|Solid State Chemistry
| Gamma Ray
We are interested in the design, synthesis, synthetic methodology, in- depth characterization, manipulation and potential applications of new substances with novel chemical, physical, or electrical properties. These substances range from discrete molecular compounds, to solid state inorganic materials, to organic polymeric materials. We seek to obtain deeper understanding of synthesis/structure relationships and structure/function relationships.
Exploratory Synthesis Presentation
Solid State Chemistry of Chalcogenides
Very few areas of chemical synthesis deserves the title "exploratory" more than inorganic solid state synthesis. The majority of synthetic chemists have a measure of predictability with the compounds they work with as the molecular units remain relatively intact throughout their reactions.Their goal is mainly to link one molecule to the next or to perform specific changes on molecular functional groups. The solid state synthetic chemist has almost no predictability in the reactions save for the simplest cases of elemental substitution, and even then predictions can be incorrect.
In the Kantazidis Group, we work on the development of novel synthetic methodologies for metal chalcogenide compounds. This involves the exploration of molten solids and their application as solvents for exploratory synthesis and crystal growth. It also involves the use of stable, pre-formed building blocks that end up in the final structures giving the solid a certain functionality.
In many ways we approach this chemistry from a solution chemist's outlook rather than a conventional high temperature solid state chemist's perspective. This allows for useful and profitable insights to be drawn from what is already known in coordination chemistry.
Although techniques will become more sophisticated, one
hundred percent synthetic predictability is not likely to be achieved in any
type of chemical synthesis. We will always have the unknown to frustrate,
motivate, and inspire us.
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Perovskite Solar Cells
In the Kanatzidis group, we work on the development of lead and lead-free perovskites for photovoltaic applications.Perovskite solar cells have recently engendered enormous interest since they exhibit power conversion efficiencies superior to most organic solar cells and comparable to those of commercialized c-Si solar cells and other technologies based on inorganic semiconductors. The light harvester in the device is a hybrid organic/inorganic perovskite (CaTiO3-type) compound, with a general formula AMX3, where A is a univalent (in)organic cation, M is a bivalent metal ion and X is a halide(-1) anion.
We have successfully implemented lead-free, methylammonium tin halides (CH3NH3SnX3), as a light absorber material. Efficient power conversion has been achieved for the CH3NH3SnI3-xBrx mixed-halide and CH3NH3Sn1-xPbxI3 mixed-metal solid solutions, using a device configuration of FTO/bl-TiO2/m-TiO2-perovskite/ spiro-OMeTAD/Au. Exploratory synthesis of other hybrid perovskite analogs and study of their optical and electrical properties is being actively pursued. We also focus on the study of fundamental problems in device architecture and materials engineering aiming to gain a better understanding of the underlying carrier transport dynamics. A closely related research direction we follow involves the use of inorganic halide perovskites as hole-transporting materials in dye-sensitized solar cells. The feasibility of such an approach was demonstrated by the use of CsSnI3-based compounds as a hole-transporter, leading to devices with power conversion efficiencies up to 8.5%.
Radiation detection has uses in defense, research, medical, and industrial applications. Currently used materials (e.g., Ge) are limited in practicality because operation requires cooling by liquid nitrogen to prevent thermal noise. Effective room-temperature nuclear detection materials are required to have large resistivity values, wide band gaps, and high density for low dark current, high signal to noise ratio, and high stopping power, respectively. Recent progress in development of nuclear radiation detector materials has been accelerated with the introduction of dimensional reduction by our group as a new approach for producing high density, wide band gap semiconductor detector materials. Additionally, single crystalline specimens of these materials are being grown and processed with the quality and sizes appropriate for detector materials.
The challenge to create high efficiency thermoelectric (TE) materials lies in achieving simultaneously high electronic conductivity, high thermopower and low thermal conductivity in the same material. These properties define the thermoelectric figure of merit ZT = (S2σ /κ )T; where S is the thermopower, σ the electronic conductivity, κ the thermal conductivity, and T the temperature. The first three quantities are determined by the details of the electronic structure and scattering of charge carriers (electrons or holes) and thus are not independently controllable parameters. The thermal conductivity has a contribution from lattice vibrations, κlat which is called the lattice thermal conductivity. Thus κ = κe + κlat , where κe is the carrier thermal conductivity.
In the Kanatzidis group, we aim to synthesize new bulk materials with high ZTs greater than the historic threshold of ZT=1 with two general approaches. By adding nanostructure to the matrix of the pristine material, the lattice thermal conductivity component in the denominator of ZT can be significantly reduced , resulting in dramatic increases in ZT. A different, less understood method for increasing ZT is to increase the power factor S2σ by increasing the thermopower of material without depressing the electronic conductivity. The relationship between thermopower and electrical conductivity is quite complex, but it may be possible to alter one and not the other with band structure engineering.
Our group focuses on the lead chalcogenides, PbTe, PbSe, and PbS and Bi2Te3, and recently SnTe.
Bi2Te3 is one of the major constituents in the best thermoelectric materials for use near room temperature. It is the most promising compound for the application of thermoelectric materials for refrigeration. Solid solution alloying with Sb2Te3 and Sb2Se3 can improve the thermoelectric figure of merit. Triggered by this, our group focuses on the p- and the n- type Bi2Te3 – based semiconductors aiming the optimization of their thermoelectric properties.
PbTe thermoelectric materials are the champions of high ZT. Our group has synthesized several materials with ZTs above 1.7, and recently as high as 2.2 By doping PbTe with PbS and Na, we can control nanostructure formations (to reduce κlat) while concurrently modifying the electronic structure to significantly enhance the thermoelectric powerfactor. In the PbTe-PbS 12%-Na 2% system, Na produces shape controlled cubic PbS nanostructures which help reduce κlat, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS to allow for enhancement of the power factor to give a ZT of 1.8 at 800K (Girard, et. al, J. Am. Chem. Soc., 2011, 133, 15588).
Another novel high ZT system, PbTe-PbSr 2%-Na 2% utilizes endotaxially arranged SrTe nanocrystals incorporated in the PbTe matrix to reduce κlatwithout having a detrimental effect on carrier mobility unlike previous nanostructured thermoelectrics. The endotaxial SrTe nanoprecipitates inhibit the heat flow in the system, but the crystallographic alignment of the SrTe and PbTe does not affect the hole mobility, allowing a large power factor to be achieved concurrently with a very low thermal conductivity. This decoupling of phonon and electron transport allows the system to reach ZT=2.2 at 900K (Biswas, et. al, Nature, 2012, 489,414.)
PbSe Based materials
The binary narrow band gap semiconductor PbSe combines several attractive features for potential thermoelectric applications such as a favorable electronic valence band structure that is comprised of two sub-bands with very different effective masses in similarity to that of PbTe, lower cost, and higher operation temperatures. Although high ZTs have been reported for several n-type PbSe compositions (ZT = ~0.9 at 900 K : Androulakis, et. al, Phys. Rev. B., 2011, 83, 195209 and ZT = ~1.3 at 900 K : Androulakis, et. al, J. Am. Chem. Soc., 2011, 133, 28), theoretical calculations suggest that heavily doped p-type PbSe can potentially show a ZT ~2 at 1000 K (Parker, et. al, Phys. Rev. B., 2010, 82,035204).
PbS Based materials
PbTe based thermoelectric materials show high performance at room and middle temperature range (600-850K), however, they contain significant amount of Te, which is a scarce element in the crust of the earth. Hence the Te price is likely to rise sharply if Te based thermoelectric materials reach mass markets. A broad search for more inexpensive alternatives is therefore warranted. S is very earth abundant compared to Te and Se, and the Pb is very strongly bound to S, making it environmentally safe. Recently, we proposed PbS as an ideal candidate for widespread application of environmentally stable and affordable thermoelectric material system because high performance in both n-type (ZT~1.1 at 923 K) and p-type (ZT~1.2 at 923 K) can be achieved. We showed that the lattice thermal conductivity can be reduced to very low levels through nanostructuring with metal sulfides such as Bi;2S;3 and Sb;2S;3 in n-type and SrS and CaS in p-type (Zhao, et. al. J. Am. Chem. Soc., 2011, 133, 20476-20487 and Zhao, et. al. J. Am. Chem. Soc., 2012, 134, 7902-7912.)\
SnSe is the world’s least thermally conductive crystalline material. Heat cannot travel well through this material because of its very “soft”, accordion-like layered structure which does not transmit vibrations well. It reminds us of the TV commercial for posture-pedic mattress where one can jump up and down on one side of the mattress and a few feet away a glass of wine does not feel the vibrations. By analogy SnSe can get hot on one side and the other side remains cool. The cool side does not feel the vibrations (also known as phonons). In SnSe this means that all heat must go to the other side of the crystal “riding” on the electronic carriers, not lattice vibrations. Thus, the hot carriers can generate useful electricity during their transport. That is enabled by the high thermoelectric power of SnSe. The poor ability to carry heat through its lattice enables the resulting record high thermoelectric conversion efficiency.
This outstanding property can be exploited in solid state electronic devices which can directly convert heat to electricity. Places where there is high heat and high temperature such as exhaust gas from coal and gas-fired power plans, automobile exhaust, high temperature processing (glass and brick industries, etc).
Li-Dong Zhao, Shih-Han Lo, Yongsheng Zhang, Hui Sun, Gangjian Tan, Ctirad Uher, C. Wolverton, Vinayak P. Dravid, Mercouri G. Kanatzidis. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature, 2014; 508 (7496): 373 DOI: 10.1038/nature13184
Interest in TE materials is not new but the need for new materials is increasing and the next decade will be critical in the development of this field. Taking into account the difficulty if identifying the "right" compound and optimizing to ZT > 1, long term sustainable planning is necessary and close collaboration between chemists, physicists and engineers is key to success. Such collaborative approaches have been the paradigm in which the present effort is base
Layered Metal Sulfides
Layered metal sulfide materials of the general formula A2xMxSn3-xS6 (x=0.5-0.95; A = Li+, Na+, K+, Rb+; M = Mg2+, Mn2+, Zn2+, Fe2+) made of inexpensive, non-toxic elements that can be easily prepared on a large scale with high purity are highly desireable. Using solid-state or hydrothermal synthesis techniques these materials are extremely stable in an air atmosphere and water. Layered metal sulfide materials exhibit facile and highly selective ion-exchange properties for cations of great environmental concern such as Cs+ and Sr2+; the radioactive isotopes of which are the major contaminants in the fission product of nuclear wastes. They are also extremely capable to clean the water from soft heavy metal ions (e.g. Hg2+, Pb2+, Cd2+, Ag+), which constitute a serious health threat for humans and other species.
Mesostructured non-oxidic solids
Mesoporous materials have attracted considerable attention in the last decade, because of their immense technological potential as catalysts, adsorbents and hosts for large molecules. The mesoporous silicates of the MCM-n family with ordered pore structure, reported by the Mobil group using long chain organic molecules as 'structure directing' agents, have spawned a new era in open framework materials. Porous low band-gap non-oxidic systems, such as chalcogenides are also of great interest because new applications could be foreseen by combining the porous nature with their semiconducting and opto-electronic properties in fields of photocatalysis nanotechnology etc.
Sulfide selenide and telluride-based and other non-oxidic open-framework materials are intriguing compounds. Apart from the fact that they possess a very diverse and therefore interesting structural chemistry, these materials might act as special "zeolites" that combine physical properties typical for semiconductors.
In one instance, we have synthesized two new mesostructured
metal germanium sulfides with hexagonal framework
organization, incorporating [Ge4S10]4- anions
and tetrahedral Ga3+ and In3+ cations. CP molecules
act as templating agents and occupy the closest-packed cylindrical pores of
the framework. The CPGaGeS and CPInGeS emit intense green light when excited
across the band gap and presage the potential of such systems in light-emitting
and optoelectronic devices. Current work focuses on experiments aimed at surfactant
removal to render the pores in these systems accessible.
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Intermetallic compounds are compounds composed of two or more metallic or semimetallic elements that adopt a different crystal structure than their constituent elements and exhibit metallic behavior. They exhibit more localized, covalent bonding than elemental metals. Complex intermetallic compounds exhibit a wide variety of diverse structural, electronic, magnetic, and mechanical properties. Many intermetallics exhibit unusual properties such as superconductivity, heavy fermion behavior, and interesting magnetic ordering.
While these compounds are interesting, their formation is nearly impossible to predict. For this reasons, our group focuses on exploratory projects to increase the library of known compounds and draw parallels between structural features and properties. Here again we approach this chemistry from a solution chemist's viewpoint rather than a conventional high temperature solid state chemist's perspective by using the flux technique to explore phase space. We take advantage of the improved diffusion and lower reaction temperatures that the flux technique permits to novel, complex phases that are not accessible by more traditional solid-state techniques.
Recently, our group has used molten Al, Ga, In, and Sn to discover a variety of complex intermetallic compounds. We reported that the compounds REFe4Al9Si6 (RE = Er, Tb), RENiAL4Ge2 (RE = Y, Sm) and REFe2+xAl7-xSi8 (RE = Ce, Pr, Nd, Sm) form in liquid Al. We have also reported that reactions in liquid In can yield Yb5Ni4Ge10, a phase closely related to a number of intermetallic superconductors. Additionally, we have discovered a series of compounds that can be made in Ga flux, and exhibit interesting tuning of magnetic properties by changing electron counts with Ge substitution in Y4Mn1-xGa12-yGey.
A new family of non-oxide, chalcogen-based aerogels, called chalcogels, has been synthesized through simple metathesis reaction by our previous group member. These chalcogels, because of their potential high affinity for halide gases and their high porosity, have shown the properties of desulfurization catalysis and heavy metal ion sequestration. We utilize these properties of chalcogels to mimic naturally occurring processes such as photosynthesis, and nitrogen fixation.
Using proteins and other biological structures as starting points, we investigate porous chalcogenide framework that can contain immobilized redox-active centers ([Fe4S4]2+, [Mo2Fe6S8(SPh)3Cl6]3–), linked by various thiostannate linking blocks ([SnnS2n+2; n=1,2,4]) covalently, and light-harvesting photo-redox dye molecules in a larger superstructure. These multifunctional chalcogels have shown to electrocatalytically reduce protons and carbon disulfide, and photochemically produce hydrogen. When the third additional transitional metal (Ni, Co) is incorporated into the gels as solid-state analogues of NiFe or NiFeS reaction centers in enzymes, the effectiveness in transformations of carbon dioxide has been increased.
These gels have a high degree of synthetic flexibility and allow a wide range of light-driven processes relevant to the production of solar fuels
For examples of work in each of these areas see List of Publications.