[Show abstract][Hide abstract] ABSTRACT: The ability to design and confer control over the kinetics of the processes involved in the mechanisms of artificial molecular machines is at the heart of the challenge to create ones that can carry out useful work on their environment, just as Nature is wont to do. As one of the more promising forerunners of prototypical artificial molecular machines, chemists have developed bistable redox-active donor-acceptor mechanically interlocked molecules (MIMs) over the past couple of decades. These bistable MIMs generally come in the form of rotaxanes, molecular compounds that constitute a ring mechanically interlocked around a dumbbell-shaped component, or catenanes, which are composed of two mechanically interlocked rings. As a result of their interlocked nature, bistable MIMs possess the inherent propensity to express controllable intramolecular, large-amplitude, and reversible motions in response to redox stimuli. In this Account, we rationalize the kinetic behavior in the ground state for a large assortment of these types of bistable MIMs, including both rotaxanes and catenanes. These structures have proven useful in a variety of applications ranging from drug delivery to molecular electronic devices. These bistable donorÀacceptor MIMs can switch between two different isomeric states. The favored isomer, known as the ground-state co-conformation (GSCC) is in equilibrium with the less favored metastable state co-conformation (MSCC). The forward (k f) and backward (k b) rate constants associated with this ground-state equilibrium are intimately connected to each other through the ground-state distribution constant, K GS . Knowing the rate constants that govern the kinetics and bring about the equilibration between the MSCC and GSCC, allows researchers to understand the operation of these bistable MIMs in a device setting and apply them toward the construction of artificial molecular machines. The three biggest influences on the ground-state rate constants arise from (i) ground-state effects, the energy required to breakup the noncovalent bonding interactions that stabilize either the GSCC or MSCC, (ii) spacer effects, where the structures overcome additional barriers, either steric or electrostatic or both, en route from one co-conformation to the other, and (iii) the physical environment of the bistable MIMs. By managing all three of these effects, chemists can vary these rate constants over many orders of magnitude. We also discuss progress toward achieving mechanostereoselective motion, a key principle in the design and realization of artificial molecular machines capable of doing work at the molecular level, by the strategic implementation of free energy barriers to intramolecular motion.
Accounts of Chemical Research 01/2014; 47(2):482-493. · 20.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability, not only to control relative movements of components in molecules, but also and to drive their components preferentially in one direction relative to each other using versatile stimuli, is one of the keys to future technological applications. Herein, we describe a wholly synthetic, small-molecule system which, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by the two termini - one a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unit - of a constitutionally asymmetric dumbbell, which can impair threading/dethreading of a pseudorotaxane, and (ii) controls the ring's affinity for a 1,5-dioxynaphthalene binding site located at the dumbbell's central core. The formation and subsequent dissociation of the pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction relatively defined has been demonstrated by (i) spectroscopic (1H NMR and UV/vis) means and cyclic voltammetry, as well as with (ii) DFT calculations and by (iii) comparison with control compounds in the shape of constitutionally symmetrical pseudorotaxanes, one with two positively charged and the other with two neutral ends. Operation of the system relies solely on reversible, yet stable, noncovalent bonding interactions. Moreover, in the presence of a photosensitizer, visible light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.
Journal of the American Chemical Society 10/2013; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: After the manner in which co-enzymes often participate in the binding of substrates in the active sites of enzymes, pillararene - a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges - modifies the binding properties of cucurbituril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbituril - a macrocycle comprised of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillararene and cucurbituril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two rotaxanes and one rotaxane in > 90% yields inside two hours at 55 oC in acetonitrile. Since the hydroxyl groups on pillararene and the carbonyl groups on cucurbituril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillararene can be trapped on the dumbbell components of the rotaxanes. In the case of the rotaxane, it is possible to isolate a compound containing two pillararene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillararene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.
Journal of the American Chemical Society 09/2013; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use of high-quality graphene as a local probe in combination with photoexcitation is described to establish a deep mechanistic understanding of charge generation/quenching processes underlying the graphene/environment interface. By combing a no-destructive bottom-up assembly technique with senstive graphene-based transistors, a bistable  rotaxane-graphene hybrid device is produced which exhibits a symmetric mirror-image photoswitching effect with logic capabilities.
[Show abstract][Hide abstract] ABSTRACT: Flame-melting rapid-cooling reactions were used to synthesize a number of pure phases of the Cs2MIIMIV3Q8 family (MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs2MIIMIV3Q8 compounds presented here crystallize in the noncentrosymmetric space group P212121, except for Cs2ZnGe3S8, which crystallizes in the centrosymmetric space group P21/n. The structures contain chains of corner-sharing MIIQ4 and MIVQ4 tetrahedra linked by edge-sharing MIV2Q6 dimers to give a two-dimensional structure. All phases are structurally similar to the AMIIIMIVQ4 (A = alkali metal, Tl; MIII = Al, Ga, In; MIV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered MII and MIV sites as opposed to the occupational disorder of MIII and MIV over all tetrahedral sites present in AMIIIMIVQ4. The structural trends of the Cs2MIIMIV3Q8 family are discussed, along with a systematic study of their optical properties. Density functional theory (DFT) electronic structure calculations were performed using the projector augmented wave method to further investigate the trends in the band gaps of the Cs2MIIMIV3Se8 (MII = Mg, Zn; MIV = Ge, Sn) compounds. The experimental diffuse reflectance UV–vis spectroscopy results show that the Mg compounds have smaller band gaps than those containing Zn for both the Ge and the Sn families whereas the DFT calculations show the opposite trend. Cs2HgSn3Se8 was studied as a representative example of this family using differential thermal analysis and melts congruently at 595 °C. Crystal growth of this compound using the Bridgman method resulted in a polycrystalline ingot from which plate crystals 2 mm × 3 mm could be cleaved. The band gap of the compounds varies from a narrow 1.07 eV for Cs2ZnGe3Te8 to a wide 3.3 eV for Cs2ZnGe3S8 and Cs2CdGe3S8 making this family a potentially useful source of materials for a variety of electronic applications. Cs2HgSn3Se8 crystals exhibit photoconductivity response where the photoexcited electron and hole show mobility-lifetime products on the order of 3.69 × 10–5 cm2/V and (μτ)h = 7.78 × 10–5 cm2/V, respectively.
Chemistry of Materials. 08/2013; 25(16):3344–3356.
[Show abstract][Hide abstract] ABSTRACT: The chemical concept of lattice hybridization was applied to identify new chalcohalide compounds as candidates for X-ray and γ-ray detection. Per this approach, compound semiconductor materials with high density and wide band gaps can be produced that can absorb and detect hard radiation. Here, we show that the mixed chalcogenide–halide compound Tl6SI4 is a congruently melting, mechanically robust chalcohalide material with strong photoconductivity response and an impressive room-temperature figure of merit. Tl6SI4 crystallizes in the tetragonal P4/mnc space group, with a = 9.1758(13) Å, c = 9.5879(19) Å, V = 807.3(2) Å3, and a calculated density of 7.265 g·cm–3. The new material requires a more simplified crystal growth compared to the leading system Cd0.9Zn0.1Te, which is the benchmark room-temperature hard radiation detector material. We successfully synthesized Tl6SI4 crystals to produce detector-grade wafers with high resistivity values (1010 Ω·cm) and high-resolution detection of X-ray spectra from an Ag (22 keV) source.
Chemistry of Materials. 07/2013; 25(14):2868–2877.
[Show abstract][Hide abstract] ABSTRACT: Two new compounds CsCdInQ3 (Q = Se, Te) have been synthesized using a polychalcogenide flux. CsCdInQ3 (Q = Se, Te) crystals are promising candidates for X-ray and γ-ray detection. The compounds crystallize in the monoclinic C2/c space group with a layered structure, which is related to the CsInQ2 (Q = Se, Te) ternary compounds. The cell parameters are: a = 11.708(2) Å, b = 11.712(2) Å, c = 23.051(5) Å, β = 97.28(3)° for CsCdInSe3 and a = 12.523(3) Å, b = 12.517(3) Å, c = 24.441(5) Å, β = 97.38(3)° for CsCdInTe3. Both the Se and Te analogues are wide-band-gap semiconductors with optical band gaps of 2.4 and 1.78 eV for CsCdInSe3 and CsCdInTe3, respectively. High-purity polycrystalline raw material for crystal growth was synthesized by the vapor transfer method for CsCdInQ3. Large single crystals up to 1 cm have been grown using the vertical Bridgman method and exhibit photoconductive response. The electrical resistivity of the crystals is highly anisotropic. The electronic structure calculation within the density functional theory (DFT) framework indicates a small effective mass for the carriers. Photoconductivity measurements on the as grown CsCdInQ3 crystals gives high carrier mobility-lifetime (μτ) products comparable to other detector materials such as α-HgI2, PbI2, and CdxZn1–xTe (CZT).
Chemistry of Materials. 05/2013; 25(10):2089–2099.
[Show abstract][Hide abstract] ABSTRACT: Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure-matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1-xSex/ZnS1-xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (light hole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power factor.
Journal of the American Chemical Society 05/2013; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe in detail a strategy for creating foldamers in which interactions between mechanically interlocked components dictate the single-molecule assembly of a folded secondary structure. This unique folding motif is based on a flexible polyether dumbbell bearing 1,5-dioxynaphthalene (DNP) donors, which folds its way through a series of cyclobis(paraquat-p-phenylene) (CBPQT4+) acceptor rings in a serpentine fashion to enable extended donor-acceptor (D-A) stacking between DNP and the electron-poor 4,4'-bipyridinium (BIPY2+) units in CBPQT4+. These oligorotaxanes can be prepared in a wide range of sizes, with molecular weights up to >15000 Da, on account of novel one-pot reactions we developed to generate the necessary oligo-DNP precursors. The product distributions from the final kinetically-controlled stoppering reactions are highly biased towards oligorotaxanes in which approximately half of the DNP units are encircled by rings, a fact which can be rationalized if the dominant solution-state struc
Chemical Science 02/2013; 4:1470-1483. · 8.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homocatenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homocatenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.
[Show abstract][Hide abstract] ABSTRACT: A homologous series of rotaxanes, in which cyclobis(paraquat-p-phenylene) (CBPQT4+) serves as the ring component, while the dumbbell components all contain single 4,4'-bipyridinium (BIPY2+) units centrally located in the midst of oligomethylene chains of varying lengths, have been synthesized by taking advantage of radical templation and copper-free azide-alkyne 1,3-dipolar cycloadditions in the formation of their stoppers. Cyclic voltammetry, UV/Vis spectroscopy and mass spectrometry reveal that the BIPY•+ radical cations in this series of rotaxanes are stabilized against oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY2+ units in the ring and dumbbell components gives rise to enhanced Coulombic repulsion, destabilizing the ground-state co-conformations of the fully oxidized forms of these rotaxanes. The smallest rotaxane with only three methylene groups on each side of its dumbbell component is found to exist under ambient conditions in a monoradical state, a situation which does not persist in acetonitrile solution at least in the case of its longer analogues. 1H NMR Spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT4+ rings over the BIPY2+ units in the dumbbells rise linearly with increasing oligomethylene chain lengths across the series of rotaxanes. These findings provide a new way of producing highly stabilized BIPY•+ radical cations and open up more opportunities to use stable organic radicals as building blocks for the construction of paramagnetic materials and conductive molecular electronic devices.
Journal of the American Chemical Society 11/2012; · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Semiconductor γ-ray detectors have broad applications, yet finding superior detector materials is a challenge because of its contradictory requirements. Here, we investigated a large set of native defects in Cs2Hg6S7 that has been suggested as a promising candidate for detector materials. Using first-principles calculations, we showed that S-vacancy and HgCs-antisite defect provide life-time limiting deep levels, and Cs-vacancy forms a shallow acceptor level, resulting in low resistivity. To decrease such detrimental effects, concentrations of defects and carriers were examined in various chemical environments, which reveal that carrier densities can be extremely reduced by adjusting Cs partial pressure.
[Show abstract][Hide abstract] ABSTRACT: The new layered compound CsHgInS3 was synthesized using solid state and flux synthesis techniques. The compound is a semiconductor and shows promising properties for X-ray and γ-ray detection. It features a layered structure that crystallizes in the monoclinic space group C2/c with cell parameters: a = 11.2499(7) Ǻ, b = 11.2565(6) Ǻ, c = 22.146(1) Ǻ, β = 97.30(5)°, V = 2781.8(4) Ǻ3, and Z = 8. CsHgInS3 is isostructural to Rb2Cu2Sn2S6, where the Hg, In, and Cs atoms occupy the Cu, Sn, and Rb sites, respectively. Large single crystals with dimension up to 5 mm were grown with a vertical Bridgman method as well as a horizontal traveling heater method. CsHgInS3 has a γ-ray attenuation length comparable to commercial Cd1–xZnxTe and a band gap value of 2.30 eV. The electrical resistivity of CsHgInS3 is anisotropic with values of 98 GΩ cm and 0.33 GΩ cm perpendicular and parallel to the (001) plane, respectively. The mobility-lifetime product (μτ) of electrons and holes estimated from photoconductivity measurements on the as-grown crystals were (μτ)e = 3.6 × 10–5 cm2 V–1 and (μτ)h = 2.9 × 10–5 cm2 V–1, respectively. Electronic structure calculations at the Density Functional Theory level were performed based on the refined crystal structure of CsHgInS3 and show a direct gap with the conduction band near the Fermi level being highly dispersive, suggesting a relatively small carrier effective mass for electrons.
[Show abstract][Hide abstract] ABSTRACT: The mechanism governing the redox-stimulated switching behavior of a tristable rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4′-bipyridinium (BIPY2+) unit, is described. The BIPY2+ unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT4+ ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable rotaxanes and one bistable catenane composed of CBPQT4+ rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY2+ unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable rotaxanes and catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY•+ units in the CBPQT2(•+) ring is oxidized to the BIPY2+ dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.
Journal of the American Chemical Society 09/2012; 134(39):16275–16288. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The heavy element semiconductor compound Cs2Hg6S7 is of interest as a potential wide gap semiconductor for gamma ray detection. To determine electrically active defects and their energy levels, photoconductivity (PC) spectroscopy was carried out over the temperature range of 90-295 K. The low temperature spectrum exhibits photoconductive transitions at 1.495, 1.61, 1.66, and 1.68 eV. The optical transitions are tentatively attributed to defects with levels located at energies of 50, 70, 120, and 240 meV from the band edge. A superlinear dependence of photocurrent on illumination intensity is observed that is attributed to a two-center recombination process that involves shallow traps and recombination centers. Near band edge photoluminescence (PL) was observed over the temperature range of 24–80 K. The spectrum revealed three defect related emission bands located at 1.68, 1.66, and 1.62 eV, whose ionization energies are 57 meV, 78 meV, and 115 meV, respectively. From the temperature and excitation dependencies of the observed peak intensities and energies, the radiative recombination mechanisms of the bands were attributed to transitions involving excitons bound to neutral and ionized acceptors. Good agreement of the defect level energies determined by PL and PC were noted, indicating that they were of the same origin. The defects were tentatively attributed to metal vacancies that form shallow acceptor levels.
Journal of Applied Physics 09/2012; 112(6). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In one fell swoop, polyrotaxanes comprising up to 64 rings can be synthesized as a result of cucurbituril-templated 1,3-dipolar azide-alkyne cycloadditions accelerated in the presence of cyclodextrins as a consequence of self-sorting and positive cooperativity, brought about by hydrogen bonding. Mixing six components in one pot affords a heterorotaxane in one minute in quantitative yield.
Angewandte Chemie International Edition 09/2012; · 13.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two redox-active bistable catenanes composed of macrocyclic polyethers of different sizes incorporating both electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4'-bipyridinium (BIPY(2+)) units, interlocked mechanically with the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)), were obtained by donor-acceptor template-directed syntheses in a threading-followed-by-cyclization protocol employing Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloadditions in the final mechanical-bond forming steps. These bistable catenanes exemplify a design strategy for achieving redox-active switching between two translational isomers, which are driven (i) by donor-acceptor interactions between the CBPQT(4+) ring and DNP, or (ii) radical-radical interactions between CBPQT(2(•+)) and BIPY(•+), respectively. The switching processes, as well as the nature of the donor-acceptor interactions in the ground states and the radical-radical interactions in the reduced states, were investigated by single-crystal X-ray crystallography, dynamic (1)H NMR spectroscopy, cyclic voltammetry, UV/vis spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy. The crystal structure of one of the catenanes in its trisradical tricationic redox state provides direct evidence for the radical-radical interactions which drive the switching processes for these types of mechanically interlocked molecules (MIMs). Variable-temperature (1)H NMR spectroscopy reveals a degenerate rotational motion of the BIPY(2+) units in the CBPQT(4+) ring for both of the two catenanes, that is governed by a free energy barrier of 14.4 kcal mol(-1) for the larger catenane and 17.0 kcal mol(-1) for the smaller one. Cyclic voltammetry provides evidence for the reversibility of the switching processes which occurs following a three-electron reduction of the three BIPY(2+) units to their radical cationic forms. UV/vis spectroscopy confirms that the processes driving the switching are (i) of the donor-acceptor type, by the observation of a 530 nm charge-transfer band in the ground state, and (ii) of the radical-radical ilk in the switched state as indicated by an intense visible absorption (ca. 530 nm) and near-infrared (ca. 1100 nm) bands. EPR spectroscopic data reveal that, in the switched state, the interacting BIPY(•+) radical cations are in a fast exchange regime. In general, the findings lay the foundations for future investigations where this radical-radical recognition motif is harnessed in bistable redox-active MIMs in order to achieve close to homogeneous populations of co-conformations in both the ground and switched states.
Journal of the American Chemical Society 07/2012; 134(28):11709-20. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It's just an illusion: Above a critical chain length, where oligomers contain five or more recognition units, apparently infinite donor-acceptor polypseudorotaxanes are formed in the solid state (see picture). X-ray crystallographic analyses of three different examples have shown that although the oligomeric chains are undoubtedly discrete and monodisperse, they nevertheless appear to be infinite in the crystal.
Angewandte Chemie International Edition 06/2012; 51(29):7231-5. · 13.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.
Journal of the American Chemical Society 05/2012; 134(20):8579-87. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cs2Hg6S7 is a promising compound for X-ray and γ-ray detection with a band gap of 1.63 eV. A new method is reported for the synthesis of Cs2Hg6S7, along with growth studies of large Cs2Hg6S7 crystals of dimensions up to several centimeters using the Bridgman method. Our growth technique gives reproducible crystals with high resistivity (106 ohm·cm). Crystals grown in this work exhibit figure of merit mobility lifetime (μτ) products comparable to commercial cadmium zinc telluride (CZT). The Cs2Hg6S7 crystals exhibit a strong photoluminescence signal at low temperature in the range 1.50–1.75 eV. The thermal properties of the crystal, including the thermal expansion coefficient and the thermal conductivity, have been characterized. The nature of defects affecting the charge transport properties in the as-grown Cs2Hg6S7 crystals is discussed. The noncentrosymmetric character of the tetragonal crystal structure (space group P42nm) gives rise to a nonlinear optical second harmonic generation response.