Unconventional Semiconductors Research Articles

Chalcogenide Perovskites and Perovskite-Based Chalcohalide as Photoabsorbers: A Study of Their Properties, and Potential Photovoltaic Applications.

In 2015, a class of unconventional semiconductors, Chalcogenide perovskites, remained projected as possible solar cell materials. The MAPbI3 hybrid lead iodide perovskite has been considered the best so far, and due to its toxicity, the search for potential alternatives was important. As a result, chalcogenide perovskites and perovskite-based chalcohalide have recently been considered options and potential thin-film light absorbers for photovoltaic applications. For the synthesis of novel hybrid perovskites, dimensionality tailoring and compositional substitution methods have been used widely. The study focuses on the optoelectronic properties of chalcogenide perovskites and perovskite-based chalcohalide as possibilities for future photovoltaic applications.

Realization of BaZrS3 chalcogenide perovskite thin films for optoelectronics

BaZrS3 is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of 1.7–1.8 eV, a very strong light-matter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many fundamental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of BaZrS3 thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of 1019-1020 cm−3. Depending on the processing temperature, the Hall mobility ranges from 2.1 to 13.7 cm2/Vs. The absorption coefficient is > 105 cm−1 at photon energy >1.97 eV. Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that BaZrS3 is a promising candidate, these results potentially unleash the family of chalcogenide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes.

Anisotropic Arm Growth in Unconventional Semiconductor CdSe/CdS Nanotetrapod Synthesis Using Core/Shell CdSe/CdS as Seeds

Anisotropic semiconductor CdSe/CdS tetrapods exhibit unique dimension-dependent quantum confinement and efficient charge separation/transport along the arms, making them useful in a wide range of optoelectronic applications. Traditionally, the CdSe/CdS tetrapods were produced by anisotropic CdS growth on the zinc blende CdSe core, induced by ligands that selectively bind to specific facets of CdSe. In this work, we report an unconventional method to grow CdSe/CdS nanotetrapods using wurtzite CdSe/CdS core/shell nanocrystals as seeds, without the aid of selective-binding ligands. The CdS layer thickness in the seeds and the Cd and S precursor concentrations have a significant impact on the morphology of CdS growth. The tetrapods we obtained have one arm much longer than the other three. Through a detailed transmission electron microscopy study, we found the anisotropic arms resulted from the stacking faults occurred in the growth, which induced a local phase change from wurtzite to zinc blende at one end o...

Molecular Donor-Bridge-Acceptor Strategies for High-Capacitance Organic Dielectric Materials.

Donor-bridge-acceptor (DBA) systems occupy a rich history in molecular electronics and photonics. A key property of DBA materials is their typically large and tunable (hyper)polarizabilities. While traditionally, classical descriptions such as the Clausius-Mossotti formalism have been used to relate molecular polarizabilities to bulk dielectric response, recent work has shown that these classical equations are inadequate for numerous materials classes. Creating high-dielectric organic materials is critically important for utilizing unconventional semiconductors in electronic circuitry. Employing a plane-wave density functional theory formalism, we investigate the dielectric response of highly polarizable DBA molecule-based thin films. Such films are found to have large dielectric response arising from cooperative effects between donor and acceptor units when mediated by a conjugated bridge. Moreover, the dielectric response can be systematically tuned by altering the building block donor, acceptor, or bridge structures and is found to be nonlinearly dependent on electric field strength. The computed dielectric constants are largely independent of the density functional employed, and qualitative trends are readily evident. Remarkably large computed dielectric constants >15.0 and capacitances >6.0 μF/cm(2) are achieved for squaraine monolayers, significantly higher than in traditional organic dielectrics. Such calculations should provide a guide for designing high-capacitance organic dielectrics that should greatly enhance transistor performance.

Maximizing the dielectric response of molecular thin films via quantum chemical design.

Developing high-capacitance organic gate dielectrics is critical for advances in electronic circuitry based on unconventional semiconductors. While high-dielectric constant molecular substances are known, the mechanism of dielectric response and the fundamental chemical design principles are not well understood. Using a plane-wave density functional theory formalism, we show that it is possible to map the atomic-scale dielectric profiles of molecule-based materials while capturing important bulk characteristics. For molecular films, this approach reveals how basic materials properties such as surface coverage density, molecular tilt angle, and π-system planarity can dramatically influence dielectric response. Additionally, relatively modest molecular backbone and substituent variations can be employed to substantially enhance film dielectric response. For dense surface coverages and proper molecular alignment, conjugated hydrocarbon chains can achieve dielectric constants of >8.0, more than 3 times that of analogous saturated chains, ∼2.5. However, this conjugation-related dielectric enhancement depends on proper molecular orientation and planarization, with enhancements up to 60% for proper molecular alignment with the applied field and an additional 30% for conformations such as coplanarity in extended π-systems. Conjugation length is not the only determinant of dielectric response, and appended polarizable high-Z substituents can increase molecular film response more than 2-fold, affording estimated capacitances of >9.0 μF/cm2. However, in large π-systems, polar substituent effects are substantially attenuated.

Bistetracene: an air-stable, high-mobility organic semiconductor with extended conjugation.

We report the synthesis and characterization of "bistetracene", an unconventional, linearly extended conjugated core with eight fused rings. The annellation mode of the system allows for increased stability of the conjugated system relative to linear analogues due to the increased number of Clar aromatic sextets. By attaching the appropriate solubilizing substituents, efficient molecular packing with large transfer integrals can be obtained. The electronic structure calculations suggest these large polycyclic aromatic hydrocarbons (PAHs) exhibit excellent intrinsic charge transport properties. Charge carrier mobilities as large as 6.1 cm(2) V(-1) s(-1) and current on/off ratios of 10(7) were determined experimentally for one of our compounds. Our study provides valuable insight into the design of unconventional semiconductor compounds based on higher PAHs for use in high-performance devices.

Hybrid Gate Dielectric Materials for Unconventional Electronic Circuitry

Recent advances in semiconductor performance made possible by organic π-electron molecules, carbon-based nanomaterials, and metal oxides have been a central scientific and technological research focus over the past decade in the quest for flexible and transparent electronic products. However, advances in semiconductor materials require corresponding advances in compatible gate dielectric materials, which must exhibit excellent electrical properties such as large capacitance, high breakdown strength, low leakage current density, and mechanical flexibility on arbitrary substrates. Historically, conventional silicon dioxide (SiO2) has dominated electronics as the preferred gate dielectric material in complementary metal oxide semiconductor (CMOS) integrated transistor circuitry. However, it does not satisfy many of the performance requirements for the aforementioned semiconductors due to its relatively low dielectric constant and intransigent processability. High-k inorganics such as hafnium dioxide (HfO2) or zirconium dioxide (ZrO2) offer some increases in performance, but scientists have great difficulty depositing these materials as smooth films at temperatures compatible with flexible plastic substrates. While various organic polymers are accessible via chemical synthesis and readily form films from solution, they typically exhibit low capacitances, and the corresponding transistors operate at unacceptably high voltages. More recently, researchers have combined the favorable properties of high-k metal oxides and π-electron organics to form processable, structurally well-defined, and robust self-assembled multilayer nanodielectrics, which enable high-performance transistors with a wide variety of unconventional semiconductors. In this Account, we review recent advances in organic-inorganic hybrid gate dielectrics, fabricated by multilayer self-assembly, and their remarkable synergy with unconventional semiconductors. We first discuss the principals and functional importance of gate dielectric materials in thin-film transistor (TFT) operation. Next, we describe the design, fabrication, properties, and applications of solution-deposited multilayer organic-inorganic hybrid gate dielectrics, using self-assembly techniques, which provide bonding between the organic and inorganic layers. Finally, we discuss approaches for preparing analogous hybrid multilayers by vapor-phase growth and discuss the properties of these materials.

Junction phenomena for unconventional semiconductors

Abstract We consider polaronic effects at the FET junctions based on molecular crystals or conducting polymers. In the molecular crystal, Coulomb interaction of free electrons with surface polar phonons of the dielectric layer results in the formation of a long-range surface polaron. The effect is strongly enhanced by the bias electric field. For the junction with conducting polymers, new allowed electronic bands appear inside the band gap instead of the usual band bending. The bias electric field and the injected charge penetrate into the polymer via creation of the soliton lattice which period changes with the distance from the contact surface. For an isotropic material, we estimate conditions for the injected charge to be captured in a close proximity to the junction surface thus providing a high efficiency for the possible dielectrics–metal transition.

Exciton formation and dissociation in mass-asymmetric electron-hole plasmas

First-principle path integral Monte Carlo simulations were performed in order to analyze correlation effects in complex electron-hole plasmas, particularly with regard to the appearance of excitonic bound states. Results are discussed in relation to exciton formation in unconventional semiconductors with large electron hole mass asymmetry.

Charge-density patching method for unconventional semiconductor binary systems.

A motif based charge patching method is presented for large system electronic structure calculations. It produces ab initio quality charge densities for large systems without actually doing self-consistent calculations for them. It represents a general faster alternative to the conventional O(N) methods. This method is applied here to unconventional semiconductor binary systems, and the resulting eigenenergies are found to be almost the same as the original ab initio eigenenergies (with 20-50 meV errors).

Very large-scale arrays of biomolecules

ArrayChips are semiconductor devices that enable rapid and flexible fabrication of very large-scale arrays of biomolecules. Array fabrication is controlled by electronic hardware on the chip and is software configurable. This feature enables very rapid turnaround times of new array designs for testing and optimization. Biomolecules in the array are synthesized in situ or immobilized at selected addressable locations. Synthesis occurs directly within the interstices of a hydrated and highly porous membrane that overlays the ArrayChip. Synthesis is accomplished in situ using reagents that are generated electrochemically at individually addressable microelectrodes on ArrayChips. We have developed technologies for in situ synthesis of arrays of oligonucleotides, peptides and small molecules. ArrayChips obviate any need for microfluidics or other unconventional semiconductor hardware. First-generation ArrayChips have microelectrode array densities of over 1,000 individually addressable sites per cm2. Second-generation hardware designs have over 500,000 individually addressable sites per cm2.

Conjugated and electroluminescent polymers

Several advances were reported in the field of conjugated polymer semiconductors over the last year. Major breakthroughs relate to the achievement of high electroluminescence efficiency via exploitation of phosphorescence, of high efficiency photovoltaic cells, and of a better understanding of the properties of charged and neutral excitations in this class of unconventional semiconductors.

The unconventional semiconductors represent two novel structure types of polythiometalates: syntheses, structures and electrical properties of {[W3Ag3S12]·[La(DMAC)5(H2O)3·(DMAC)4]}n and {[W8Ag10S32]·[M(DEF)8]2}n (M = La, Nd) †

Polymeric transition metal chalcogenides have attracted much attention because of their possible properties directly derived from their peculiar structures. Compared with polyoxometalates, polythiometalate chemistry is still very young. We report here the syntheses, structural characterizations and electrical properties of the novel polythiometalates, {[W3Ag3S12]·[La(DMAC)5(H2O)3·(DMAC)4]}n 1 and {[W8Ag10S32]·[M(DEF)8]2}n(M = La 2 and M = Nd 3). The complexes were obtained by self-assembly of (NH4)WS4 and AgNO3 with Ln(NO3)3 (Ln = La, Nd) in different solvents: DMAC (DMAC = N,N′-dimethylacetamide), DEF (DEF = N,N′-diethylformamide). The title compounds represent two novel structure types of W/Ag/S polythiometalates, 1: a single-stranded helix chain; 2 and 3 are isomorphous: equal step zigzag chains with a double square unit. The unusual 1 has two helices: an anion chain and a cation chain simultaneously exist in the unit cell and the anion helix is defined by a pitch (length per turn) of 17.65 Å, and a radius of circumscribed cylinder of 8.88 Å. Room temperature conductivities of 1, 2/3 are 1.16 × 10–4, 2.58 × 10–4 S cm–1.