Geometry, Phase Stability, and Electronic Properties of Isolated Selenium Chains Incorporated in a Nanoporous Matrix

Abstract

We report the fabrication process of isolated one-dimensional Se chains incorporated in the matrix of AlPO4-5 single crystals and the experimental investigation of the geometry, phase stability, electronic properties, and electron-phonon coupling effect of these Se chains. The structure of the helical Se chains inside the channels is discussed on the basis of X-ray scattering measurements. Thermal analysis and temperature-dependent micro-Raman measurements show that Se single chains are flexible and can convert from a weak distorted phase into another phase with strongly disordered structure ("melting" state) around 340 K. Since the electrons are confined in the one-dimensional channels, the absorption band of the Se chain is obviously blue shifted compared with that of trigonal Se. With increasing temperature, this band shifts linearly to the lower energy side, in sharp contrast to the nonlinear temperature coefficient of trigonal Se, which is attributed to the greatly diminished interchain interaction and the weakening of the electron-optical phonon coupling in a low-dimensional system. In the vicinity of the absorption band, both first-order and second-order Raman signals for the Se chain are enhanced, due to the strong electron-phonon coupling when the excitation laser energy matches the electronic transition in isolated Se chains.

Elemental chains

“…Thereby, they may also end up in stabler helical structures. The fact that the Se chains are so soft can also explain why the experimental value of the dihedral angle for Se helices inside the channels of the nanoporous host, as reported by Li et al [48], differs significantly from our optimized value.…”

“…In a recent study, Li et al [48] reported the synthesis of isolated selenium chains inside the channels of a nanoporous host. According to their study, the chains are helical with bond lengths of 4.50 a.u.…”

Negative compressibility of selenium chains confined in the channels of AlPO4-5 single crystals

“…The inner diameter of the channel is 7.3 Å, and the separation distance between two neighboring channels is 13.7 Å. As shown in figure 1(a), selenium species are incorporated into the AlPO 4 -5 crystal channels by the vapor phase diffusion method as described previously [7]- [9]. Previous extended x-ray absorption fine structure (EXAFS) and photoacoustic spectroscopy (PAS) studies on the Se-Se distances have confirmed that single chains of Se are indeed being confined in the assembling zeolite template [10,11], mordenite with a channel diameter of 6.7 Å.…”

“…The Se chains formed inside AFI with a diameter of 7.3 Å are studied by x-ray scattering, which determines the diameter of the chain to be 3.4 Å. If we consider that the van der Waals radius of the chain is 1.9 Å, only the formation of single chains is possible inside the channel of AFI [9].…”

Selenium-loaded zeolite (ZSM-5): I. Structures and fundamental properties

“…However, in some studies, powderlike zeolites have been employed, which may exhibit substantial surface effects. 18,28,34 In addition, there is a variety of zeolites with pore shapes and covalencies, 5 while most of previous studies have employed zeolites containing ionic pores, [15][16][17][18][19][20][21][22][23][24][25][26][27][29][30][31][32][33][34] which may mask covalent properties of chalcogens. Accordingly, further work relating atomic structures with macroscopic properties in chalcogen-loaded covalent zeolites seems intriguing.…”