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Field-tunable valley coupling in a dodecagonal semiconductor quasicrystal
Quasicrystals are characterized by atomic arrangements having long-range order without periodicity. Van der Waals bilayers provide an opportunity to controllably vary the atomic alignment between two layers from a periodic moiré crystal to an aperiodic quasicrystal. Here, in this work, we reveal that in a dodecagonal WSe 2 quasicrystal, two separate valleys in separate layers are brought arbitrarily close in momentum space through higher-order Umklapp scatterings. A modest perpendicular electric field is then sufficient to induce strong interlayer valley hybridization, manifested as another hybrid excitonic doublet. Concurrently, we observe the disappearance of the trion that exists at low field, which we attribute to a modified spatial distribution of the wavefunction due to the quasicrystal potential. This is possibly a precursor to localization. Our findings highlight the ability of incommensurate systems to bring any pair of momenta into close proximity, thereby introducing opportunities for valley engineering.
Wigner polarons probe the dynamics of a Wigner crystal in a monolayer semiconductor
Wigner crystals—lattices made purely of electrons—provide a platform for studying correlation-driven quantum phase transitions. Despite extensive research, accessing the internal dynamics of Wigner crystals has remained challenging, with most experiments probing only static order or collective motion. Here we demonstrate optical probing and the manipulation of zero-field Wigner crystals and elucidate their static and dynamic properties in the frequency domain. We observe optical resonances that we identify as Wigner polarons—quasiparticles formed when the electron lattice is locally distorted by exciton–Wigner crystal coupling. We further achieve all-optical control of spins in the Wigner crystal, thereby directly probing valley-dependent Wigner polaron scattering well above the magnetic ordering temperature and in the absence of any external magnetic field. Lastly, we show optical melting of the Wigner crystal and observe different responses of the umklapp (static) and Wigner polaron (dynamic) resonances to optical excitation. Our results provide an avenue for understanding electron dynamics and achieving ultrafast optical control of interaction-driven quantum phase transitions in strongly correlated electron systems.
Sub-molecular structural relaxation at a physisorbed interface with monolayer organic single-crystal semiconductors
Arranging molecules into highly symmetric, topological crystal structures has been recognized as the best approach to functionalize electronic properties in molecular crystals, where the constituent molecules have been assumed to be rigid in shape. Here, in striking contrast, we demonstrate that the molecules in a monolayer organic crystal can undergo a significant deformation in proximity to the substrate, which is reflected by an asymmetry in the electron density profile. X-ray reflectivity and X-ray absorption spectroscopies in conjunction with density-functional theory calculations reveal that the highly planarized π-core are deformed into a bent shape, while the bulk lattice parameters are maintained. The molecular shape change is found to be perfectly suppressed in a bilayer single crystal, which leads to a 40% increase in mobility in the bilayer crystal. Our finding of a unique, sub-molecular scale shape change in monolayer single crystals can offer possibilities for functionalizing electrical properties via nano-scale physisorption.
MnFe0.5Ru0.5O3: an above-room-temperature antiferromagnetic semiconductor
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LaMer's 1950 model of particle formation: a review and critical analysis of its classical nucleation and fluctuation theory basis, of competing models and mechanisms for phase-changes and particle formation, and then of its application to silver halide, semiconductor, metal, and metal-oxide nanoparticles
Are classical nucleation theory and the 1950 LaMer model of particle formation supported for a wide range of particle formations, or do competing models in the form of chemical reaction mechanisms have better experimental support? Read on to find out.
Doping molecular organic semiconductors by diffusion from the vapor phase
Spiro-OMeTAD can be doped by infiltration of F 4 TCNQ from the vapor phase without causing crystallization. Optical spectroscopy and thermopower measurements examined the question of the formation of dication states by charge transfer doping.
Thermoelectric transport of semiconductor full-Heusler VFe 2 Al
Seebeck coefficient of VFe 2 Al over a wide range of doping levels can be explained only with a small band-gap ( E g ) range of 0.02–0.04 eV. This E g value is also consistent with high-temperature resistivity data of nominally stoichiometric VFe 2 Al.
Particle formation mechanisms supported by in situ synchrotron XAFS and SAXS studies: a review of metal, metal-oxide, semiconductor and selected other nanoparticle formation reactions
Following the definition of and four primary criteria for obtaining reliable, disproof-based mechanisms of particle formation, 74 literature papers utilizing synchrotron XASF and SAXS are analyzed with an emphasis on 8 case studies.
Dynamic processes in transient phases during self-assembly of organic semiconductor thin films
Understanding and manipulating crystallization processes has been an important challenge for solution-processed organic thin films, both for fundamental studies and for fabricating thin films with near-intrinsic charge transport properties. We report an in situ X-ray scattering study of the crystallization of 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C 10 ) during its deposition from solution. At temperatures modestly below the smectic-E/crystalline phase boundary, the crystallization goes through a transient liquid crystal state before reaching the final stable crystalline phase. Significant dynamics occur in the first few seconds of the transition, which are observed through fluctuations in the X-ray scattering intensity, and are correlated with the time interval that the transient thin film coexists with the evaporating solvent. The transition to the stable crystalline phase takes minutes or even hours under these conditions, which may be a result of the asymmetry of the molecule. Transient phases are of potential interest for applications, since they can act as a route to self-assembly of organic thin films. However, our observations show that the long-lived monolayer-stacked intermediate state does not act as a template for the bilayer-stacked crystalline phase. Rather, the grain structure is replaced through nucleation, where the nucleation free-energy barrier is related to a potential barrier that prevents molecules to flip their long axis by 180°.
Near infrared bioimaging and biosensing with semiconductor and rare-earth nanoparticles: recent developments in multifunctional nanomaterials
This review highlights the very recent examples of near infrared contrast agents employed for multivariate diagnostics, multimodal imaging, and theranostic. Considerations on how to further advance these probes towards real-life use are also given.
Synthetic control over polymorph formation in the d-band semiconductor system FeS 2
Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS 2 .
Aqueous processing of organic semiconductors enabled by stable nanoparticles with built-in surfactants
The introduction of oligoether side chains onto a polymer backbone can help to stabilise polymeric dispersions in water without the necessity of surfactants or additives when conjugated polymer nanoparticles are prepared. A series of poly(3-hexylthiophene) (P3HT) derivatives with different content of a polar thiophene derivative 3-((2-methoxyethoxy)methyl)thiophene was interrogated to find the effect of the polar chains on the stability of the formed nanoparticles, as well as their structural, optical, electrochemical, and electrical properties. Findings indicated that incorporation of 10–20 percent of the polar side chain led to particles that are stable over a period of 42 days, with constant particle size and polydispersity, however the particles from the polymer with 30 percent polar side chain showed aggregation effects. The polymer dispersions showed a stronger solid-like behaviour in water with decreasing polar side chain content, while thin film deposition from water was found to afford globular morphologies and crystallites with more isotropic orientation compared to conventional solution-processed films. As a proof-of-principle, field-effect transistors were fabricated directly from the aqueous dispersions demonstrating that polymers with hydrophilic moieties can be processed in water without the requirement of surfactants.
Theoretical simulation and experimental verification of the competition between different recombination channels in GaN semiconductors
We simulate the time-resolved decay process of the yellow band in GaN based on the first principles, and the simulated decay lifetimes are confirmed in our experiments.
Material descriptors for thermoelectric performance of narrow-gap semiconductors and semimetals
Simple descriptors to search for low-temperature thermoelectric materials.
Sub-4 nm mapping of donor–acceptor organic semiconductor nanoparticle composition
The first report of sub-4 nm mapping of donor : acceptor nanoparticle composition in eco-friendly colloidal dispersions for organic electronics.
Designing complex Pb 3 SBr x I 4− x chalcohalides: tunable emission semiconductors through halide-mixing
Quaternary chalcohalides, Pb 3 SBr x I 4− x (1 ≤ x ≤ 3), exhibit tunable photoluminescence emission through halide-mixing with variable amounts of Br − and I − anions in their structure.