Christos D Malliakas | Northwestern University (original) (raw)

Papers by Christos D Malliakas

Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI... more Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.

Crystal Growth & Design, Oct 19, 2017

Tl 6 SeI 4 is a promising wide-bandgap semiconductor for room-temperature highenergy photon detec... more Tl 6 SeI 4 is a promising wide-bandgap semiconductor for room-temperature highenergy photon detection. Because of the air-sensitive Tl precursor or Tl-based binary precursors used in the synthesis, this material can contain deleterious Tl oxide impurities. These impurities lead to problems during syntheses and crystal growth including glass attack, tube rupture, and parasitic nucleation, which subsequently deteriorate detector performance. In this work, we present a facile way to chemically reduce Tl oxides and eliminate oxygen impurities in Tl 6 SeI 4 by adding high-purity graphite powder during synthesis. The addition of carbon leads to reduction of the residual Tl oxides and formation of CO 2. The resistivity and hard radiation detection performance for 122 keV γ-rays of Tl 6 SeI 4 single crystals was drastically improved. The improvement in the crystallinity was also confirmed by a narrower near-band-edge emission band in the photoluminescence spectra. We confirmed that the reaction between Tl oxide and graphite occurs, and propose a mechanism which is highly effective in substantially reducing oxide impurities from Tl-containing precursors. First-principles DFT calculations reveal that the presence of interstitial oxygen atoms (O int) leads to the formation of a deep level located near the middle of the gap, which can act as carrier traps detrimental to detector performance. The calculations also indicate that graphite addition is safe for detector performance because all carbon-induced defects have high formation energy and are not likely to appear in lattice. ■ INTRODUCTION Wide-band-gap compound semiconductors are of great interest for room temperature hard radiation (X-rays, γ-rays and α-particles) detection applications including homeland security, medical imaging, and non-proliferation of nuclear materials. 1, 2 However, very few compounds have been identified as hard radiation detector materials, as a set of strict physical property requirements must be simultaneously met to generate spectroscopic detection performance. 1, 2 Even the most successful room temperature detection material, Cd 0.9 Zn 0.1 Te (CZT), 3 still suffers

Physical Review Materials, Oct 7, 2019

The cubic intermetallic compound Ba 10 Ti 24 Bi 39 has a Ba 6 Bi 16 polyhedral cage with a Bi gue... more The cubic intermetallic compound Ba 10 Ti 24 Bi 39 has a Ba 6 Bi 16 polyhedral cage with a Bi guest atom encapsulated inside. The compound can be formulated as Ba 5 Ti 12 Bi 19+x when x signifies the extra Bi atoms filling cages. It crystallizes in a complex noncentrosymmetric cubic structure in space group P-43m with cell parameter a = 12.6787(4) Å. The guest Bi atoms distribute diffusely in the cages and seem to play a role in stabilizing the crystal structure. The magnetic susceptibility of this compound shows a weak temperature dependence with a positive slope coefficient. The charge transport properties as a function of temperature exhibit two competing components which are in charge of positive and negative magnetoresistances. Electronic band-structure calculations reveal the complex multiband hybridization of Ti/Bi orbitals near the Fermi surface, which may play a role in the enormous electron-electron scattering in this material evidenced by the large Kadowaki-Woods ratio.

Nature Synthesis, Aug 15, 2022

Chemistry of Materials, Jul 30, 2015

ABSTRACT

Crystal Growth & Design, Apr 5, 2016

We assess the mercury chalcohalide compound, β-Hg 3 S 2 Cl 2 , as a potential semiconductor mater... more We assess the mercury chalcohalide compound, β-Hg 3 S 2 Cl 2 , as a potential semiconductor material for X-ray and γ-ray detection. It has a high density (6.80 g/cm 3), wide band gap (2.56 eV) and crystallizes in the cubic Pm-3n space group with a three-dimensional structure comprised of [Hg 12 S 8 ] cubes with Cl atoms located within and between the cubes, featuring a trigonal pyramidal SHg 3 as the main building block. First-principle electronic structure calculations at the density functional theory level predict that the compound has closely lying indirect and direct band gaps. We have successfully grown transparent, single crystals of β-Hg 3 S 2 Cl 2 up to 7 mm diameter and 1 cm long using a new approach by the partial decomposition of the quaternary Hg 3 Bi 2 S 2 Cl 8 compound followed by the formation of β-Hg 3 S 2 Cl 2 and an impermeable top layer, all happening in-situ during vertical Bridgman growth. The decomposition process was optimized by varying peak temperatures and temperature gradients using a 2 mm/h translation rate of the Bridgman technique. Formation of the quaternary Hg 3 Bi 2 S 2 Cl 8 followed by its partial decomposition into β-Hg 3 S 2 Cl 2 was confirmed by in-situ temperature-dependent synchrotron powder diffraction studies. The single crystal samples obtained had resistivity of 10 10 Ω•cm and mobility-lifetime products of electron and hole carriers of 1.4(4) x 10-4 cm 2 /V and 7.5(3) x 10-5 cm 2 /V, respectively. Further, an appreciable Ag X-ray photoconductivity response was observed showing the potential of β-Hg 3 S 2 Cl 2 as a hard radiation detector material.

Journal of the American Chemical Society, Nov 13, 2009

Chemistry of Materials, May 1, 2015

Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a comp... more Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a complex problem, especially when approaching room temperature. Here, we demonstrate the important role of defects in the magnetic properties of semiconductors by reporting the structures and properties of the iron chalcogenides (BaF) 2 Fe 2−x Q 3 (Q = S, Se), which exhibit anomalous magnetic properties that are correlated with defects in the Fe-sublattice. The compounds form in both long-range ordered and disordered polytypes of a new structure typified by the alternate stacking of fluorite (BaF) 2 2+ and (Fe 2−x Q 3) 2− layers. The latter layers exhibit an ordered array of strong Fe−Fe dimers in edge-sharing tetrahedra. Given the strong Fe−Fe interaction, it is expected that the Fe−Fe dimer is antiferromagnetically coupled, yet crystals exhibit a weak ferromagnetic moment that orders at relatively high temperature: below 280−315 K and 240−275 K for the sulfide and selenide analogues, respectively. This transition temperature positively correlates with the concentration of defects in the Fe-sublattice, as determined by single-crystal X-ray diffraction. Our results indicate that internal defects in Fe 2−x Q 3 layers play an important role in dictating the magnetic properties of newly discovered (BaF) 2 Fe 2−x Q 3 (Q = S, Se), which can yield switchable ferromagnetically ordered moments at or above room temperature.

European Journal of Inorganic Chemistry, 2010

A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) is characte... more A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) is characterized by combined X‐ray single‐crystal and high‐resolution transmission‐electron microscopy (HRTEM). Similar nanosize domains with variable sizes and complex internal structures are common to all three compounds. Although a partial ordering of domains for the bulk of K2In12Se19 is dominating, the observed ordering patterns in microdomains range from total random orientation to a pattern with a ninefold superstucture (rare precursor phase not stable under HRTEM conditions). In spite of testing various synthesis conditions it was not possible to avoid these unusual structural features for K2In12Se19, which are apparently intrinsic. The formation of significantly larger domains is observed for K2In12Se19–xTex and K2–yTlyIn12Se19 and results in a twofold superstructure that can be observed with X‐ray diffraction also on a macroscopic scale. (NH4)2In12Se19 is a special case where an initial weak ordering is observed that is characterized by ring X‐ray reflections forming hexagons around certain reciprocal lattice positions. This pattern has apparent similarities to K2In12Se19 but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K2In12Se19–xTex and K2–yTlyIn12Se19. The reason was identified as a combined process of domain broadening and NH3 evaporation. As observed for K2In12Se19 at T > 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In2Q3 (Q: chalcogen) shows strong structural similarities to K2In12Se19 with respect to the internal structure of the nanodomains. Their three‐dimensional arrangement, however, and the resulting superstructure are closer related to K2In12Se19–xTex and K2–yTlyIn12Se19.

Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wi... more Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I SICs (e.g., AgI, Ag2Se, etc.) are defined by an abrupt transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a twodimensional type I SIC, a-KAg3Se2 by scattering techniques and complementary simulations. Quasielastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag + ions are confined to sub-nanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that comprise the immobile charge-balancing layers. Our work thus extends the known classes of SICs and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

Nature Materials, Jul 22, 2021

Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wi... more Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I SICs (e.g., AgI, Ag2Se, etc.) are defined by an abrupt transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a twodimensional type I SIC, -KAg3Se2 by scattering techniques and complementary simulations. Quasielastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag + ions are confined to sub-nanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that comprise the immobile charge-balancing layers. Our work thus extends the known classes of SICs and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

Crystal Growth & Design, Oct 11, 2016

Hg3Se2Br2 is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm3) and crystall... more Hg3Se2Br2 is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm3) and crystallizes in the monoclinic space group C2/m with cell parameters of a = 17.496 (4) A, b = 9.3991 (19) A, c = 9.776(2) A, β = 90.46(3)°, V = 1607.6(6) A3. It melts congruently at a low temperature, 566 °C, which allows for an easy single crystal growth directly from the stoichiometric melt. Single crystals of Hg3Se2Br2 up to 1 cm long have been grown using the Bridgman method. Hg3Se2Br2 single crystals exhibit a strong photocurrent response when exposed to Ag X-ray and blue diode laser. The resistivity of Hg3Se2Br2 measured by the two probe method is on the order of 1011 Ω·cm, and the mobility-lifetime product (μτ) of the electron and hole carriers estimated from the energy spectroscopy under Ag X-ray radiation are (μτ)e ≈ 1.4 × 10–4 cm2/V and (μτ)h ≈ 9.2 × 10–5 cm2/V. Electronic structure calculations at the density functional theory level indicate a direct band gap and a relatively small effective mass for carrie...

Inorganic Chemistry, Oct 9, 2013

The new compounds TlHg6Q4Br5 (Q = S, Se) are reported along with their syntheses, crystal structu... more The new compounds TlHg6Q4Br5 (Q = S, Se) are reported along with their syntheses, crystal structures, and thermal and optical properties, as well as electronic band structure calculations. Both compounds crystallize in the tetragonal I4/m space group with a = 14.145(1) Å, c = 8.803(1) Å, and dcalc = 7.299 g/cm(3) for TlHg6S4Br5 (compound 1) and a = 14.518(2) Å, c = 8.782(1) Å, and dcalc = 7.619 g/cm(3) for TlHg6Se4Br5 (compound 2). They consist of cuboid Hg12Q8 building units interconnected by trigonal pyramids of BrHg3, forming a three-dimensional structure. The interstitial spaces are filled with thallium and bromide ions. Compounds 1 and 2 melt incongruently and show band gaps of 3.03 and 2.80 eV, respectively, which agree well with the calculated ones. First-principles electronic structure calculations at the density functional theory level reveal that both compounds have indirect band gaps, but there also exist direct transitions at energies similar to the indirect gaps.

ChemInform, Apr 1, 2016

Single crystals of La2Re3B7 (I) and La3Re2B5 (II) are grown from mixtures containing the elements... more Single crystals of La2Re3B7 (I) and La3Re2B5 (II) are grown from mixtures containing the elements in the La:Re:B molar ratio of 1:3:5 covered by a La/Ni eutectic (compound (I)) or 1:2:1 with 1 equiv.

ChemInform, Oct 15, 2015

A Layered Wide-Gap Oxychalcogenide Semiconductor.-The new title compound is synthesized in low yi... more A Layered Wide-Gap Oxychalcogenide Semiconductor.-The new title compound is synthesized in low yield from a stoichiometric mixture of CaO and FeSe (alumina crucible, 1000 C, 10 h) and characterized by single crystal XRD, SEM, optical spectroscopy, resistivity measurements, and DFT band structure calculations. The material crystallizes in the orthorhombic space group Pnma with Z = 4. The structure is built up of a quasi-two-dimensional network of corrugated infinite layers of corner-sharing FeSe2O2 tetrahedra. Ca 2+ cations are sandwiched between the layers. The material shows semiconducting behavior with an indirect band gap of about 1.8 eV and an activation energy of 0.

ChemInform, Jul 1, 2016

Mixed-Valent NaCu 4Se3: A Two-Dimensional Metal.-The new title compound is synthesized in quantit... more Mixed-Valent NaCu 4Se3: A Two-Dimensional Metal.-The new title compound is synthesized in quantitative yield by solid state reaction of a stoichiometric mixture of CuSe, Cu 2Se, and Na (alumina crucible, 800 C, 24 h, followed by annealing at 600 C for 3 d). NaCu4Se3 crystallizes in the trigonal space group R3m with Z = 3 (RbCd 4As3-type structure, single crystal XRD). The structure contains  2 [Cu4Se3] slabs separated by Na + cations. The compound is formally Se 2-/Semixed-valent. DFT electronic band structure calculations and physical property measurements reveal p-type metallic behavior.-(

CsPbBr3 has direct band gap (orange color, 2.25 eV), high density (4.85 g/cm3), attenuation coeff... more CsPbBr3 has direct band gap (orange color, 2.25 eV), high density (4.85 g/cm3), attenuation coefficient comparable to CZT, and high resistivity ~10^9 ohm∙cm. These fundamental physical properties of CsPbBr3well meet the requirements for gamma-ray detector materials. CsPbBr3 exhibits the carrier mobility-lifetime product in the order of 10^-4 cm2/V promising enough to be further developed for practical applications. The major challenge in the process to further enhance the detection performance is the carrier traps present at a deep level of the energy gap which should be minimized. We report the synthesis, purification, crystal growth and physical characterization of the CsPbBr3 crystals obtained by new processes we developed for highly pure materials with reduced carrier traps. The starting binary materials were prepared by reaction of Cs2CO3/HBr and Pb(ac)2/HBr in aqueous solution. Purification of materials was performed by sublimation, bromination with HBr gas, and filtration of molten materials. Large single crystals were grown by the vertical Bridgman and EelectroDynamic Gradient method and cut to the dimensions appropriate for assessment of the material for gamma-ray detector applications. All characterization including optical characteristics, charge transport properties, photoconductivity, and gamma-ray spectroscopy from the new single crystals of CsPbBr3 will be presented. In addition, the charge carrier traps profile has been studied for this compound by Deep-Level Transient Spectroscopy (DLTS), Thermally Stimulated Luminescence (TSL), and Photoluminescence (PL) and will be presented.

Chemistry of Materials, Jul 11, 2017

The 2D material KCu 3-x Se 2 was synthesized using both a K 2 Se 3 flux and directly from the ele... more The 2D material KCu 3-x Se 2 was synthesized using both a K 2 Se 3 flux and directly from the elements. It crystallizes in the CsAg 3 S 2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu 3-x Se 2 ]layers separated by K + ions. Thermal analysis indicates that KCu 3-x Se 2 melts congruently at ~755 °C. UV-Vis spectroscopy showed an optical band gap of ~1.35 eV, that is direct in nature as confirmed by electronic structure calculations. Electronic transport measurements on single crystals yield an in-plane resistivity of ~6×10-1 Ωcm at 300 K that has a complex temperature-dependence. Seebeck coefficient measurements are consistent with a doped, p-type semiconductor (S = +214 µV K-1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm 2 V-1 s-1) holes caused by relatively flat valence bands with substantial Cu 3d character and a Page 1 of 30 ACS Paragon Plus Environment Chemistry of Materials 2 significant concentration of Cu ion vacancy defects (p ~10 19 cm-3) in this material. Electronic band structure calculations show that electrons should be significantly more mobile in this structure type.

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We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototyp... more We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototypical incommensurate Charge Density Wave (CDW) material. A comparative study of the low-energy electronic structures of 2H-TaS2 and two other related compounds, 2H-TaSe2 and 2H-NbSe2, identifies several generic features of their CDW orders. Firstly, Fermi surface (FS) nesting alone doesn’t seem to give rise to the CDW instability in these compounds. Secondly, partial gapping of the underlying FS surface in the CDW state is common to each of these materials. Finally, the CDW energy gap, unlike the energy gap in a superconductor, is not symmetric with respect to the chemical potential.

Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI... more Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.

Crystal Growth & Design, Oct 19, 2017

Tl 6 SeI 4 is a promising wide-bandgap semiconductor for room-temperature highenergy photon detec... more Tl 6 SeI 4 is a promising wide-bandgap semiconductor for room-temperature highenergy photon detection. Because of the air-sensitive Tl precursor or Tl-based binary precursors used in the synthesis, this material can contain deleterious Tl oxide impurities. These impurities lead to problems during syntheses and crystal growth including glass attack, tube rupture, and parasitic nucleation, which subsequently deteriorate detector performance. In this work, we present a facile way to chemically reduce Tl oxides and eliminate oxygen impurities in Tl 6 SeI 4 by adding high-purity graphite powder during synthesis. The addition of carbon leads to reduction of the residual Tl oxides and formation of CO 2. The resistivity and hard radiation detection performance for 122 keV γ-rays of Tl 6 SeI 4 single crystals was drastically improved. The improvement in the crystallinity was also confirmed by a narrower near-band-edge emission band in the photoluminescence spectra. We confirmed that the reaction between Tl oxide and graphite occurs, and propose a mechanism which is highly effective in substantially reducing oxide impurities from Tl-containing precursors. First-principles DFT calculations reveal that the presence of interstitial oxygen atoms (O int) leads to the formation of a deep level located near the middle of the gap, which can act as carrier traps detrimental to detector performance. The calculations also indicate that graphite addition is safe for detector performance because all carbon-induced defects have high formation energy and are not likely to appear in lattice. ■ INTRODUCTION Wide-band-gap compound semiconductors are of great interest for room temperature hard radiation (X-rays, γ-rays and α-particles) detection applications including homeland security, medical imaging, and non-proliferation of nuclear materials. 1, 2 However, very few compounds have been identified as hard radiation detector materials, as a set of strict physical property requirements must be simultaneously met to generate spectroscopic detection performance. 1, 2 Even the most successful room temperature detection material, Cd 0.9 Zn 0.1 Te (CZT), 3 still suffers

Physical Review Materials, Oct 7, 2019

The cubic intermetallic compound Ba 10 Ti 24 Bi 39 has a Ba 6 Bi 16 polyhedral cage with a Bi gue... more The cubic intermetallic compound Ba 10 Ti 24 Bi 39 has a Ba 6 Bi 16 polyhedral cage with a Bi guest atom encapsulated inside. The compound can be formulated as Ba 5 Ti 12 Bi 19+x when x signifies the extra Bi atoms filling cages. It crystallizes in a complex noncentrosymmetric cubic structure in space group P-43m with cell parameter a = 12.6787(4) Å. The guest Bi atoms distribute diffusely in the cages and seem to play a role in stabilizing the crystal structure. The magnetic susceptibility of this compound shows a weak temperature dependence with a positive slope coefficient. The charge transport properties as a function of temperature exhibit two competing components which are in charge of positive and negative magnetoresistances. Electronic band-structure calculations reveal the complex multiband hybridization of Ti/Bi orbitals near the Fermi surface, which may play a role in the enormous electron-electron scattering in this material evidenced by the large Kadowaki-Woods ratio.

Nature Synthesis, Aug 15, 2022

Chemistry of Materials, Jul 30, 2015

ABSTRACT

Crystal Growth & Design, Apr 5, 2016

We assess the mercury chalcohalide compound, β-Hg 3 S 2 Cl 2 , as a potential semiconductor mater... more We assess the mercury chalcohalide compound, β-Hg 3 S 2 Cl 2 , as a potential semiconductor material for X-ray and γ-ray detection. It has a high density (6.80 g/cm 3), wide band gap (2.56 eV) and crystallizes in the cubic Pm-3n space group with a three-dimensional structure comprised of [Hg 12 S 8 ] cubes with Cl atoms located within and between the cubes, featuring a trigonal pyramidal SHg 3 as the main building block. First-principle electronic structure calculations at the density functional theory level predict that the compound has closely lying indirect and direct band gaps. We have successfully grown transparent, single crystals of β-Hg 3 S 2 Cl 2 up to 7 mm diameter and 1 cm long using a new approach by the partial decomposition of the quaternary Hg 3 Bi 2 S 2 Cl 8 compound followed by the formation of β-Hg 3 S 2 Cl 2 and an impermeable top layer, all happening in-situ during vertical Bridgman growth. The decomposition process was optimized by varying peak temperatures and temperature gradients using a 2 mm/h translation rate of the Bridgman technique. Formation of the quaternary Hg 3 Bi 2 S 2 Cl 8 followed by its partial decomposition into β-Hg 3 S 2 Cl 2 was confirmed by in-situ temperature-dependent synchrotron powder diffraction studies. The single crystal samples obtained had resistivity of 10 10 Ω•cm and mobility-lifetime products of electron and hole carriers of 1.4(4) x 10-4 cm 2 /V and 7.5(3) x 10-5 cm 2 /V, respectively. Further, an appreciable Ag X-ray photoconductivity response was observed showing the potential of β-Hg 3 S 2 Cl 2 as a hard radiation detector material.

Journal of the American Chemical Society, Nov 13, 2009

Chemistry of Materials, May 1, 2015

Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a comp... more Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a complex problem, especially when approaching room temperature. Here, we demonstrate the important role of defects in the magnetic properties of semiconductors by reporting the structures and properties of the iron chalcogenides (BaF) 2 Fe 2−x Q 3 (Q = S, Se), which exhibit anomalous magnetic properties that are correlated with defects in the Fe-sublattice. The compounds form in both long-range ordered and disordered polytypes of a new structure typified by the alternate stacking of fluorite (BaF) 2 2+ and (Fe 2−x Q 3) 2− layers. The latter layers exhibit an ordered array of strong Fe−Fe dimers in edge-sharing tetrahedra. Given the strong Fe−Fe interaction, it is expected that the Fe−Fe dimer is antiferromagnetically coupled, yet crystals exhibit a weak ferromagnetic moment that orders at relatively high temperature: below 280−315 K and 240−275 K for the sulfide and selenide analogues, respectively. This transition temperature positively correlates with the concentration of defects in the Fe-sublattice, as determined by single-crystal X-ray diffraction. Our results indicate that internal defects in Fe 2−x Q 3 layers play an important role in dictating the magnetic properties of newly discovered (BaF) 2 Fe 2−x Q 3 (Q = S, Se), which can yield switchable ferromagnetically ordered moments at or above room temperature.

European Journal of Inorganic Chemistry, 2010

A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) is characte... more A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) is characterized by combined X‐ray single‐crystal and high‐resolution transmission‐electron microscopy (HRTEM). Similar nanosize domains with variable sizes and complex internal structures are common to all three compounds. Although a partial ordering of domains for the bulk of K2In12Se19 is dominating, the observed ordering patterns in microdomains range from total random orientation to a pattern with a ninefold superstucture (rare precursor phase not stable under HRTEM conditions). In spite of testing various synthesis conditions it was not possible to avoid these unusual structural features for K2In12Se19, which are apparently intrinsic. The formation of significantly larger domains is observed for K2In12Se19–xTex and K2–yTlyIn12Se19 and results in a twofold superstructure that can be observed with X‐ray diffraction also on a macroscopic scale. (NH4)2In12Se19 is a special case where an initial weak ordering is observed that is characterized by ring X‐ray reflections forming hexagons around certain reciprocal lattice positions. This pattern has apparent similarities to K2In12Se19 but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K2In12Se19–xTex and K2–yTlyIn12Se19. The reason was identified as a combined process of domain broadening and NH3 evaporation. As observed for K2In12Se19 at T > 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In2Q3 (Q: chalcogen) shows strong structural similarities to K2In12Se19 with respect to the internal structure of the nanodomains. Their three‐dimensional arrangement, however, and the resulting superstructure are closer related to K2In12Se19–xTex and K2–yTlyIn12Se19.

Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wi... more Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I SICs (e.g., AgI, Ag2Se, etc.) are defined by an abrupt transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a twodimensional type I SIC, a-KAg3Se2 by scattering techniques and complementary simulations. Quasielastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag + ions are confined to sub-nanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that comprise the immobile charge-balancing layers. Our work thus extends the known classes of SICs and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

Nature Materials, Jul 22, 2021

Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wi... more Superionic conductors (SICs) possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I SICs (e.g., AgI, Ag2Se, etc.) are defined by an abrupt transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a twodimensional type I SIC, -KAg3Se2 by scattering techniques and complementary simulations. Quasielastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag + ions are confined to sub-nanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that comprise the immobile charge-balancing layers. Our work thus extends the known classes of SICs and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

Crystal Growth & Design, Oct 11, 2016

Hg3Se2Br2 is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm3) and crystall... more Hg3Se2Br2 is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm3) and crystallizes in the monoclinic space group C2/m with cell parameters of a = 17.496 (4) A, b = 9.3991 (19) A, c = 9.776(2) A, β = 90.46(3)°, V = 1607.6(6) A3. It melts congruently at a low temperature, 566 °C, which allows for an easy single crystal growth directly from the stoichiometric melt. Single crystals of Hg3Se2Br2 up to 1 cm long have been grown using the Bridgman method. Hg3Se2Br2 single crystals exhibit a strong photocurrent response when exposed to Ag X-ray and blue diode laser. The resistivity of Hg3Se2Br2 measured by the two probe method is on the order of 1011 Ω·cm, and the mobility-lifetime product (μτ) of the electron and hole carriers estimated from the energy spectroscopy under Ag X-ray radiation are (μτ)e ≈ 1.4 × 10–4 cm2/V and (μτ)h ≈ 9.2 × 10–5 cm2/V. Electronic structure calculations at the density functional theory level indicate a direct band gap and a relatively small effective mass for carrie...

Inorganic Chemistry, Oct 9, 2013

The new compounds TlHg6Q4Br5 (Q = S, Se) are reported along with their syntheses, crystal structu... more The new compounds TlHg6Q4Br5 (Q = S, Se) are reported along with their syntheses, crystal structures, and thermal and optical properties, as well as electronic band structure calculations. Both compounds crystallize in the tetragonal I4/m space group with a = 14.145(1) Å, c = 8.803(1) Å, and dcalc = 7.299 g/cm(3) for TlHg6S4Br5 (compound 1) and a = 14.518(2) Å, c = 8.782(1) Å, and dcalc = 7.619 g/cm(3) for TlHg6Se4Br5 (compound 2). They consist of cuboid Hg12Q8 building units interconnected by trigonal pyramids of BrHg3, forming a three-dimensional structure. The interstitial spaces are filled with thallium and bromide ions. Compounds 1 and 2 melt incongruently and show band gaps of 3.03 and 2.80 eV, respectively, which agree well with the calculated ones. First-principles electronic structure calculations at the density functional theory level reveal that both compounds have indirect band gaps, but there also exist direct transitions at energies similar to the indirect gaps.

ChemInform, Apr 1, 2016

Single crystals of La2Re3B7 (I) and La3Re2B5 (II) are grown from mixtures containing the elements... more Single crystals of La2Re3B7 (I) and La3Re2B5 (II) are grown from mixtures containing the elements in the La:Re:B molar ratio of 1:3:5 covered by a La/Ni eutectic (compound (I)) or 1:2:1 with 1 equiv.

ChemInform, Oct 15, 2015

A Layered Wide-Gap Oxychalcogenide Semiconductor.-The new title compound is synthesized in low yi... more A Layered Wide-Gap Oxychalcogenide Semiconductor.-The new title compound is synthesized in low yield from a stoichiometric mixture of CaO and FeSe (alumina crucible, 1000 C, 10 h) and characterized by single crystal XRD, SEM, optical spectroscopy, resistivity measurements, and DFT band structure calculations. The material crystallizes in the orthorhombic space group Pnma with Z = 4. The structure is built up of a quasi-two-dimensional network of corrugated infinite layers of corner-sharing FeSe2O2 tetrahedra. Ca 2+ cations are sandwiched between the layers. The material shows semiconducting behavior with an indirect band gap of about 1.8 eV and an activation energy of 0.

ChemInform, Jul 1, 2016

Mixed-Valent NaCu 4Se3: A Two-Dimensional Metal.-The new title compound is synthesized in quantit... more Mixed-Valent NaCu 4Se3: A Two-Dimensional Metal.-The new title compound is synthesized in quantitative yield by solid state reaction of a stoichiometric mixture of CuSe, Cu 2Se, and Na (alumina crucible, 800 C, 24 h, followed by annealing at 600 C for 3 d). NaCu4Se3 crystallizes in the trigonal space group R3m with Z = 3 (RbCd 4As3-type structure, single crystal XRD). The structure contains  2 [Cu4Se3] slabs separated by Na + cations. The compound is formally Se 2-/Semixed-valent. DFT electronic band structure calculations and physical property measurements reveal p-type metallic behavior.-(

CsPbBr3 has direct band gap (orange color, 2.25 eV), high density (4.85 g/cm3), attenuation coeff... more CsPbBr3 has direct band gap (orange color, 2.25 eV), high density (4.85 g/cm3), attenuation coefficient comparable to CZT, and high resistivity ~10^9 ohm∙cm. These fundamental physical properties of CsPbBr3well meet the requirements for gamma-ray detector materials. CsPbBr3 exhibits the carrier mobility-lifetime product in the order of 10^-4 cm2/V promising enough to be further developed for practical applications. The major challenge in the process to further enhance the detection performance is the carrier traps present at a deep level of the energy gap which should be minimized. We report the synthesis, purification, crystal growth and physical characterization of the CsPbBr3 crystals obtained by new processes we developed for highly pure materials with reduced carrier traps. The starting binary materials were prepared by reaction of Cs2CO3/HBr and Pb(ac)2/HBr in aqueous solution. Purification of materials was performed by sublimation, bromination with HBr gas, and filtration of molten materials. Large single crystals were grown by the vertical Bridgman and EelectroDynamic Gradient method and cut to the dimensions appropriate for assessment of the material for gamma-ray detector applications. All characterization including optical characteristics, charge transport properties, photoconductivity, and gamma-ray spectroscopy from the new single crystals of CsPbBr3 will be presented. In addition, the charge carrier traps profile has been studied for this compound by Deep-Level Transient Spectroscopy (DLTS), Thermally Stimulated Luminescence (TSL), and Photoluminescence (PL) and will be presented.

Chemistry of Materials, Jul 11, 2017

The 2D material KCu 3-x Se 2 was synthesized using both a K 2 Se 3 flux and directly from the ele... more The 2D material KCu 3-x Se 2 was synthesized using both a K 2 Se 3 flux and directly from the elements. It crystallizes in the CsAg 3 S 2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu 3-x Se 2 ]layers separated by K + ions. Thermal analysis indicates that KCu 3-x Se 2 melts congruently at ~755 °C. UV-Vis spectroscopy showed an optical band gap of ~1.35 eV, that is direct in nature as confirmed by electronic structure calculations. Electronic transport measurements on single crystals yield an in-plane resistivity of ~6×10-1 Ωcm at 300 K that has a complex temperature-dependence. Seebeck coefficient measurements are consistent with a doped, p-type semiconductor (S = +214 µV K-1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm 2 V-1 s-1) holes caused by relatively flat valence bands with substantial Cu 3d character and a Page 1 of 30 ACS Paragon Plus Environment Chemistry of Materials 2 significant concentration of Cu ion vacancy defects (p ~10 19 cm-3) in this material. Electronic band structure calculations show that electrons should be significantly more mobile in this structure type.

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We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototyp... more We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototypical incommensurate Charge Density Wave (CDW) material. A comparative study of the low-energy electronic structures of 2H-TaS2 and two other related compounds, 2H-TaSe2 and 2H-NbSe2, identifies several generic features of their CDW orders. Firstly, Fermi surface (FS) nesting alone doesn’t seem to give rise to the CDW instability in these compounds. Secondly, partial gapping of the underlying FS surface in the CDW state is common to each of these materials. Finally, the CDW energy gap, unlike the energy gap in a superconductor, is not symmetric with respect to the chemical potential.