Se8 Rings Research Articles

Enhanced charge transport in 2D inorganic molecular crystals constructed with charge‐delocalized molecules

AbstractOutstanding charge transport in molecular crystals is of great importance in modern electronics and optoelectronics. The widely adopted strategies to enhance charge transport, such as restraining intermolecular vibration, are mostly limited to organic molecules, which are nearly inoperative in 2D inorganic molecular crystals currently. In this contribution, charge transport in 2D inorganic molecular crystals is improved by integrating charge‐delocalized Se8 rings as building blocks, where the delocalized electrons on Se8 rings lift the intermolecular orbitals overlap, offering efficient charge transfer channels. Besides, α‐Se flakes composed of charge‐delocalized Se8 rings possess small exciton binding energy. Benefitting from these, α‐Se flake exhibits excellent photodetection performance with an ultrafast response rate (~5 μs) and a high detectivity of 1.08 × 1011 Jones. These findings contribute to a deeper understanding of the charge transport of 2D inorganic molecular crystals composed of electron‐delocalized inorganic molecules and pave the way for their potential application in optoelectronics.

Se(IV) reduction and extracellular biosynthesis of Nano-Se(0) by Shewanella oneidensis MR-1 and Shewanella putrefaciens

Microbial reduction of Se(IV) to Se(0) is extremely important for the detoxification of selenium oxyanion, but the reductive mechanism is still controversial. In this study, the reduction of Se(IV) by Shewanella oneidensis MR-1 and Shewanella putrefaciens was systematically investigated. The results showed that excessive Se(IV) severely inhibited the growth of microorganisms and Se(IV) reduction. The extracellular accumulation of Nano-Se(0) particles was shown to be approximately 250–650 nm in diameter. Riboflavin facilitated Se(IV) reduction, while rotenone, as an electron inhibitor, could completely inhibit Se(IV) reduction, validating the extracellular electron transport process for biosynthesized Nano-Se(0). Importantly, the amorphous Nano-Se(0) synthesized by S. oneidensis MR-1 were mixtures of helical chains and Se8 rings, while those synthesized by S. putrefaciens were mainly composed of helical chains. FTIR results demonstrated that Nano-Se(0) particles were wrapped by some proteins, organic substances and lipids, exhibiting superior dispersibility and biocompatibility. Biogenic Nano-Se(0) particles can also be considered for technological applications in various fields.

Advanced all-solid-state lithium-selenium batteries enabled by selenium-nitrogen doped hierarchic meso-microporous carbon nanospheres composite cathode

Selenium, an element belonging to the same group in the periodic table as sulfur, has a high electronic conductivity (1 × 10−5 S/cm) and a high volumetric energy density (3253 mAh/cm3), which is a prospective cathode material for high-energy all-solid-state rechargeable batteries. However, its wide use is hindered by large volume expansion and low utilization rate. In this work, Se-infused nitrogen-doped hierarchical meso-microporous carbon composites (Se/NHPC) are prepared by a melt-diffusion process. Amorphous Se is uniformly dispersed in meso-micropores of NHPC with a high mass loading of 81%. All-solid-state Li-Se batteries fabricated by using Se/NHPC as the cathode, a Li-In alloy as the anode, and Li6PS5Cl as the solid-state electrolyte, deliver a highly reversible capacity of 621 mAh/g (92% of theoretical capacity), a good rate capability and a high capacity retention value of 80.9% after 100 cycles. It is found that the capacity decay of Se cathode is mainly related to the interfacial degradation and the separation of Se from the carbon substrate, as suggested by the continuous increase of interfacial resistance and the structural transformation from amorphous Sen chains to Se8 rings initial discharge/charge cycle and then to the trigonally crystalline Se chains structure after the long-term cycles.

Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

We encapsulated selenium (Se) into AFI and LTA zeolites via Se vapour adsorption at ∼450 °C and studied Raman and optical absorption spectra (RS and OAS, respectively) of zeolite single crystals with incorporated Se. At high Se loading density (D), Se8 rings are formed in the AFI channels along with Se chains. The rings are oriented by their 4-fold axis parallel to channels. Se8 is also formed in the LTA large cavities with its 4-fold axis oriented parallel to the 4-fold axis of LTA at any D up to ∼8 Se atoms per cavity (at/c). However, at D > 0.5 at/c, nearly unexplored Se12 rings oriented by their 3-fold axis parallel to the LTA 3-fold axis are formed along with Se8 in neighboring cavities. We discriminate between Se8 and Se12 rings via 1) RS frequencies; 2) RS and OAS band intensities vs. D; 3) RS band intensities vs. excitation wavelength and light polarization configuration. With an increase in D, the number of Se12 rings grows faster than that of Se8. We extract OAS of Se8 and Se12 from the experimental AFI-Se and LTA-Se OAS and show that the 3.0–3.3 eV absorption of Se12 is enhanced compared to that of Se8. This absorption is responsible for the observed resonant Raman effect of Se12 at the 514.5 nm wavelength excitation. Both LTA-confined Se8 and Se12 display rather high thermal stability.

Experimental and theoretical studies of the solid-state performance of electrodeposited Yb2O3/As2Se3 nanocomposite films

The influence of the rare-earth oxide (Yb2O3) on arsenic selenide (As–Se) host is currently investigated and documented in this study. Nanocomposite films were grown using the electrodeposition (ED) technique with the addition of 1 – 4 mol% Yb2O3. The film’s optical structural, optical, morphological, and electronic features were studied. The host matrix presents the monoclinic structure whereas the composite films present two phases –cubic and orthorhombic crystal structure. Crystallites within nano sizes (36.2 nm) were recorded for the pristine film while the composite films showed sizes ranging from 37.2 to 48.1 nm. Moreover, the Raman spectra revealed low energy Raman phonons characteristic of Se8 rings, As4Se4 cages, As–Se hetero-polar bonds whose cleavage was due to the diffusion of the Yb ions into the host matrix as shown by the Raman shift. More so, the refractive index peaked at n = 2.66 in the λ interval of 350–600 nm, while the SEM images showed nano-balls and nanoclusters which are fairly homogeneous. PL studies exhibited multimode emission bands spanning the UV–Vis–NIR regions. First-principles calculations were employed to investigate the structural properties and electronic structure of the pristine (As–Se) film, as well as Yb contribution. It was found that Yb addition leads to reduce the bandgap energy and consequently the lone pair p-states of Se contribute nearby to Fermi energy level. Hence, the experimental and theoretical results of films afford potential applications in phase-change memory (PCM) and downconverting solar cell window layer.

Pressure-Induced Superconductivity and Flattened Se6 Rings in the Wide Band Gap Semiconductor Cu2I2Se6.

The two major classes of unconventional superconductors, cuprates and Fe-based superconductors, have magnetic parent compounds, are layered, and generally feature square-lattice symmetry. We report the discovery of pressure-induced superconductivity in a nonmagnetic and wide band gap 1.95 eV semiconductor, Cu2I2Se6, with a unique anisotropic structure composed of two types of distinct molecules: Se6 rings and Cu2I2 dimers, which are linked in a three-dimensional framework. Cu2I2Se6 exhibits a concurrent pressure-induced metallization and superconductivity at ∼21.0 GPa with critical temperature (Tc) of ∼2.8 K. The Tc monotonically increases within the range of our study reaching ∼9.0 K around 41.0 GPa. These observations coincide with unprecedented chair-to-planar conformational changes of Se6 rings, an abrupt decrease along the c-axis, and negative compression within the ab plane during the phase transition. DFT calculations demonstrate that the flattened Se6 rings within the CuSe layer create a high density of states at the Fermi level. The unique structural features of Cu2I2Se6 imply that superconductivity may emerge in anisotropic Cu-containing materials without square-lattice geometry and magnetic order in the parent compound.

Bismuth thickness-dependent structural and electronic properties of Bi/As2Se3 bilayer thin films

The structural, microstructural and electronic band gap properties of bilayer Bi/As2Se3 thin films prepared on glass substrate have been discussed in detail. The thin films prepared by thermal evaporation technique under high vacuum were characterized by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM) and UV–visible–NIR spectrophotometer. The bilayer films with small Bi thickness showed amorphous nature, while large Bi thickness film led to the development of sharp XRD peaks corresponding to Bi and Bi2Se3 phase. The band gap of the studied film decreased with increasing Bi layer thickness which is explained on the basis of high concentration of localized defect states in the band gap region. The FESEM images showed the smooth and homogeneous surface for all the films. The Raman spectra showed the different structural units like AsSe3 pyramidal unit and Se8 rings that change with the Bi layer thickness. The present study reveals the interface diffusion of Bi into As2Se3 layer and interaction of Bi with Se due to exposure of the films to UV radiation.

Structure of Amorphous Selenium by 2D 77Se NMR Spectroscopy: An End to the Dilemma of Chain versus Ring

AbstractAmorphous selenium, owing to its tremendous technological importance and perhaps to its chemical simplicity, has been studied for nearly a century and yet an unequivocal structural description of this material remains lacking to date. The primary controversy regarding the structure of amorphous Se relates to the relative fraction of Se atoms residing in chains versus in Se8 rings. Herein we present the results of a two‐dimensional solid‐state 77Se nuclear magnetic resonance (NMR) spectroscopic study of the chain and ring crystalline allotropes of Se as well as of amorphous Se to unequivocally demonstrate that 1) the Se8 rings and the chains are characterized by their unique 77Se NMR signatures and 2) the structure of amorphous Se consists exclusively of chains.

Structure of Amorphous Selenium by 2D 77 Se NMR Spectroscopy: An End to the Dilemma of Chain versus Ring.

Amorphous selenium, owing to its tremendous technological importance and perhaps to its chemical simplicity, has been studied for nearly a century and yet an unequivocal structural description of this material remains lacking to date. The primary controversy regarding the structure of amorphous Se relates to the relative fraction of Se atoms residing in ∞1Se chains versus in Se8 rings. Herein we present the results of a two-dimensional solid-state 77 Se nuclear magnetic resonance (NMR) spectroscopic study of the chain and ring crystalline allotropes of Se as well as of amorphous Se to unequivocally demonstrate that 1) the Se8 rings and the ∞1Se chains are characterized by their unique 77 Se NMR signatures and 2) the structure of amorphous Se consists exclusively of ∞1Se chains.

Molecular origin of aging of pure Se glass: Growth of inter-chain structural correlations, network compaction, and partial ordering.

Glass transition width W of pure Se narrows from 7.1(3) °C to 1.5(2) °C and the non-reversing enthalpy of relaxation (ΔHnr) at Tg increases from 0.23(5) cal/g to 0.90(5) cal/g upon room temperature aging for 4 months in the dark as examined in modulated differential scanning colorimetry (MDSC) at low scan rates. In Raman scattering, such aging leads the A1 mode of Sen-chains (near 250 cm-1) to narrow by 26% and its scattering strength to decrease as the strength of modes of correlated chains (near 235 cm-1) and of Se8 rings (near 264 cm-1) systematically grows. These calorimetric and Raman scattering results are consistent with the "molecular" chains of Sen, predominant in the fresh glass, reconstructing with each other to compact and partially order the network. Consequences of the aging induced reconstruction of the long super-flexible and uncorrelated Sen-chains are also manifested upon alloying up to 4 mol. % of Ge as revealed by a qualitative narrowing (by 25%) of the Raman vibrational mode of the corner-sharing GeSe4 tetrahedra and a blue-shift of the said mode by nearly 1 cm-1 in 194 cm-1. But, at higher Ge content (x> 6%), as the length of Sen chain-segments across Ge cross-links decreases qualitatively (⟨n⟩ < 8), these aging induced chain-reconstruction effects are suppressed. The width of Tg increases beyond 15 °C in binary GexSe100-x glasses as x> 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid.

An economical method for amorphous selenium thick films preparation: e-beam evaporation

We report a controllable, economical and rapid preparation of amorphous selenium (a-Se) thick films by using e-beam evaporation technique. Characterization including X-ray diffraction, Raman and FTIR spectroscopy confirm the amorphous nature of the as-grown films. The films in thickness as high as 267µm were obtained with a deposition rate of 2.11–2.3µm/min. A Raman shift peak at 252.4cm−1 and the absorption of the vibration band at 487.9 and 737.7cm−1 in FTIR spectra are related to the disordered Se chains and Se8 rings assigned to a-Se. All the results indicate that the e-beam evaporation is a promising technique for a-Se films fabrication.

Uncharged Selenium Rings as Terminal and Bridging Ligands in Selenium‐Rich Iridium Clusters

AbstractPurple air‐stable crystals of Ir8Se40Br10 were obtained by reacting Ir in an excess of Se and SeBr4 at 300 °C for two weeks. The monoclinic crystal structure [space group P21/c, a = 1371.9(1) pm, b = 1108.1(1) pm, c = 2058.2(1) pm, β = 106.897(2)° at 296(1) K] consists of uncharged clusters Ir8Se40Br10 = (Ir3+)8(Se22–)(Se42–)6(Se7)2(Br–)10. The iridium(III) cations are coordinated octahedrally by selenium and bromide ions. While the oligoselenide anions provide high intra‐cluster connectivity, μ6‐η2 in the case of Se22– and μ4‐η3 for Se42–, the terminal uncharged Se7 rings have boat conformation and are η1‐coordinating. About 10 % of the Se7 rings are substituted by Se6 rings with chair conformation. A similar reaction using SeCl4 yielded vermillion air‐stable crystals of Ir2Se9Cl6. X‐ray diffraction on a single‐crystal revealed a tetragonal lattice (space group I41/a) with a = 1229.8(1) pm and c = 2365.5(2) pm at 296(1) K. In the Se9(IrCl3)2 cluster, a crown‐shaped uncharged Se9 ring coordinates two iridium(III) cations as a bridging bis‐tridentate ligand. Three terminal chloride ions complete the distorted octahedral coordination of each transition metal atom.

Cs3Se22, a Selenium‐rich Caesium Polyselenide containing Se8 Crowns and $^{3}_{\infty}\rm [Se_{6}^{3-}]$ Anions with Radical Character

AbstractCs3Se22 can be prepared together with the known compounds Cs2Sn2Se6 and Cs4Sn2Se6 by reacting Cs2CO3 and Sn in a CH3OH/en solution at 120 °C. The presence of CuCl and the macrocyclic ligand 1,10‐dithia‐18‐crown‐6 are essential for product formation. The caesium polyselenide consists of anionic nets $^{3}_{\infty}\rm [Se_{6}^{3-}]$ that are separated by two layers of discrete Se8 rings. Within the polymeric anions pairs of Se3·− radical anions and Se32− anions are linked through secondary interactions.

Resonant Raman study of confined Se single helix and Se8 rings

Electron-phonon coupling in Se species confined in the nanoporous matrix has been investigated by using resonant Raman spectra performed with several different laser lines from deep blue to near infrared. The spectra strongly depend on the energies of the excitation laser lines. The one-phonon symmetric A1 modes for Se single helix and Se8 rings are enhanced in the vicinity of their absorption bands. Detailed analysis shows that the Raman band in the high-frequency range of 450–550cm−1 is composed of three individual second-order Raman bands for the confined Se species. These two-phonon Raman shifts occur at twice the frequency shift of the first-order Raman lines, and their intensities are also enhanced when the excitation laser energy matches an electronic transition in Se nanospecies.

An Air-Stable Selenium/Zeolite Nanocomposite

A novel selenium/faujasite nanocomposite was synthesized on the basis of Rb+-exchanged zeolite Y. The confined selenium has an optical band gap of 2.62 eV and exhibits a significantly improved stability in air when compared to other selenium/faujasite composites. Its unique Raman spectrum consists of 12 well-resolved modes, which are attributed to Se8 (D4d) crowns and stable Rb+ coordination complexes with this ring molecule. The Rb+ coordination has a strong structure-directing effect on the confined Se, promoting a complete cyclization of the Se inside the cavities of zeolite Y. The extraordinary stability of this composite is credited to the matching dimensions of the Se8 rings and the aluminosilicate framework as well as to the coordinative immobilization of the Se clusters.

Structure study of Se species in channels of AlPO4-5 crystals

The thermal adsorption/desorption process of selenium species in the channels of AlPO4-5 (AFI) single crystals is studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC). There exist two peaks in the TG and DSC curves, indicating that two different structures of Se species, helical chain and 8-member ring, coexist in the AFI channels. Polarized Raman and optical absorption spectra show that the helical chains are highly aligned along the channel direction, while the Se8 rings are randomly oriented in the channels. The helical chains have a lowest optical absorption band at 2.6 eV, which is strongly polarization dependent, while the 8-member rings have a lowest absorption band at 3.0 eV that varies slowly with the light polarization.

Resonance Raman scattering in In0.45Se0.55 amorphous films

Raman scattering spectra of the In0.45Se0.55 amorphous films recorded in quasi-rectangular geometry with 514, 488 and 458nm excitation lines of an ion argon laser are presented and discussed. The recorded spectra reveal a density of states character with some contribution of a molecular one. The strongest band at about 125cm−1 appears to be superposition of two components peaking at about 115 and 127cm−1, where the former is attributed to the crystal related zone center mode and the latter one to Se8 rings distributed locally. The 115cm−1 component is the strongest in the RS spectra recorded with a 514nm laser line, whereas the 127cm−1 one being the strongest when recorded with 458nm line. With 488nm excitation the intensities of the two components are nearly the same. The intensity increase of the 125cm−1 band is seen to increase with increasing wavelength of the excitation line. The observed intensity variations of the 125cm−1 band and its two components are interpreted in terms of resonance enhancement due to involvement of two electronic levels of the studied amorphous system with energies equal to 2.4 and 2.7eV, close to energies of the 514 and 458nm photons, respectively.

Isolated Se6Rings in the Voids of a Weakly Interacting, Electroneutral, and Crystalline SiO2Matrix: Combined Experimental and Theoretical Study

For the first time a selenium species has been successfully trapped in the voids of an electroneutral SiO2 modification, namely, in the porosil decadodecasil 3R (DDR). Raman spectroscopy in combination with theoretical ab initio calculations at the SCF and MP2 levels of theory have been used to assign the observed spectral bands exclusively to Se6 rings with D3d symmetry. The A1g(r) and Eg(r) mode frequencies of the matrix-isolated Se6 rings are considerably higher than those of bulk Se6, suggesting that intermolecular correlations are lost for the perfectly isolated molecules in the microporous host matrix. By an EXAFS analysis of Se K-spectra, Se−Se bond lengths of 2.34 Å and next-nearest Se···Se distances of 3.61 Å are obtained, in very good agreement with the theoretical ab initio studies. EXAFS analysis in addition reveals that the interaction between the SiO2 framework and the embedded Se6 rings are weak, resulting in a novel host−guest compound with nearly unperturbed Se6 guest molecules. The crystalline electroneutral host thus acts similarly to a noble gas matrix at low temperatures.

Powerful Templating Effect in Rb/Pd/Sex Promoted by Crown Ether-like [Rb(Se8)]+ Coordination. Formation of Rb2[Pd(Se4)2]·Se8: A Layered Pd Polyselenide with “Encapsulated” Eight-Membered Selenium Rings

A novel layered Rb−Pd polyselenide has been synthesized by methanothermal reaction of Pd2+ with Sex2- (from Se2- + Se) in the presence of Rb2CO3. Rb2[Pd(Se4)2]·Se8 (I) reveals a unique structure with infinite [Pd(Se4)2]2- sheet polyanions acting as a host for neutral Se8 rings. The black shiny rectangular crystals of Rb2[Pd(Se4)2]·Se8 are insoluble in H2O and common organic solvents. The compound crystallizes in the noncentrosymmetric space group P4̄b2 (No. 117) with a = 12.6748(8) Å, c = 6.9273(7) Å, Z = 2, and V = 1112.9(2) Å3. The [Pd(Se4)2]2- sheet anion contains Pd2+ ions with a distorted square planar coordination geometry to four terminal Se atoms of Se42- chains. The cocrystallized neutral Se8 rings have a crown-like conformation as known from the elemental Se8 forms. Through coordination to Rb+ ions they build [Rb(Se8)+]n “cationic chains” running in the c direction. Rb2[Pd(Se4)2]·Se8 therefore differs fundamentally from its “neighboring” alkali metal Pd polyselenides, K2PdSe10 and Cs2PdSe8, which each possess a three-dimensional structure with two interpenetrating [Pd(Sex)2]2- frameworks. Optical spectroscopic data and the thermal stability of the compound are discussed.

Raman and X-ray absorption study of selenium incorporated into the channels of mordenite: Dependence on the ion exchange and the method of incorporation

Influence of ion exchange and method of selenium incorporation into mordenite channels on the structure of incorporated selenium species are studied by Raman and X-ray absorption spectroscopy. It is shown that Se chains prepared by Se vapor adsorption are more regular than Se chains prepared by pressure injection of liquid Se. Concentration ratio between Se chains and Se 6 rings is found to be practically independent of the method of preparation but dependent on the type of cations in mordenite channels. Substitution of Na cations by larger monovalent cations leads to a decrease in Se chains concentration compared to Se 6 rings concentration and to an increase in average SeSe bond length. X-ray absorption spectrum of M-Se for polarization of radiation perpendicular to the channels is shown to be determined mainly by Se6 rings, and that for polarization parallel to the channels is a superposition of the absorption of chains and rings. Selenium incorporated into H-form of mordenite is found to be located not only in the original mordenite channels, but also in meso-channels organized due to partial destruction of the interchannel walls.