an advanced model to deal with the internal structure of excitons. 2 The exciton Bohr radius is estimated to be Ϸ0.2 nm. As for dynamical aspects, excitons can propagate along linear chains without being self-trapped, because exciton-phonon coupling is exceptionally small.…”

Doping effects and electronic states in C60-polymers

“…We note that the operator at the lattice sites of the four membered rings is one of the relevant linear combinations of the effective σ-like components, assuming a possibility of local σ-conjugations at the four membered rings. The similar assumption of the σ--5 -conjugation has been used in Si-based polymers, for example, in [13]. We, however, use the term "conjugation" for simplicity in this paper, because the local σ-conjugations can be regarded as a part of the global conjugations which are extended over the system.…”