Se Substitution Research Articles

Visualizing superconductivity in a doped Weyl semimetal with broken inversion symmetry

The Weyl semimetal $\mathrm{Mo}{\mathrm{Te}}_{2}$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5 K, but suppresses the ${T}_{d}$ structure phase that is essential for the emergence of the Weyl state. A microscopic understanding of the possible coexistence of enhanced superconductivity and the ${T}_{d}$ phase has not been established so far. Here, we use scanning tunneling microscopy to study an optimally doped superconductor $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ with bulk ${T}_{c}\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. By means of quasiparticle interference imaging, we identify the existence of a low-temperature ${T}_{d}$ phase with broken inversion symmetry where superconductivity globally coexists. Furthermore, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of the existing chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in $\mathrm{Mo}{\mathrm{Te}}_{1.85}{\mathrm{Se}}_{0.15}$ make it a promising candidate for realizing topological superconductivity.

Influence of Se and Te substitutions at Tl-site of Tl(Ba,Sr)CaCu2O7 superconductor on the AC susceptibility and electrical properties

The Sr and Ba bearing Tl-1212 phase, Tl(Ba,Sr)CaCu2O7 is an interesting superconductor. The Sr only bearing TlSr2CaCu2O7 is not easily prepared in the superconducting form. The Ba only bearing TlBa2CaCu2O7 on the other hand does not show improvement in the transition temperature with elemental substitution. In this work the influence of multivalent Se (non-metal) and Te (metalloid) substitutions at the Tl-site of Tl1-xMx(Ba,Sr)CaCu2O7 (M = Se or Te) superconductors for x = 0–0.6 was studied. The samples were prepared via the conventional solid-state reaction method. XRD patterns showed a single Tl-1212 phase for x = 0 and 0.1 Se substituted samples. The critical current density at the peak temperature, Tp of the imaginary (χ”) part of the AC susceptibility (χ = χ’ +χ”), Jc-inter(Tp) for all samples was between 15 and 21 A cm−2. The highest superconducting transition temperature was shown by the x = 0.3 Se-substituted sample (Tc-onset = 104 K, Tc-zero = 89 K, Tcχ’ = 104 K and Tp = 80 K). Te suppressed the superconductivity of Tl-1212 phase. The order of highest transition temperatures are as follows: Tl1-xTex(Ba,Sr)CaCu2O7<Tl(Ba,Sr)CaCu2O7<Tl1-xSex(Ba,Sr)CaCu2O7. This work showed that Se was better than Te in improving the transition temperature and flux pinning of the Tl-1212 phase. The roles of ionic radius of Se and Te on the superconductivity of Tl(Ba,Sr)CaCu2O7 are discussed in this paper.

Reduction in low-temperature thermal conductivity of Cu2Se via substitution of Se by Te atoms

We report the structural and thermoelectric properties of the Cu2Se compound with a systematic substitution of Se by Te atom. The bulk polycrystalline Cu2Se1-xTex samples for the Te concentrations of x = 0.00, 0.02, 0.04, 0.06, and 0.08 were prepared by the solid-state reaction method. The room-temperature XRD studies revealed that the studied samples possess a monoclinic crystal structure. The oxidation state and electronic structure are confirmed by employing X-ray photoemission spectroscopy (XPS). The hardness of the samples increased systematically with an increase in Te doping concentration. The electrical resistivity of the doped samples was found to be increased in comparison to the pure sample. The Seebeck coefficient was positive throughout the temperature range under investigation, indicating that holes are the majority charge carriers in the studied compounds. The thermal conductivity of the doped samples decreased, which is presumably attributed to the point defect scattering. The systematic evolution of thermoelectric properties via substitution of Se by Te over the range (0.00 ≤ x ≤ 0.08) concludes that the overall thermoelectric power factor (PF) and figure of merit (ZT) decrease with Te substitution compared to the pristine Cu2Se compound.

Investigation of lattice anharmonicity in thermoelectric LaOBiS2–x Se x through Grüneisen parameter

The Se substitution effect on Grüneisen parameter (γ G) of thermoelectric layered compound LaOBiS2–x Se x , whose highest dimensionless figure-of-merit is 0.36 at 650 K, was investigated. From the estimated volume thermal expansion coefficient, bulk modulus, molar volume, and molar heat capacity at constant volume, γ G was calculated as 0.88, 1.46, and 1.36 for x = 0, 0.5, and 1.0, respectively. The increase in γ G by Se substitution, obtained in this study, is consistent with the results obtained in previous neutron inelastic scattering experiments and proves enhanced lattice anharmonicity by Se substitution.

Effects of Se substitution on the Schottky barrier of a MoS x Se(2−x)/graphene heterostructure

One of the most fundamental and challenging tasks to achieve high-performance ultra-thin atomic field effect transistors (FETs) is to obtain very low or even zero Schottky barrier height (SBH) at source/drain contact. Here, we propose that heteroatom substitution is an effective strategy to tune the performance of two-dimensional materials-based FETs, which is demonstrated by systematically exploring the effects of Se substitution on the structural and electronic properties, and SBH of MoS x Se(2−x)/graphene (MoS x Se(2−x)/G) heterostructures using first-principles calculations. Our findings suggest that the type and height of Schottky barrier can be adjusted by varying Se concentration. The transformation from n-type Schottky barrier to p-type Schottky barrier can be realized when the Se concentration is greater than 25%. With the increase of Se concentration, a lower p-type Schottky barrier can be obtained at the interface to achieve efficient charge transfer. Moreover, the Schottky barrier of MoS x Se(2−x)/G heterostructures with different Se concentration would disappear as the external electric field exceeds certain values. These results would provide a direction in developing high-performance FETs involving heteroatom substitution layers as contact electrodes.

Growth and electrical properties of SnS1-xSex (0 ≤ x ≤ 1) single crystals grown using the temperature gradient method

Tin selenide (SnSe) has attracted much attention due to its record ZT value in both pristine and doped crystals. This issue has renewed interest in the single-crystal form of tin-based chalcogenide compounds which possess the same layered structure as SnSe such as tin sulfide (SnS). Due to their natural abundances and low toxicities, SnSe and SnS can be alloyed at reduced the processing cost while maintaining the favorable electrical properties of SnSe. In this research, using the temperature gradient method, we successfully fabricated single crystals of SnS1-xSex with 0 ≤ x ≤ 1. High-quality crystals were obtained, and the electrical properties of those crystals were investigated. The results showed that the p-type crystals have layered structures with lattice constants changing gradually according to Vegard’s law. The value of the band gap monotonically decreased with increasing Se amount (x). The substitution of Se into S sites results in not only a large increase in the electrical conductivity but also a decrease in the Seebeck coefficient; i.e., the electrical conductivity at room temperature increased from 5 × 10–4 (x = 0) to 5.24 S⋅cm−1 (x = 0.8). The Seebeck coefficient decreased from 1069 in SnS to 525 μVK−1 in SnS0.2Se0.8 and to 481 μVK−1 in SnSe at room temperature. This work provides positive information for the growth of large-size SnS–SnSe single crystals.

Structural transformation and phase change properties of Se substituted GeTe

GeTe1−xSex (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe1−xSex thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Room temperature magnetic ordering in very dilute Iron doped with and without selenium substitutions in antimony matrix

Mossbauer spectroscopy is one of the very sensitive tools to study the hyperfine interactions in alloys and compounds. The interaction of the probe with the local environment gives information like spin state, coordination number, magnetic and quadrupole interaction etc. In the present study, 57Fe Mossbauer spectroscopy was used to study the hyperfine interaction of the FexSb1-x-ySey bulk alloy with very dilute quantity of Fe, x = 0.002. The measurements were done at room temperature. Keeping the amount of Fe constant in the bulk alloy at x = 0.002, the concentration of Se was varied from y = 0.000, 0.004, 0.016 to 0.048. Additionally, to see the effect of extremely low concentration of Fe in Sb matrix alone, a sample with Fe concentration of 0.0015 was prepared without any Se substitution in Sb. The structural study was done using X-ray diffraction technique. Neutron depolarization study was done to see if clusters of Fe were present in the samples. The XRD spectra showed Sb peaks with no other compound phases. From Mossbauer spectroscopy, at y = 0.000, x = 0.002 one magnetic site (site A) with hyperfine magnetic field of ~312 kOe was observed. When the concentration of Fe was further reduced to x = 0.0015, there was no change in the hyperfine magnetic field of ~312 kOe. On introduction of Se into the system, we observed two magnetic sites (A and B). The value of hyperfine magnetic field (HMF) for both the sites (A and B) decreases with increases in Se concentration. At higher concentration of Se (y = 0.048) an additional nonmagnetic site (site C) with quadrupole splitting of 0.51 mm/s was also observed.

Impact of selenium composition variation in CZTS solar cell

A comparative performance analysis of selenium composition variation in Cu2ZnSn(SXSe1-X)4 is analytically performed in this paper. We obtained the optimal stoichiometry (Cu2ZnSnS1.8Se2.2) where efficiency is highest. Comparative simulation brings out the trend of device characteristics variation with stoichiometry. The comparative simulation for band diagram shows the band banding at rear in CZTS whereas it is absent in CZTSe. This highlights the issue of back contact in pure sulphide CZTS. The cut off wavelength increases in QE plot as we move from CZTS to CZTSe compositions. The VOC-illumination and VOC-temperature plots shows (i) VOC saturation in CZTS and (ii) a linear rise of VOC for CZTSe. Nyquist impedance plot shows impedance decrease with selenium substitution. Se substitution enhances the chalcogenide opto-electronic properties as a whole and enhances devices performance. This enables higher efficiency of the CZTSSe devices as compared to the pure sulphide CZTS devices. An optimal stoichiometry obtained is (Cu2ZnSnS1.8Se2.2) with efficiency of 15.3 %, pure selenide CZTSe has efficiency of 13.4 % and 11.6 % for pure sulphide CZTS solar cell.

Effects of Se Doping on Thermoelectric Properties of Tetrahedrite Cu12Sb4S13−zSez

Tetrahedrite Cu12Sb4S13 is composed of abundant non-toxic components and has attracted attention as a promising thermoelectric material with low thermal conductivity at intermediate temperatures. The carrier concentration can be optimized by doping (substituting), thereby maximizing its power factor and reducing its thermal conductivity. In this study, Cu12Sb4S13−zSez (z = 0.1–0.4) compounds were synthesized using mechanical alloying and hot pressing. Our objective was to maintain a high power factor through Se doping and to reduce the lattice thermal conductivity through additional phonon scattering. X-ray diffraction analysis revealed that the lattice constant increased with an increase in Se substitution for the S sites, and all the specimens appeared as a single tetrahedrite phase. As the Se doping level increased, the carrier (hole) concentration decreased while the mobility increased. The Hall and Seebeck coefficients were both positive, indicating that Se-doped tetrahedrites exhibit p-type conduction. As the Se substitution increased, the electrical conductivity decreased, but the Seebeck coefficient increased. In addition, Se doping lowered both, the electronic and lattice thermal conductivities, which resulted in decreased thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.87 was obtained at 723 K for Cu12Sb4S12.8Se0.2 with a high power factor of 0.96 mW m−1 K−2 and a low thermal conductivity of 0.77 W m−1 K−1.

Outstanding Thermoelectric Performance of MCu3X4 (M = V, Nb, Ta; X = S, Se, Te) with Unaffected Band Degeneracy under Pressure

The demand for green energy increases day by day due to environmental concern and thermoelectric (TE) materials are one of the eco-friendly energy resources. Few authors reported high TE performance in TmCu$_3$Ch$_4$, reaching the figure of merit (ZT) above 2 at 1000K, from first-principles calculations neglecting electron-phonon scattering, spin-orbit coupling effect (SOC), and energy-dependent carrier lifetime. Here, thermoelectric transport properties of TmCu$_3$Ch$_4$ are reinvestigated through considering these parameters, and significant discrepancies are found. The ZT of p-type TaCu$_3$Te$_4$ can reach ~3 at 1000K among these compounds due to its low lattice thermal conductivity ($\kappa_l$) (0.38 W m-1 K-1). Interestingly, the value of $\kappa_l$ is reduced to 0.17 W m-1 K-1 through 1 GPa pressure while the power factor is slightly improved due to bandgap reduction, leading to an extraordinary ZT~5.5 at 1000K. Although the substitution of Se causes a slight reduction of $\kappa_l$ to ~0.3 W m-1 K-1, the power factor is reduced significantly due to the dramatic reduction of DOS near Fermi level, which leads to lower the Seebeck coefficient largely and increase electrical conductivity slightly.

Structural Insights into the Interaction of Clinically Relevant Phosphorothioate mRNA Cap Analogs with Translation Initiation Factor 4E Reveal Stabilization via Electrostatic Thio-Effect.

mRNA-based therapies and vaccines constitute a disruptive technology with the potential to revolutionize modern medicine. Chemically modified 5′ cap structures have provided access to mRNAs with superior translational properties that could benefit the currently flourishing mRNA field. Prime examples of compounds that enhance mRNA properties are antireverse cap analog diastereomers that contain an O-to-S substitution within the β-phosphate (β-S-ARCA D1 and D2), where D1 is used in clinically investigated mRNA vaccines. The compounds were previously found to have high affinity for eukaryotic translation initiation factor 4E (eIF4E) and augment translation in vitro and in vivo. However, the molecular basis for the beneficial “thio-effect” remains unclear. Here, we employed multiple biophysical techniques and captured 11 cap analog-eIF4E crystallographic structures to investigate the consequences of the β-O-to-S or -Se substitution on the interaction with eIF4E. We determined the SP/RP configurations of β-S-ARCA and related compounds and obtained structural insights into the binding. Unexpectedly, in both stereoisomers, the β-S/Se atom occupies the same binding cavity between Lys162 and Arg157, indicating that the key driving force for complex stabilization is the interaction of negatively charged S/Se with positively charged amino acids. This was observed for all structural variants of the cap and required significantly different conformations of the triphosphate for each diastereomer. This finding explains why both β-S-ARCA diastereomers have higher affinity for eIF4E than unmodified caps. Binding affinities determined for di-, tri-, and oligonucleotide cap analogs suggested that the “thio-effect” was preserved in longer RNAs. Our observations broaden the understanding of thiophosphate biochemistry and enable the rational design of translationally active mRNAs and eIF4E-targeting drugs.

Boosting Thermoelectric Properties of AgBi3(SeyS1-y)5 Solid Solution via Entropy Engineering.

AgBi3S5 is an environmentally friendly n-type thermoelectric material composed of earth-abundant and nontoxic elements. It has a complex monoclinic structure with distorted NaCl-type fragments, which provide its intrinsically low thermal conductivity. However, poor electrical properties limit its overall performance. Configurational entropy engineering is an effective method to enhance thermoelectric properties. With the increase of configurational entropy, phonon point defect scattering is amplified, yielding lower lattice thermal conductivity, while the structure symmetry can also be improved, which leads to the enhanced electrical transport property. In this study, we combine carrier modulation and entropy engineering, utilizing melting-annealing and spark plasma sintering, to synthesize a series of AgBi3(SeyS1-y)5.08 bulks. Se substitution effectively increases the configurational entropy and thus dramatically decreases the thermal conductivity. Moreover, anion deficiency modulation effectively optimizes the carrier concentration and the electrical transport properties. Due to a power factor of 2.7 μW/(cm·K2) and a low thermal conductivity of 0.45 W/(m·K) at 723 K, the AgBi3(Se0.9S0.1)5.08 sample possesses the highest ZT of 0.42 at 723 K, nearly double the value of AgBi3S5.08 or pristine AgBi3S5. Our work demonstrates that apart from carrier optimization, entropy engineering opens a new avenue for enhancing the thermoelectric properties of a given material.

Anomalous Charge Density Wave State Evolution and Dome-Like Superconductivity in CuIr &lt;sub&gt;2&lt;/sub&gt;Te &lt;sub&gt;4-X&lt;/sub&gt;Se &lt;sub&gt;x&lt;/sub&gt;

The interplay between superconductivity (SC) and the charge density wave (CDW)-like order in CuIr2Te4 is systematically explored through Se substitution for Te site through the powder X-ray diffraction (PXRD), magnetic susceptibility, resistivity measurements and heat capacity. CuIr2Te4-XSex (0 ≤ x ≤ 0.5) compounds retain the same structure as the host CuIr2Te4. The unit cell parameters a and c decrease linearly with increasing Se content. Tc exhibits a weak dome-like variation as substituting Te by Se with the maximum Tc = 2.83 K for x = 0.1 followed by suppression in Tc and simultaneous decrease of the superconducting volume fraction. Unexpectedly, the CDW-like transition (TCDW) is suppressed with lower Se doping (0.025 ≤ x ≤ 0.2) but re-emerge at higher doping (0.25 ≤ x ≤ 0.5). We propose that it arises from the disorder effect by chemical doping. The lower and the upper critical field and these compounds are also addressed.

Synthesis PbFCl-Type Mixed Anion &lt;i&gt;A&lt;/i&gt;P&lt;i&gt;X&lt;/i&gt;(&lt;i&gt;A&lt;/i&gt;=Hf, &lt;i&gt;X&lt;/i&gt;=S, Se) Superconductors Related with Topological Materials by High-Pressure Technique

We performed a systematic study of the crystal structure, physical properties, and electronic structure of PbFCl-type intermetallic APX(A=Zr, Hf, X=S and Se) superconductor. We successfully synthesized single-phase polycrystalline samples for the Se substitution range of 0.4≤x≤0.8 in Zr(P2-xSex) using high pressure technique. On the other hand, S substitution range in Zr(P2-xSx) was narrow of 0.4≤x≤0.6, the S substitution range in Hf(P2-xSx) was narrow of x≈0.55, and the Se substitution range in Hf(P2-xSex) was also very narrow of x≈0.4. Zr(P2-xSex) exhibits a dome-like superconducting phase diagram for the substitution amount x. The superconducting transition temperature (Tc) is achieved at approximately x≈0.75 at which point the Tcis 6.3 K for Zr(P1.25Se0.75), 5.5 K for Hf(P1.30Se0.70), 5.0 K for Zr(P1.325S0.675), and 4.6 K for Hf(P1.50S0.50), respectively. Tcfor Zr(P1.25Se0.75) is increased from 6.3 K to 7.6 K by partially substituting a non-magnetic rare earth Sc atom for Zr. Single crystals of Zr(P1.25Se0.75) and partially substituted by Sc atom for Zr site of the ZrP2-xSexwere also grain grown using high pressure technique. Plate-like single crystal with approximate edge sizes of up to 500 × 300 × 20 μm3for (Zr0.50Sc0.50)PSe nominal composition was obtained. Tc= 8.36 K was reached and the Hc1//c-axis(0) and Hc1⊥c-axis(0) roughly determined are 0.0045 T and 0.0038 T, respectively. Assuming the type-II superconductivity in dirty limit, the Hc2//c-axis(0) value of 1.33 T was also obtained. In this presentation, the crystal growth and physical properties of this APX(A=Zr, Hf, X=S and Se) superconductor has reported.

Дефекты межслоевой поверхности и термоэлектрические свойства в слоистых пленках топологических изоляторов n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-2.7-=/SUB=-Se-=SUB=-0.15-=/SUB=-S-=SUB=-0.15-=/SUB=-

Interlayer surface defects and thermoelectric properties in layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators L. N. Lukyanova*, O. A. Usov, M. P. Volkov, V. A. Rusakov Ioffe Institute Russian academy of science, 194021 St.Petersburg, Russia *E-mail: lidia.lukyanova@mail.ioffe.ru Abstract In layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators optimized for temperatures below room temperature, the morphology of the (0001) interlayer surface and thermoelectric properties were studied. On the profiles of the (0001) surface, we identified neutral impurity defects arising from the substitution of Se and S atoms for Te atoms and donor antisite defects of tellurium at bismuth sites, which affect the thermoelectric properties. The average value of thermoelectric figure of merit in n-Bi2Te2.7Se0.15S0.15 films increases to &lt;Z&gt; ≈ 3.0•10-3 K-1 in the range 80 – 215 K, while in bulk solid solution &lt;Z&gt; ≈ 2.0•10-3 K-1. An increase in the thermoelectric figure of merit in films is associated with an increase in the energy dependence of the relaxation time due to an increase in the effective scattering parameter reff. It is shown that in films the Seebeck coefficient, the density of states effective mass m/m0, and the material parameter proportional to the power factor increase, while the lattice κL and electronic thermal conductivity κe decrease, which determines the increase in thermoelectric figure of merit.

Galectin-Glycan Interactions: Guidelines for Monitoring by 77 Se NMR Spectroscopy, and Solvent (H2 O/D2 O) Impact on Binding.

Functional pairing between cellular glycoconjugates and tissue lectins like galectins has wide (patho)physiological significance. Their study is facilitated by nonhydrolysable derivatives of the natural O‐glycans, such as S‐ and Se‐glycosides. The latter enable extensive analyses by specific 77Se NMR spectroscopy, but still remain underexplored. By using the example of selenodigalactoside (SeDG) and the human galectin‐1 and ‐3, we have evaluated diverse 77Se NMR detection methods and propose selective 1H,77Se heteronuclear Hartmann–Hahn transfer for efficient use in competitive NMR screening against a selenoglycoside spy ligand. By fluorescence anisotropy, circular dichroism, and isothermal titration calorimetry (ITC), we show that the affinity and thermodynamics of SeDG binding by galectins are similar to thiodigalactoside (TDG) and N‐acetyllactosamine (LacNAc), confirming that Se substitution has no major impact. ITC data in D2O versus H2O are similar for TDG and LacNAc binding by both galectins, but a solvent effect, indicating solvent rearrangement at the binding site, is hinted at for SeDG and clearly observed for LacNAc dimers with extended chain length.

Transition from band insulator to excitonic insulator via alloying Se into monolayer TiS3 : A computational study

First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS$_{3-x}$Se$_x$ ($x$=1 and 2). It is found that bandgap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the bandgap can be significantly decreased as compared to that of pristine TiS$_3$, the exciton binding energy just varies a little, regardless of position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with the spontaneous exciton condensation. Our work thus offers a new insight for engineering an excitonic insulator.

Hole transport nature exploration of 4,4-Difluoro-8-(C4H3X)-4-bora-3a,4a-diaza-s-indacene (X = O, S, Se) (BODIPY) systems

ABSTRACT Charge transport and electronic character of fluorophore chalcogenides 4,4-Difluoro-8-(-XC4H3)-4-bora-3a,4a-diaza-s-indacene (X = O, S, Se) (BODIPY) derivatives were examined at the molecular level. The effect of chalcogenides was investigated on the transfer integrals, electron affinity (EA), frontier molecular orbitals (FMO), ionisation energy (IE), intrinsic mobility along with reorganisation energies. The ring dissimilarities within 4,4-difluoro-8-(-XC4H3)-4-bora-3a,4a-diaza-s-indacene (X = O, 2-/3-furyl (Comp1/Comp4)); (X = S, 2-/3-thienyl (Comp2/Comp4)) and (X = Se, 2-/3-selenenyl (Comp3/Comp6)) were compared systematically. Among BODIPY chalcogenides derivatives, 2-furyl substitution is more beneficial compared with 2-thienyl, 2-selenenyl and 3-furyl as the prior moiety significantly increases intrinsic hole mobility. The –S or –Se substitution at 2- or 3- position in place of –O at equivalent position reduces the structural polarisation from neutral to cation resulting in reduced hole reorganisation energies. The doping of –S or –Se at 2-position decreases the hole transfer integral in comparison to 2-furyl-substituted derivative. The effect of chalcogen atoms on 2- or 3- position is not so significant on the HOMO/LUMO energy levels. It is expected that –O doping at 2-position in furyl is supporting p-type charge transport channel. These results demonstrated that the studied BODIPY derivatives would be efficient hole transfer materials to be used in organic semiconductor devices.

Study on charge mobility of hexathiapentacene and its selenium analogs

The relationship between molecular geometries, crystal structures and charge mobilities of hexathiapentacene (HTP) and three of its derivatives (2Se-HTP, 4Se-HTP, 6Se-HTP) were studied using the density functional theory combined with a hopping mechanism at the molecular and crystal level. The effect of Se substitution on the charge mobility was discussed. The calculated results showed that the derivatives exhibit good planarity and the molecular geometries show little variation during the charge transfer process. The electron mobility is 1.20 cm2 V-1 S-1 for HTP and 2.30 cm2 V-1 S-1 for 6Se- -HTP, which are much larger than the corresponding hole ones, indicating that HTP and 6Se-HTP are good candidates for n-type organic semiconductors. However, 2Se-HTP and 4Se-HTP have comparable hole and electron motilities and are suitable for ambipolar semiconductors.