Ordered Arrays of II/VI Diluted Magnetic Semiconductor Quantum Wires: Formation within Mesoporous MCM-41 Silica

Abstract

We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.

Cd1–xMnxS Diluted Magnetic Semiconductors as Nanostructured Guest Species in Mesoporous Thin‐Film Silica Host Media

“…[16] Recently, the successful formation of diluted magnetic semiconductor Cd 1±x Mn x S quantum nanowires inside MCM-41 SiO 2 powders was published. [10,17] A diluted magnetic semiconductor (DMS) is formed when the cation A of a semiconductor AB is partially substituted by a magnetic ion M to give an A 1±x M x B structure. In the DMS class of materials, the most-investigated compounds are (II,Mn)VI semiconductors where a fraction x of the group II cations are substituted by Mn 2+ ions.…”

“…[20] As in the corresponding Cd 1±x Mn x S nanoparticles synthesized in mesoporous MCM-41 silica powders, there is no distinct red-shift or broadening of the LO phonon bands compared with the bulk, due to phonon confinement effects. [10] …”

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

“…For all materials, a type-IV isotherm is observed, which is the typical hysteresis loop of mesoporous materials [41], indicating that the incorporation of Mo species does not destroy the structure of mesoporous SBA-15. The pore sizes were calculated from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model, which is an appropriate method for determining changes in the pore-size distribution [17,18,42] and the BarrettEmmett-Teller (BET) equation was applied for calculation of specific surface areas. The isotherms have the following characteristic features: (1) the capillary condensation step, which is caused by the decrease of the pore sizes, typically at the relative pressure p/p 0 = 0.6-0.7 for pristine SBA-15, shifts to lower relative pressure p/p 0 = 0.4-0.6 after introduction of MoO 3 particles into the pores (Fig.…”

“…In addition to the studies of the formation of metal nanoparticles and nanowires, such as Pt [3][4][5], Pd [6][7][8][9], Au [3-5, 10, 11], Ag [3], and Ti [12], the formation of binary nanomaterials such as PbS [13], CdS [14], TiN [15], and GaN [16] within the nanoscale channels of these mesoporous materials have been explored. More recently, highly ordered quantum wires of Zn 1-x Mn x S [17] and Cd 1-x Mn x S [18] have been successfully synthesized within the mesopores of SBA-15.…”

SBA-15 as Support for MoS2 and Co-MoS2 Catalysts Derived from Thiomolybdate Complexes in the Reaction of HDS of DBT

“…2.1.1 Synthesis of Pristine Mesoporous SBA-15 [22][23][24] Eight gram of poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglycol) triblock copolymer (Aldrich, pluronic, P-123) was dissolved in 240 g of water and 28.6 g of concentrated hydrochloric acid at 303 K on a water bath. After drop wise addition of 16 g of tetraethylorthosilicate (TEOS), the reaction mixture was stirred for 24 h at 303 K. The resulting gel was transferred into a Teflon bottle and heated to 353 K and 413 K for 24 h to obtain SBA-15 with 6 nm and 9 nm pores, respectively.…”