Excitonic Bands in the Optical Absorption Spectra of A₃MX₆ and A₃M₂X₉ (A = MeNH₃, Me₂NH₂, Me₃NH, Me₄N; M = Bi, Sb; X = Cl, Br, I)

Abstract

Main Group Metal Complexes. Electronic Spectra. ExcitonsThe title compounds (natural low-dimensional semiconductors) show strong excitonic optical ab sorption bands in the UV-visible spectral region, because of the dielectric confinement of excitons. as in the cases of other similar systems based on PbX5~, SnX^-. P t ' -X -P t IV-X , C d,Sv-clusters etc.During the last twenty years a large num ber of artificial low-dimensional (LD) sem iconductor systems have been fabricated and studied. They are characterized by enhanced excitonic binding energy, enhanced excitonic oscillator strength, and consequently enhanced optical nonlinearity, in comparison to the corresponding three-dimensional (3D ) systems [1]. Similar effects have been observed in some natural two-dimensional (2D ), one-dimensional (1 D) and zero-dimensional (OD) semiconductor systems. Compounds of the for mula A 2MX4 (where A = alkylammonium, phenylalkylammonium etc.; M = Pb, Sn; X = Cl, Br, I) are 2D (natural quantum well) systems [2][3][4][5][6]. Crystals of (M eNH3)3M2Br9 consist of 2D layers of polyanions, while crystals of (M e3N H )3Sb2Cl9 and (M e2N H 2)3Sb2Cl9 consist of 2D layers of SbCl3 connected by Cl-anions and sandwiched between alkylammonium cations (2D systems) [10,11]. Crystals of (M eNH3)3M2Cl9 are built up of 1 D double-chain polyanions and the alkyl ammonium cations surrounding them (I D sys tems) [11,12], Crystals of A 3MX6 are built up of octahedral anions MX^", while crystals of (M eNH3)3M2I9, (Me2NH 2)3Bi2I9, (M e4N )3M2Br9, and (Me4N)3M2Cl9 are built up of bioctahedral M2X9_ anions and the alkylammonium cations sur rounding them (0D systems) [11][12][13][14][15][16].In this paper we report results concerning the ex citonic bands in the optical absorption (OA) spectra of A3MX6 and A3M2X9 LD systems. When this work was in progress, preliminary results concerning optical absorption spectra in crystals of the 0D system (MeNH3)3Bi2I9 have been reported by others [17], The compounds were prepared by methods de scribed in the literature [10][11][12][13][14][15][16][17], and identified by elemental analyses and/or Weissenberg diagrams. Thin deposits of compounds on quartz plates were prepared by methods reported previously [4][5][6][7]. The optical absorption spectra were recorded at room temperature on a Varian 2390 model UV-vis ible spectrophotometer.Figs 1-3 show the OA spectra of thin (solid) de posits of 0D systems, and Fig. 4 shows the spectra of some 1 D and 2 D systems. One can see that the position of excitonic band (which is the low fre quency band close to the absorption edge) in the OA spectra is shifted to longer wavelengths in the order chlorides (310-332 nm) < bromides (360-409 nm) < iodides (457-500 nm). It occurs at inter mediate positions for mixed halide compounds (e.g. Fig. 2d). The excitonic bands in the spectra of Sbcontaining compounds occur at shorter wavelengths than those of the corresponding Bi-containing com pounds as it is expected from the observed spectra of Sbl3 [18] and BiI3 [19] crystals as well as ...

A step closer to the binary: the 1∞[Bi6I20]2−anion

“…Compound 3 displays another shift towards higher wavelength, again in accordance with darker color and the condensed, infinite building unit. The optical properties of 1-4 accord well with previous reports on hybrid iodidobismuthates ranging from isolated [BiI 6 ] 3− octahedra,34 to dinuclear [Bi 2 I 9 ] 3− units,35 hexanuclear [Bi 6 I 22 ] 4− units 36 and infinite chain anions 1 ∞ [BiI 4 ] − (ref. 13, 37) and 1 ∞ [BiI 5 ] − .…”

Excitonic Bands in the Optical Absorption Spectra of (Bu4N)CuBr2, (Et4N)2Cu2Br4, (Pr4N)2Cu4Br6, (Bu4N)2Cu2I4, (Me4N)Cu2I3, (Pr4N)4Ag4I8, (Me4N)Ag2I3, (Et4N)Ag2Br3, and Similar Compounds

“…In the case of Me4NAg2l3, the structure was found to be little different from that in the literature [23], The space group Pcmn, a= 10.066, b = 7.426, c= 17.739 A, V= 1326 A ' [19] was found. Thin deposits (of these compounds as well as of the corresponding conventional compounds) are mainly prepared by translational or rotational rubbing the crystals on quartz plates (see [9] and refs therein). The optical absorption spectra of these deposits were recorded on a Varian 2390 model UV-visible-near-IR and /or on a Perkin Elmer Lambda 19 spectrophotometer.…”

Preparation and Characterization of (C6H5CH2CH2NH3)4BiI7 · H2O, (C6H5CH2CH2NH3)3BiBr6 and (C6H5CH2CH2NH3)3BiCl6

“…One can see that the peak positions of (low fre quency) excitonic bands occur at 474, 380 and 327 nm, respectively. There is a blue shift of the peaks in comparison to those of (Ph4P)3Bi5I 18 (515 nm), [13,19] [or (M eNH3)3Bi2I9 (497 nm)], (Me4N )3Bi2Br9 (388 nm), and (M e4N )3Bi2Cl9 (329 nm) [1], respectively. The reason is that B il7, BiBr6 and BiCl6 species are smaller in size than Bi5I I8 [or Bi2I9], Bi2Br9 and Bi2Cl9, respec tively.…”

“…Recently, the optical absorption bands arising from the exci tonic states of these compounds have been investi gated [1,2]. The excitonic bands in the optical absorption spectra of (M eNH3)3Bi2I9 [6], (Me4N )3Bi2Br9 [3][4][5] and (Me4N)3Bi2Cl9" [3,5,6], for example, occur at 497, 388 and 329 nm, respec tively, while the corresponding bands of Sb-analogues [3, 5 -7 ] occur at 457, 360 and 310, respec tively [1]. However, little is known about the preparations, structures, and properties of com pounds (C"H->,!+lN H 3)3MX6 (with n > 2) and (C6H5CH2CH 2NH 3)3MX6 [9][10][11][12][13], Some com pounds of the formulae (C"H2"+1NH 3)3Bi2X9 and (C"H2,I+1NH 3)3BiX6 have been prepared as poly crystalline precipitates [13].…”