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.

Impact of molecular and packing structure on the charge-transport properties of hetero[8]circulenes

The charge transfer mobility of hetero[8]circulenes tends to increase with O/NH or S/Se substitution and benzoannelation.

Synthesis PbFCl-Type Mixed Anion APX(A=Hf, X=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.

Diketopyrrolopyrrole Derivatives Functionalized with N-Annulated PDI and Se-Annulated PDI by Direct (Hetero)Arylation Methods

<p>We report on the synthesis of diketopyrrolopyrrole (DPP) derivatives functionalized with N-annulated and Se-annulated perylene diimide (NPDI and SePDI, respectively) <i>via </i>direct (hetero)arylation methods. DPP is symmetrically bifunctionalized with SePDI (SePDI–DPP–SePDI) and unsymmetrically functionalized with SePDI and NPDI (SePDI–DPP–NPDI). The effects of Se substitution compared to N substitution on physical, electrochemical, and optical properties are investigated along with performance as non-fullerene acceptors in photovoltaic devices. It is found that Se substitution increases the electron affinity of the p-conjugated molecule and blue shifts the optical absorption spectra, observations that were supported by computational analysis. Steric strain between the PDI endcap and DPP core prevent complete electronic communication along the p-conjugated backbone and results in the unsymmetrical compound, SePDI‒DPP‒NPDI, having electronic and optical properties that are a linear combination of both the symmetrical SePDI and NPDI based compounds. Different is that the SePDI‒DPP‒NPDI compound has a distinct melt observed at 343 °C and organic photovoltaic devices based on this compound had lower than expected open-circuit voltages, suggesting a unique solid-state packing arrangement. SePDI-based compounds performed worse than the NPDI-based compound in organic photovoltaic devices using the donor polymer PTB7-Th. </p>

Diketopyrrolopyrrole Derivatives Functionalized with N-Annulated PDI and Se-Annulated PDI by Direct (Hetero)Arylation Methods

<p>We report on the synthesis of diketopyrrolopyrrole (DPP) derivatives functionalized with N-annulated and Se-annulated perylene diimide (NPDI and SePDI, respectively) <i>via </i>direct (hetero)arylation methods. DPP is symmetrically bifunctionalized with SePDI (SePDI–DPP–SePDI) and unsymmetrically functionalized with SePDI and NPDI (SePDI–DPP–NPDI). The effects of Se substitution compared to N substitution on physical, electrochemical, and optical properties are investigated along with performance as non-fullerene acceptors in photovoltaic devices. It is found that Se substitution increases the electron affinity of the p-conjugated molecule and blue shifts the optical absorption spectra, observations that were supported by computational analysis. Steric strain between the PDI endcap and DPP core prevent complete electronic communication along the p-conjugated backbone and results in the unsymmetrical compound, SePDI‒DPP‒NPDI, having electronic and optical properties that are a linear combination of both the symmetrical SePDI and NPDI based compounds. Different is that the SePDI‒DPP‒NPDI compound has a distinct melt observed at 343 °C and organic photovoltaic devices based on this compound had lower than expected open-circuit voltages, suggesting a unique solid-state packing arrangement. SePDI-based compounds performed worse than the NPDI-based compound in organic photovoltaic devices using the donor polymer PTB7-Th. </p>

Study on charge mobility of hexathiapentacene and its selenium analogs

The relationship between molecular geometries, crystal structures and charge mobilities of hexathiapentacene (HTP) and its three derivatives (2Se-HTP, 4Se-HTP, 6Se-HTP) were studied with density functional theory combined with hopping mechanism in 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 have 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 semiconductor. However, 2Se-HTP and 4Se-HTP have comparable hole and electron mobilities and are suitable for ambipolar semiconductor.