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Low band gap

Lingjie Du, Sheng Wang, Diego Scarabelli, Loren N Pfeiffer, Ken W West, Saeed Fallahi, Geoff C Gardner, Michael J Manfra, Vittorio Pellegrini, Shalom J Wind, Aron Pinczuk
The interplay between electron-electron interactions and the honeycomb topology is expected to produce exotic quantum phenomena and find applications in advanced devices. Semiconductor-based artificial graphene (AG) is an ideal system for these studies that combines high-mobility electron gases with AG topology. However, to date, low-disorder conditions that reveal the interplay of electron-electron interaction with AG symmetry have not been achieved. Here, we report the creation of low-disorder AG that preserves the near-perfection of the pristine electron layer by fabricating small period triangular antidot lattices on high-quality quantum wells...
August 17, 2018: Nature Communications
Leah Y Kuritzky, Claude Weisbuch, James S Speck
The possibility of a III-nitride LED with 100% or greater wall-plug efficiency is examined considering recent observations of the phenomenon for smaller bandgap mid-IR LEDs under extremely low-bias operation [Phys. Rev. Lett. 108, 1 (2012)]. Thermoelectric pumping of carriers by lattice heat enables ≥ 100% WPE, but this effect is relatively weaker for the wider band gap III-nitrides. This work assesses the electrical and optical performance of several state-of-the-art nitride devices and summarizes the requirements and prospects for approaching ≥ 100% WPE for III-nitride LEDs operating at technologically relevant current densities (> 1 A/cm2 )...
June 25, 2018: Optics Express
A Pustogow, Y Saito, E Zhukova, B Gorshunov, R Kato, T-H Lee, S Fratini, V Dobrosavljević, M Dressel
The electrodynamic response of organic spin liquids with highly frustrated triangular lattices has been measured in a wide energy range. While the overall optical spectra of these Mott insulators are governed by transitions between the Hubbard bands, distinct in-gap excitations can be identified at low temperatures and frequencies, which we attribute to the quantum-spin-liquid state. For the strongly correlated β^{'}-EtMe_{3}Sb[Pd(dmit)_{2}]_{2}, we discover enhanced conductivity below 175  cm^{-1}, comparable to the energy of the magnetic coupling J≈250  K...
August 3, 2018: Physical Review Letters
Zhao Liu, Haidi Wang, Jiuyu Sun, Rujie Sun, Z F Wang, Jinlong Yang
Since the discovery of graphene, two-dimensional (2D) materials have paved new ways to design high-performance nanoelectronic devices. To facilitate applications of such devices, there are three key requirements that a material needs to fulfill: sizeable band gap, high carrier mobility, and robust environmental stability. However, among the most popular 2D materials studied in recent years, graphene is gapless, hexagonal boron nitride has a very large band gap, transition metal dichalcogenides have low carrier mobility, and black phosphorene is ambience-sensitive...
August 17, 2018: Nanoscale
Wei Zhou, Fan Sui, Guo-Hua Zhong, Guanming Cheng, Mingyue Pan, Chunlei Yang, Shuangchen Ruan
Bulk CsPbI3 has an orthorhombic to cubic phase transition at high temperature above 320 oC. Recently, cubic phase CsPbI3 quantum dots (QDs) have been synthesized and remain stable at room temperature, showing merits of the most suitable bandgap and high efficiency light absorption for solar cells. However, vital properties of cubic CsPbI3 including lattice dynamics and stability at high temperature remain less explored. In this work, we fabricate cubic phase CsPbI3 QDs of 20 nm size, then investigate its lattice dynamic and thermal stability from 300 to 700 K...
August 14, 2018: Journal of Physical Chemistry Letters
Ehsan Soheyli, Reza Sahraei, Golamreza Nabiyouni, Fereshteh Nazari, Reza Tabaraki, Behnaz Ghaemi
In this paper, an aqueous-based approach is introduced for facile, fast, and green synthesis of gradient-alloyed Fe-doped ZnSe(S)@ZnSe(S) core:shell QDs with intense and stable emission. Co-utilizing of co-nucleation and growth doping strategies, along with systematic optimization of emission intensity, provide a well-controllable/general method to achieve internally doped-quantum dots (d-dots) with intense emission. Results indicated that the alloyed ZnSe(S)@ZnSe(S) core:shell QDs have a gradient structure which consists of the Se-rich core and the S-rich shell...
August 14, 2018: Nanotechnology
Swati Kumari, Ramona Gutkowski, Joao Junqueira, Aleksander Kostka, Katharina Hengge, Christina Scheu, Wolfgang Schuhmann, Alfred Ludwig
The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron co-sputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90Ox to Fe79V21Ox) and thickness gradients (from 140 nm to 425 nm). These material libraries were investigated by high throughput characterization techniques in terms of composition, structure, optical and photoelectrochemical properties in order to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density...
August 13, 2018: ACS Combinatorial Science
Rui Xu, Nan Gao, Hongdong Li, Dongchao Qiu, Qiliang Wang, Shaoheng Cheng
Two-dimensional rhombic silicene (named r-silicene) bonded on the diamond (111) surface with a (1 × 1) configuration is predicted by density functional theory. The planar structure of r-silicene has P6/mmm symmetry, and it is dynamically and thermodynamically stable on the diamond surface. r-Silicene has a metallic character, which is different to both bulk cubic Si and hexagonal silicene. The hybrid structure of r-silicene/diamond consists of a metallic surface and a wide band gap semiconductor diamond substrate, providing a novel low-dimensional composite based on silicon-diamond to realize high performance nanodevices...
August 13, 2018: Physical Chemistry Chemical Physics: PCCP
Daniel J Rizzo, Gregory Veber, Ting Cao, Christopher Bronner, Ting Chen, Fangzhou Zhao, Henry Rodriguez, Steven G Louie, Michael F Crommie, Felix R Fischer
Topological insulators are an emerging class of materials that host highly robust in-gap surface or interface states while maintaining an insulating bulk1,2 . Most advances in this field have focused on topological insulators and related topological crystalline insulators3 in two dimensions4-6 and three dimensions7-10 , but more recent theoretical work has predicted the existence of one-dimensional symmetry-protected topological phases in graphene nanoribbons (GNRs)11 . The topological phase of these laterally confined, semiconducting strips of graphene is determined by their width, edge shape and terminating crystallographic unit cell and is characterized by a [Formula: see text] invariant12 (that is, an index of either 0 or 1, indicating two topological classes-similar to quasi-one-dimensional solitonic systems13-16 )...
August 2018: Nature
Evert Jan Baerends
The relation of Kohn-Sham (KS) orbital energies to ionization energies and electron affinities is different in molecules and solids. In molecules, the local density approximation (LDA) and generalized gradient approximations (GGA) approximate the exact ionization energy (I) and affinity (A) rather well with self-consistently calculated (total energy based) ILDFA and ALDFA , respectively. The highest occupied molecular orbital (HOMO) energy and lowest unoccupied molecular orbital (LUMO) energy, however, differ significantly (by typically 4-6 eV) from these quantities, ϵH LDFA (mol)>-I(mol)≈-ILDFA (mol), ϵL LDFA (mol)<-A(mol)≈-ALDFA (mol)...
August 7, 2018: Journal of Chemical Physics
Achyut J Raghavendra, Wren E Gregory, Tyler J Slonecki, Yongchang Dong, Indushekhar Persaud, Jared M Brown, Terri F Bruce, Ramakrishna Podila
Background: Although optical spectroscopy promises improved lateral resolution for cancer imaging, its clinical use is seriously impeded by background fluorescence and photon attenuation even in the so-called two-photon absorption (2PA) imaging modality. An efficient strategy to meet the clinical cancer imaging needs, beyond what two-photon absorption (2PA) offers, is to use longer excitation wavelengths through three-photon absorption (3PA). A variety of fluorescent dyes and nanoparticles (NPs) have been used in 3PA imaging...
2018: International Journal of Nanomedicine
Hiroo Suzuki, Noritada Ogura, Toshiro Kaneko, Toshiaki Kato
Graphene nanoribbon (GNR), also known as 1-dimensional graphene, with a non-zero band gap has a huge potential for various electrical and optoelectrical applications because of its high transparency, flexibility, controllable band gap, and unique edge states. Recent advances in the synthesis of GNR enable us to show the possibility of GNRs as future high performance electrical devices. However, the applicability of GNRs to optoelectrical devices is unclear. Here we report that suspended GNR devices can show persistent photoconductivity (PPC) with long decay time (over 72 h) and adequate environmental stability...
August 7, 2018: Scientific Reports
Vasudevan Iyer, Yong P Chen, Xianfan Xu
Topological insulators are promising candidates for optically driven spintronic devices, because photoexcitation of spin polarized surface states is governed by angular momentum selection rules. We carry out femtosecond midinfrared spectroscopy on thin films of the topological insulator Bi_{2}Te_{2}Se, which has a higher surface state conductivity compared to conventionally studied Bi_{2}Se_{3} and Bi_{2}Te_{3}. Both charge and spin dynamics are probed utilizing circularly polarized light. With a sub-band-gap excitation, clear helicity-dependent dynamics is observed only in thin (<20  nm) flakes...
July 13, 2018: Physical Review Letters
Yangguang Zhong, Qi Wei, Zhen Liu, Qiuyu Shang, Liyun Zhao, Ruiwen Shao, Zhepeng Zhang, Jie Chen, Wenna Du, Chao Shen, Jun Zhang, Yanfeng Zhang, Peng Gao, Guichuan Xing, Xinfeng Liu, Qing Zhang
Lead Iodide (PbI2 ) is a layered semiconductor with direct band gap holding great promises in green light emission and detection devices. Recently, PbI2 planar lasers are demonstrated using hexagonal whispering-gallery-mode microcavities, but the lasing threshold is quite high. In this work, lasing from vapor phase deposition derived PbI2 trapezoidal nanoplatelets (NPs) with threshold that is at least an order of magnitude lower than the previous value is reported. The growth mechanism of the trapezoidal NPs is explored and attributed to the synergistic effects of van der Waals interactions and lattice mismatching...
July 31, 2018: Small
Asish Parbatani, Eui Sang Song, Fan Yang, Bin Yu
Low bulk band gaps and conductive surface electronic states of tetradymite topological insulators (TTI) make them potential candidates for next generation ultra-broadband photodevices. Here, we demonstrate a broadband and self-biased photodiode based on a Sb2Te3-Si heterostructure. A low-cost thermal evaporation technique was employed to fabricate the photodiode. The self-biased nature of the photodiode was due to the built-in potential at the Sb2Te3-Si interface. Upon characterizing the Sb2Te3 nanocrystalline film via AFM, SEM, EDX, and XPS it was found that the film exhibited p-type behavior due to antimony vacancies or antisites...
August 9, 2018: Nanoscale
Filipe Matusalem, Marcelo Marques, Lara K Teles, Alessio Filippetti, Giancarlo Cappellini
Dialkali halides are materials of great interest from both fundamental and technological viewpoints, due to their wide transparency range. The accurate determination of their electronic, excitation and optical properties in bulk and low dimensional systems is therefore of crucial importance. Moreover, it is a challenge from the theoretical point of view to deal with quasiparticle band structure calculations for such large energy gap materials, requiring very expensive methods for achieving a desirable accuracy...
July 27, 2018: Journal of Physics. Condensed Matter: An Institute of Physics Journal
Zhipeng Meng, Suli Wu, Bingtao Tang, Wei Ma, Shufen Zhang
The photonic stop bands of colloidal crystals appear as structural colors, which are potentially useful for display devices, colorimetric sensors, optical filters, paints, and photonic papers. However, low durability and pale colors caused by the undesired scattering of light seriously limit their practical applications. In this article, a polydimethylsiloxane (PDMS)/photonic crystal (PC)/PDMS sandwich structure was designed as a free standing structural colored film with good durability and brilliant color...
August 2, 2018: Nanoscale
Christopher L Davies, Juliane Borchert, Chelsea Q Xia, Rebecca L Milot, Hans Kraus, Michael B Johnston, Laura M Herz
Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI3 (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI3 . While MAPbI3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI3 vary smoothly over a range of 100-160 K in accordance with a more gradual transition...
July 27, 2018: Journal of Physical Chemistry Letters
Sahar Keshtkar, Alimorad Rashidi, Mohammad Kooti, Mojtaba Askarieh, Sepideh Pourhashem, Ebrahim Ghasemy, Nosrat Izadi
In this study, SnO2 quantum dots-fullerene (SnO2 QDs-C60 ) nanohybrid as novel sensing material was synthesized by a simple hydrothermal method. The structure and morphology of the synthesized sample were studied by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The prepared hybrid was used as gas sensors for detection of different gasses including 70 ppm H2 S, 1% methane, and 1% propane at low temperatures of 100-200 °C. The results indicated that the SnO2 QDs-C60 nanohybrid has high response and high selectivity to 70 ppm H2 S, 1% methane, and 1% propane gasses at low temperatures...
October 1, 2018: Talanta
Meng Shen, Lingxia Zhang, Jianlin Shi
A metal-free photocatalyst, graphitic carbon nitride (GCN) with a moderate band gap catering for visible-light excitation, shows amazing potential in various photocatalytic applications. Carbon dioxide reduction using diversified photocatalysts has been attracting increasing public attention and the extensively studied GCN is one of the most promising photocatalysts. However, because of the low concentration and high recombination rate of photogenerated carriers, and some other disadvantages of the pristine GCN photocatalyst, the solar-to-fuel conversion efficiency is too low for practical use...
October 12, 2018: Nanotechnology
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