Topological Properties By Atomic Buckling in Bismuth Bi (110)

by Tommy on 12/10/2015
Bismuth (011) Surface Bond Angles

Bismuth (011) Surface Bond Angles

Topological Properties Determined by Atomic Buckling in Self-Assembled Ultrathin Bi(110), Yunhao Lu, Wentao Xu, Mingang Zeng, Guanggeng Yao, Lei Shen, Ming Yang, Ziyu Luo, Feng Pan, Ke Wu, Tanmoy Das, Pimo He, Jianzhong Jiang, Jens Martin, Yuan Ping Feng, Hsin Lin and Xue-sen Wang, Nano Lett., 15 (1), 80–87 (12 December 2014), DOI:10.1021/nl502997v

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

This was before the first of the year. I have not been paying attention. So I went to the UW (The University of Wisconsin at Madison) and downloaded a bunch of stuff on this, as far back as 15 years ago, to try and get caught up. I have identified several international groups working on this.

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Phase Space of 2D Topological Superfluids Analyzed

by Tommy on 11/10/2015

Topological superfluids on a square optical lattice with non-Abelian gauge fields: effects of next-nearest-neighbor hopping in the BCS-BEC evolution, M. Iskin

We consider a two-component Fermi gas with attractive interactions on a square optical lattice, and study the combined effects of Zeeman field, spin-orbit coupling and next-nearest-neighbor hopping on the ground-state phase diagrams in the entire BCS-BEC evolution. In particular, we first classify and distinguish all possible superfluid phases by the momentum-space topology of their zero-energy quasiparticle/quasihole excitations, and then numerically establish a plethora of quantum phase transitions in between. These transitions are further signalled and evidenced by the changes in the corresponding topological invariant of the system, i.e., its Chern number. Lastly, we find that the superfluid phase exhibits a reentrant structure, separated by a fingering normal phase, the origin of which is traced back to the changes in the single-particle density of states.

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NASA Congressional Hearing With No NASA Representatives

by Tommy on 9/10/2015

Doug Cooke and Dan Dumbacher don’t even work at NASA anymore and are the two people most responsible for this debacle since Michael Griffin and his henchmen half baked it into its pathetic existence. Yet the Eddie Bernice Johnson and Donna Edwards show still continues.

Epic Congressional Fail.

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The SpaceX NASA Spaceflight NSF Chris Bergin Tweet Thing

by Tommy on 8/10/2015

I’m probably in a good position to make a wild, uniformed, speculative guess on this.

Oh … forget it. Start here:

Billionaires, schmillionaires.

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Electromagnetically Tuned Plasmon Behaviors in 2D Silicene

by Tommy on 7/10/2015

Feature-rich electronic excitations in external fields of 2D silicene, Jhao-Ying Wu, Szu-Chao Chen, Godfrey Gumbs and Ming-Fa Lin

Electronic Coulomb excitations in monolayer silicene are investigated by using the Lindhard dielectric function and a newly developed generalized tight-binding model (G-TBM). G-TBM simultaneously contains the atomic interactions, the spin-orbit coupling, the Coulomb interactions, and the various external fields at an arbitrary chemical potential. We exhibit the calculation results of the electrically tunable magnetoplasmons and the strong magnetic field modulation of plasmon behaviors. The two intriguing phenomena are well explained by determining the dominant transition channels in the dielectric function and through understanding the electron behavior under the multiple interactions (intrinsic and external). A further tunability of the plasmon features is demonstrated with the momentum transfer and the Fermi energy. The methodological strategy could be extended to several other 2D materials like germanene and stanene, and might open a pathway to search a better system in nanoplasmonic applications.

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Bismuth (110) Surface, Edge and Layer Reconstruction Studied

by Tommy on 7/10/2015
Bismuth (110) Atomic Lattice Layer Structure

Bismuth (110) Atomic Lattice Layer Structure

First-principles and spectroscopic studies of Bi (110) films: Thickness-dependent Dirac modes and property oscillations, G. Bian, X. Wang, T. Miller, T.-C. Chiang, P. J. Kowalczyk, O. Mahapatra, and S. A. Brown, Phys. Rev. B 90, 195409 (7 November 2014), doi:10.1103/PhysRevB.90.195409

The electronic structure of Bi (110) thin films as a function of film thickness is investigated by first-principles calculations, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy. Energy minimization in the calculation reveals significant atomic relaxation and rebonding at the surface. The calculated surface energy for the relaxed structures indicates that films consisting of odd numbers of atomic layers are inherently unstable and tend to bifurcate into film domains consisting of neighboring even numbers of atomic layers. This theoretical trend agrees with experimental observations. The results can be explained by the presence of unsaturated pz dangling bonds on the surfaces of films of odd-numbered atomic layers only. These pz dangling bonds form a Dirac-cone feature near the Fermi level at the M¯ point as a consequence of the interplay of mirror symmetry and spin-orbit coupling. Films consisting of even numbers of atomic layers exhibit a band gap at M¯ instead.

This is looking more doable all the time.

This too is a must read paper.

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Oxidation Behavior of an Unusual Allotrope of Bismuth (110)

by Tommy on 7/10/2015

Anisotropic oxidation of bismuth nanostructures: Evidence for a thin film allotrope of bismuth, P. J. Kowalczyk, D. Belic, O. Mahapatra, S. A. Brown, E. S. Kadantsev, T. K. Woo, B. Ingham and W. Kozlowski, Appl. Phys. Lett. 100, 151904 (10 April 2012), doi:10.1063/1.3701166

We present evidence that ultra-thin Bi (110) nanostructures oxidise from the edges, and that their top surfaces remain unoxidised. Even after prolonged oxidation, clean (unoxidised) bismuth is present in nanostructures that are less than 5 monolayers thick. Since the (110) surface of bulk bismuth is known to be readily oxidised, this is strong evidence for a thin film allotrope of bismuth. We present a comparison with calculated structures and the structures of polymeric nitrogen, which suggests that the allotrope is one of several complex or hybrid paired-layer structures.

Moving quickly onward again, I see some recognizable names now.

This is one I will have to read.

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The Bismuth Nanofilm PhD Thesis of Chu Xinjun at Singapore

by Tommy on 6/10/2015

STM investigations of self-assembled bismuth nanostructures and ultra-fine gold nanparticles, Chu Xinjun, PhD Thesis, National University of Singapore (2010)

In-situ scanning tunneling microscopy (STM) has been utilized to investigate the growth of bismuth nanorods (single/multi- layer, straight/branched), ultra-thin Bi nanowires, Bi superstructures, and ultra-fine Au nanoparticles (NPs) on various substrates. When deposited on MoS2(0001), before the height exceeds the critical thickness, Bi form Bi(110) nanobelts (nanoribbons). Straight Bi nanorods can be obtained at low Bi flux and deposition amount, while at high Bi flux, multi-layer branched nanostructures form. A structural transformation from Bi(110) to Bi(111) was observed when the Bi(110) film thickness exceeds 8-Bi(110) monolayer. Other measurements such as scanning electron microscopy (SEM) and low energy electron diffraction (LEED) were used to characterize the orientation distribution of Bi nanobelts. In addition, Bi nanostructures deposited on highly-oriented pyrolytic graphite (HOPG) were studied by low temperature scanning tunneling spectroscopy (LT-STS). Thickness dependent local density of states (LDOS) on Bi(110) layers with different thickness was observed, which may result from the structural relaxation and transformation from Black-P like Bi(110) to bulk-like one. Using a molecular layer 3,4,5,10-perylene tetracarboxylic dianhydride (PTCDA) on MoS2(0001) as a template, ultra-thin Bi nanowires can be synthesized. Bi first grow into NWs with single atomic layer thickness and aligned orientation and then develop into 4- or 6-layer Bi(110) NWs at larger deposition amounts. The NWs grow along three directions of the ordered molecular layer. Due to the side wall passivation by PTCDA, the growth of width of NWs is greatly depressed and hence NWs with large length-to-width ratio (LWR) can be obtained. Using LEED and STM, three structural phases were revealed when Bi deposited on Ru(0001), with Bi coverage ranged from sub-monolayer (ML) to a few ML. A loosely rectangular superlattice (2 × v3) formed at the initial growth stage. After more Bi was deposited, a hexagonal (v7 × v7)R19.1° superlattice was observed. When Ru(0001) was saturated with this (v7 × v7)R19.1°-Bi, it acts as a buffer layer and the surface becomes rather inert. With additional Bi deposited, Bi(110) thin film is formed on this inert substrate. Using PTCDA as a surfactant layer, size-tunable ultra-fine Au NPs can be synthesized on MoS2. The PTCDA overlayer can greatly increase the nucleation density of Au NPs and prevent fine NPs from aggregating into larger particles. Molecular scale STM images show that Au atoms nucleate and grow into NPs underneath the PTCDA layer and lift the molecules to the top of the NPs. Moreover, by annealing the sample, PTCDA molecules can desorb from the MoS2 surface first and then desorb from the top of Au NPs at a higher temperature. By controlling the deposition amount of Au, the size of Au NPs can be tuned. In addition, interaction of Au NPs with PTCDA was investigated in-situ by X-ray photoelectron spectroscopy (XPS), and charge transfer from Au NPs to PTCDA was observed, which indicates that these Au NPs may have new chemical properties.

Double bingo. Twice in one day.

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Facet Dependent Photocatalytic Activity of Bismuth Oxyiodide

by Tommy on 6/10/2015
Facet Dependent Photocatalytic Activity Of Bismuth Oxyiodide

Facet Dependent Photocatalytic Activity Of Bismuth Oxyiodide

Facet-Dependent Catalytic Activity of Nanosheet-Assembled Bismuth Oxyiodide Microspheres in Degradation of Bisphenol A, Meilan Pan, Haijun Zhang, Guandao Gao, Lu Liu and Wei Chen, Environ. Sci. Technol., 2015, 49 (10), 6240–6248 (21 April 2015), DOI:10.1021/acs.est.5b00626

Photocatalysts with different exposed facets often exhibit different photochemical performances, but the underlying mechanisms are not fully understood. In this study, we synthesized two nanosheet-assembled bismuth oxyiodide (BiOI) microspheres with exposed (110) and (001) facets, respectively, to further investigate facet-dependent photocatalytic activity. Our experimental results showed that the BiOI microspheres with exposed (110) facets exhibited much greater catalytic activity than the BiOI microspheres with exposed (001) facets in the degradation of bisphenol A under visible light irradiation. Density functional theory calculation revealed that the (110) facets can adsorb a greater amount of O2 and, thus, form more O2  and OH radicals than the (001) facets. The electron spin resonance spectroscopy and radical scavenging experiments verified that the BiOI microspheres with exposed (110) facets could produce a greater amount of O2 radicals than the BiOI microspheres with exposed (001) facets, and more importantly, between the two BiOI products, only the BiOI microspheres with exposed (110) facets could generate OH radicals directly. The facet-dependent radical formation mechanisms were previously unidentified. The findings of this study may have important implications for the understanding of the facet-dependent photochemical performance of photocatalysts and the design of novel catalytic materials with inorganic nanostructures.

Moving quickly forward and onward. This has been verified in bismuth vanadate as well.

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4, Rengui Li, Fuxiang Zhang, Donge Wang, Jingxiu Yang, Mingrun Li, Jian Zhu, Xin Zhou, Hongxian Han and Can Li, Nature Communications, 4, 1432 (5 February 2013), doi:10.1038/ncomms2401

Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.

See also:!divAbstract

First-principles studies on facet-dependent photocatalytic properties of bismuth oxyhalides (BiOXs), Haijun Zhang, Lu Liu and Zhen Zhou, RSC Adv., 2012, 2, 9224-9229 (28 August 2012), DOI:10.1039/C2RA20881D

The photocatalytic properties were compared for the {001}, {110} and {010} facets of bismuth oxyhalides (BiOXs) through density functional theory (DFT) computations. X-terminated bulk-like {001} facets with clear boundary of [Bi2O2] and halogen slabs result in high thermodynamic stability and efficient separation of photo-induced e – h+ pairs. Moreover, surface O vacancies, which act as e – h+ recombination centers, are energetically unfavorable within {001} facets. BiX – terminated {110} and other facets with surface O vacancies introduce deep defect levels to the band gap, which are detrimental to the separation of e – h+ pairs. These findings can better understand the origin of facet-dependent photocatalytic activities in BiOXs, and provide guidance for the design of high-efficiency photocatalysts.

Very nice analysis here.

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The Bismuth (110) PhD Thesis of Ojas Mahapatra at Canterbury

by Tommy on 6/10/2015

A scanning probe microscopy (SPM) study of Bi(110) nanostructures on highly oriented pyrolytic graphite (HOPG), Ojas Mahapatra, PhD Thesis, University of Canterbury, Christchurch, New Zealand (2013)

This research work is aimed at understanding the electronic properties of Bi(110) nanostructures. This study chiefly uses Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS) and Non Contact Atomic Force Microscope (NCAFM) to investigate the geometric and electronic structure of Bi(110) islands on highly oriented pyrolytic graphite (HOPG) substrate. STM measurements are the primary focus of the thesis which involves imaging the bismuth islands and study of its atomic structure. STM images of the Bi(110) islands reveal a ‘wedding cake’ profile of the bismuth islands that show paired layers on top of a base. I(V) (Current vs voltage) data was acquired via STS techniques and its first derivative was compared to DFT calculations. The comparison implied the presence of a dead wetting layer which was present only underneath the bismuth islands. We observed bilayer damped oscillations in the surface energy that were responsible for the stability of paired layers in Bi(110) islands. Interesting Moiré pattern arising out of misorientation between the substrate and the overlayer are also observed in STM images on some bismuth islands. Bright features pertaining to enhanced LDOS (local density of states) were observed on the perimeter of the bismuth islands and stripes in the STM images and STS dI/dV maps which appear at energies around the Fermi level. The bright features which we termed as ‘bright beaches (BB)’ are also observed on grain boundaries and defects that suggest that they are related to termination of the chain of bismuth atoms. The Bi(110) islands and stripes were observed to form preferred widths with a well defined periodicity. This peculiar phenomenon was attributed to a lateral quantum size effect (QSE) that results from a Fermi wave vector with appropriate shifts in Fermi energy. The widths of the islands prefer to adjust themselves at the nodes of this in-plane Fermi wavelength. NaCl deposited on a HOPG substrate forms cross shaped islands which were used as spacers to limit the interaction between the bismuth films and the underlying HOPG substrate. The NaCl islands are transparent to the flow of tunneling current and allow STS measurements. The LDOS of Bi/HOPG was very similar to the LDOS of Bi deposited on NaCl/HOPG which suggests that the wetting layer underneath the bismuth islands plays an important role in decoupling the film from the underlying substrate.

PDF Copy of the Ojas Mahapatra’s PhD Dissertation.

Bingo! It takes me a while to dig this stuff up. I’m a slow learner.

It sure beats pounding the pavement, though!

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A Virtual Zoo of Phosphorene Polymorphs Predicted

by Tommy on 6/10/2015
Phosphorene Polymorphs

Phosphorene Polymorphs

Nine New Phosphorene Polymorphs with Non-Honeycomb Structures: A Much Extended Family, Menghao Wu, Huahua Fu, Ling Zhou, Kailun Yao and Xiao Cheng Zeng, Nano Lett., 2015, 15 (5), 3557–3562 (6 April 2015), DOI:10.1021/acs.nanolett.5b01041

We predict a new class of monolayer phosphorus allotropes, namely, ε-P, ζ-P, η-P, and θ-P. Distinctly different from the monolayer α-P (black) and previously predicted β-P (Phys. Rev. Lett. 2014, 112, 176802), γ-P, and δ-P (Phys. Rev. Lett. 2014, 113, 046804) with buckled honeycomb lattice, the new allotropes are composed of P4 square or P5 pentagon units that favor tricoordination for P atoms. The new four polymorphs, together with five additional hybrid polymorphs, greatly enrich the phosphorene structures, and their stabilities are confirmed by first-principles calculations. In particular, the θ-P is shown to be equally stable as the α-P (black) and more stable than all previously reported phosphorene polymorphs. Prediction of nonvolatile ferroelastic switching and structural transformation among different polymorphs under strains points out their potential applications via strain engineering.

It’s obvious now that bond angle is the key parameter here.

Behold the Polymorphic Phosphoronics revolution!

Strain engineering is the next big thing.

What a topological nightmare!

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Bi+ Impurity Center NIR Near Infrared Emission Confirmed

by Tommy on 6/10/2015
Roadmap For The Identification Of Bi+ Emitters

Roadmap For The Identification Of Bi+ Emitters

NIR photoluminescence of bismuth-doped CsCdBr3 – The first ternary bromide phase with a univalent bismuth impurity center, Alexey N. Romanov, Alexander A. Veber, Daria N. Vtyurina, Mikhail S. Kouznetsov, Ksenia S. Zaramenskikh, Igor S. Lisitsky, Zukhra T. Fattakhova, Elena V. Haula, Pavel A. Loiko, Konstantin V. Yumashev and Vladimir N. Korchak, Journal of Luminescence, 167, 371–375 (1 November 2015), doi:10.1016/j.jlumin.2015.07.020

Single crystals of Bi+-doped ternary bromide CsCdBr3 were prepared.
Broadband NIR photoluminescence was observed from Bi+-doped CsCdBr3.
Single optical center is responsible for NIR emission in Bi+-doped CsCdBr3.

Single crystals of ternary bromide phase CsCdBr3 doped with univalent bismuth cations are prepared for the first time by the Bridgman method. Bi+ impurity center emits a broadband long-lived near-infrared photoluminescence with a maximum at ~ 1053 nm. The characteristics of this photoluminescence and its relations with the energy spectrum of Bi+ impurity center are discussed. A comparison of Bi+ photoluminescence in CsCdBr3 and ternary chlorides (studied previously) is performed.

See Also:

NMR, ESR, and Luminescence Characterization of Bismuth Embedded Zeolites Y, Hong-Tao Sun, Yoshio Sakka, Naoto Shirahata, Yoshitaka Matsushita, Kenzo Deguchi and Tadashi Shimizu, J. Phys. Chem. C, 2013, 117 (12), 6399–6408 (28 February 2013), DOI:10.1021/jp401861c

Thermal treatment of bismuth-embedded zeolite Y yields luminescent Bi+ substructures without the formation of metallic nanoparticles. The structural and photophysical features of the resulting zeolite Y have been thoroughly characterized by using extensive experimental techniques including nuclear magnetic resonance (NMR), electron spin resonance (ESR), 2-dimentional excitation–emission and absorption spectra. NMR and ESR results indicate that some Al and oxygen are expelled from the zeolite Y framework after undergoing thermal treatment. The detailed analyses of luminescence and absorption spectra, coupled with TDDFT calculations, suggest that all Bi+ substructures (i.e., Bi44+, Bi33+, Bi22+, and Bi+) are optically active in the near-infrared (NIR) spectral range. It is found that Bi+, Bi22+, Bi33+, and Bi44+ units result in NIR emissions peaking at ca. 1050, 1135, 1145, and 1240/1285 nm, respectively. The emission lineshapes under diverse excitation wavelengths greatly depend on the Bi concentration and annealing temperature, as a result of the change in the relative concentration and the spatial distribution, as well as local structural features of Bi active species. Specifically, the above analyses imply that the reducing agents for Bi3+ are water molecules as well as framework oxygen. These findings represent an important contribution to the understanding of the processes involved in the formation of Bi+ and of the luminescence mechanisms of Bi+ substructures in zeolite Y frameworks, which are not only helpful for the in-depth understanding of experimentally observed photophysical properties in other Bi-doped materials but also important for the development of novel photonic material systems activated by other p-block elements.

Well that’s settled. Now imagine a high density array of these things excited to higher states.

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Electronic and Lattice Structures of Group V Monolayers

by Tommy on 6/10/2015

Two-Dimensional Pnictogen Honeycomb Lattice: Structure, On-Site Spin-Orbit Coupling and Spin Polarization, Jason Lee, Wen-Chuan Tian, Wei-Liang Wang and Dao-Xin Yao, Scientific Reports 5, 11512 (30 June 2015), doi:10.1038/srep11512

Because of its novel physical properties, two-dimensional materials have attracted great attention. From first-principle calculations and vibration frequencies analysis, we predict a new family of two-dimensional materials based on the idea of octet stability: honeycomb lattices of pnictogens (N, P, As, Sb, Bi). The buckled structures of materials come from the sp3 hybridization. These materials have indirect band gap ranging from 0.43 eV to 3.7 eV. From the analysis of projected density of states, we argue that the s and p orbitals together are sufficient to describe the electronic structure under tight-binding model, and the tight-binding parameters are obtained by fitting the band structures to first-principle results. Surprisingly large on-site spin-orbit coupling is found for all the pnictogen lattices except nitrogen. Investigation on the electronic structures of both zigzag and armchair nanoribbons reveals the possible existence of spin-polarized ferromagnetic edge states in some cases, which are rare in one-dimensional systems. These edge states and magnetism may exist under the condition of high vacuum and low temperature. This new family of materials would have promising applications in electronics, optics, sensors, and solar cells.

This article is also open and thus is an excellent place to start.

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Pairing Fluctuations in the BCS-BEC Crossover Regime

by Tommy on 5/10/2015

Specific heat and effects of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Femi gas, Pieter van Wyk, Hiroyuki Tajima, Ryo Hanai and Yoji Ohashi

We investigate the specific heat at constant volume CV in the BCS (Bardeen-Cooper-Schrieffer) – BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas above the superfluid phase transition temperature Tc. Within the framework of the strong-coupling theory developed by Nozieres and Schmitt-Rink, we show that this thermodynamic quantity is sensitive to the stability of preformed Cooper pairs. That is, while CV(TTc) in the unitary regime is remarkably enhanced by metastable preformed Cooper pairs or pairing fluctuations, it is well described by that of an ideal Bose gas of long-lived stable molecules in the strong-coupling BEC regime. Using these results, we identify the region where the system may be viewed as an almost ideal Bose gas of stable pairs, as well as the pseudogap regime where the system is dominated by metastable preformed Cooper pairs, in the phase diagram of an ultracold Fermi gas with respect to the strength of a pairing interaction and the temperature. We also show that the calculated specific heat agrees with the recent experiment on a 6Li unitary Fermi gas. Since the formation of preformed Cooper pairs is a crucial key in the BCS-BEC crossover phenomenon, our results would be helpful in considering how fluctuating preformed Cooper pairs appear in a Fermi gas, to eventually become stable, as one passes through the BCS-BEC crossover region.

Hallelujah! It’s gonna be another great week.

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Ammonium Bismuth Iodide Synthesized and Analyzed

by Tommy on 4/10/2015

Synthesis, Crystal Structure and Properties of a Perovskite-Related Bismuth Phase, (NH4)3Bi2I9, Shijing Sun, Satoshi Tominaka, Jung-Hoon Lee, Fei Xie, Paul D. Bristowe, Anthony K. Cheetham

Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to a remarkable breakthrough in photovoltaic devices. However, due to the environmental and stability concerns of the heavy metal, lead, in these perovskite based solar cells, research in the non-lead perovskite structures have been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH4)3Bi2I9, was prepared in solution and the structure was solved by single crystal X-ray diffraction. The results from DFT calculations showed the significant lone pair effect of the bismuth ion and the band gap was measured as around 2.04 eV, which is lower than the band gap of CH3NH3PbBr3. Conductivity measurement was also performed to examine the potential in the applications as an alternative to the lead containing perovskites.

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Majorana Fermion Surface Code For Quantum Computation

by Tommy on 4/10/2015

Physical Implementation of a Majorana Fermion Surface Code for Fault-Tolerant Quantum Computation, Sagar Vijay and Liang Fu

We propose a physical realization of a commuting Hamiltonian of interacting Majorana fermions realizing Z2 topological order, using an array of Josephson-coupled topological superconductor islands. The required multi-body interaction Hamiltonian is naturally generated by a combination of charging energy induced quantum phase-slips on the superconducting islands and electron tunneling. Our setup improves on a recent proposal for implementing a Majorana fermion surface code, a ‘hybrid’ approach to fault-tolerant quantum computation that combines (1) the engineering of a stabilizer Hamiltonian with a topologically ordered ground state with (2) projective stabilizer measurements to implement error correction and a universal set of logical gates. Our hybrid strategy has advantages over the traditional surface code architecture in error suppression and single-step stabilizer measurements, and is widely applicable to implementing stabilizer codes for quantum computation.

Previously I was only freaked out, now I am hallucinating.

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Topological Phases of Two Dimensional Materials Reviewed

by Tommy on 4/10/2015

Topological Phases in Two-Dimensional Materials: A Brief Review, Yafei Ren, Zhenhua Qiao and Qian Niu, Invited Review Article for Reports on Progress in Physics

Topological phases with insulating bulk and gapless surface or edge modes have attracted much attention because of their fundamental physics implications and potential applications in dissipationless electronics and spintronics. In this review, we mainly focus on the recent progress in the engineering of topologically nontrivial phases (such as Z2 topological insulators, quantum anomalous Hall effects, quantum valley Hall effects etc.) in two-dimensional material systems, including quantum wells, atomic crystal layers of elements from group III to group VII, and the transition metal compounds.

38 pages of up to date short review.

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Quantum Astrophysics, Rocket Science and Space Architecture

by Tommy on 4/10/2015
Space Cadet

Space Cadet

Quantum Astrophysicist, Rocket Scientist and Space Architect

I think I like the other one better.

Am I vain, or what?

Ok, I earned it.

Update: I’ve decided to shorten it up to Quantum Astrophysics.

Rocket Science and Space Architecture?

Meh. Old science.

I’ve moved on.

Update 2: Or maybe not.

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Systematic Stamping Technique for 2D Nanolayer Exfoliation

by Tommy on 2/10/2015

A systematic exfoliation technique for isolating large and pristine samples of 2D materials, Alexander E Mag-isa, Jae-Hyun Kim, Hak-Joo Lee and Chung-Seog Oh, 2D Materials, 2, 034017 (25 September 2015), doi:10.1088/2053-1583/2/3/034017

The device conceptualization and proof-of-concept testing of two-dimensional (2D) materials are performed with their pristine forms that are obtained through the micromechanical cleaving of bulk natural crystals, i.e., the so-called Scotch tape method. However, obtaining large 2D sheets is very difficult and time consuming. We developed a systematic exfoliation technique for producing sub-millimeter-sized (the largest lateral dimension ever reported) pristine 2D sheets with high throughput. It requires the treatment of both the bulk crystal and receiving substrate. Contrary to the conventional Scotch tape technique that involves the repeated folding and unfolding of an adhesive tape, the flake is stamped onto an adhesive tape to preserve the lateral size of the bulk crystal, to improve the surface flatness, and to reduce the amount of residue on the surface of the samples. When applied to graphene, the method produced monolayer and few layer graphene samples that were several hundreds of microns in length. Surprisingly, the biggest monolayer graphene sample of 367 μm in length was easily produced. The technique was also applied to produce pristine MoS2 and phosphorene sheets of about 45 μm and 95 μm in length, respectively.

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Full Electronic Band Structure of Phosphorene Calculated

by Tommy on 2/10/2015

Band parameters of phosphorene, L. C. Lew Yan Voon, J. Wang, Y. Zhang and M. Willatzen, Journal of Physics: Conference Series, 633, 1, 12042-12046 (1 January 2015)

Phosphorene is a two-dimensional nanomaterial with a direct band-gap at the Brillouin zone center. In this paper, we present a recently derived effective-mass theory of the band structure in the presence of strain and electric field, based upon group theory. Band parameters for this theory are computed using a first-principles theory based upon the generalized-gradient approximation to the density-functional theory. These parameters and Hamiltonian will be useful for modeling physical properties of phosphorene.

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Electronic Structure of Group IV and V Bilayer Nanosheets

by Tommy on 2/10/2015!divAbstract

Electronic Structures of Group-V − Group-IV Hetero-bilayer Structures: A First-principles Study, Yanli Wang and Yi Ding, Phys. Chem. Chem. Phys. (25 September 2015), DOI:10.1039/C5CP04815J

Recent findings of group-V nanosheets provide new building units for the van der waals hetero-nanostructures. Based on first-principles calculation, we investigate the structural and electronic properties of bilayer heteo-sheets composed of group-V (arsenene/antimonene) and group-IV (graphene/silicene) layers. These hetero-sheets exhibit typical van der Waals features with small binding energies and soft interlayer elastic constants. In the hetero-sheets, the Dirac characteristic of group-IV layer and semiconducting feature of group-V one are well preserved, which causes a Schottky contact in the metal-semiconductor interface. The Schottky barriers are always p-type in the Si-based hetero-sheets. Whereas in the C-based ones, the interfacial feature is sensitive to the interlayer distance. A tensile strain would induce a p-type−to−n-type Schottky barrier transition for the As−C hetero-sheet, while a compressive strain can cause a Schottky-to-Ohmic contact transition in the Sb−C one. Moreover, due to the inhomogeneous charge redistribution, a sizeable band gap is opened at the Dirac point of Sb−Si hetero-sheet, which could be linearly modulated by perpendicular strains around the equilibrium site. The versatile electronic structures and tunable interfacial properties enable the Group-V − Group-IV heterostructures many potential applications in nano-devices and nano-electronics.

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Hydrogenated and Functionalized Bismuth Thin Films Laid Bare

by Tommy on 30/09/2015

Functionalized Bismuth Films: Giant Gap Quantum Spin Hall and Valley-Polarized Quantum Anomalous Hall States, Chengwang Niu, Gustav Bihlmayer, Hongbin Zhang, Daniel Wortmann, Stefan Blugel and Yuriy Mokrousov, Phys. Rev. B 91, 041303(R) (20 January 2015), doi:10.1103/PhysRevB.91.041303

The search for new large band gap quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators is critical for their realistic applications at room temperature. Here we predict, based on first principles calculations, that the band gap of QSH and QAH states can be as large as 1.01 eV and 0.35 eV in an H-decorated Bi(111) film. The origin of this giant band gap lies both in the large spin-orbit interaction of Bi and the H-mediated exceptional electronic and structural properties. Moreover, we find that the QAH state also possesses the properties of quantum valley Hall state, thus intrinsically realising the so-called valley-polarized QAH effect. We further investigate the realization of large gap QSH and QAH states in an H-decorated Bi(110) film and X-decorated (X=F, Cl, Br, and I) Bi(111) films.

See also: Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene, H.-H. Chen, S. H. Su, S.-L. Chang, B.-Y. Cheng, S. W. Chen, H.-Y. Chen, M.-F. Lin and J. C. A. Huang, Scientific Reports, 5, 11623 (23 June 2015), doi:10.1038/srep11623

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High Thermoelectric ZT in a Distorted Bismuth (110) Layer

by Tommy on 29/09/2015

High thermoelectric performance originates from the weak electron-phonon coupling in the distorted Bismuth (110) layer, L. Cheng, H. J. Liu, J. Zhang, J. Wei, J. H. Liang, P. H. Jiang, D. D. Fan, L. Sun and J. Shi

The thermoelectric properties of the distorted bismuth (110) layer are investigated using first-principles calculations combined with the Boltzmann transport equation for both electrons and phonons. To accurately predict the electronic and transport properties, the quasiparticle corrections with the GW approximation of many-body effects have been explicitly included. It is found that a maximum ZT value of 6.4 can be achieved for n-type system, which is essentially stemmed from the weak scattering of electrons. Moreover, we demonstrate that the distorted Bi layer remains high ZT values at relatively broad regions of both temperature and carrier concentration. Our theoretical work confirms that the deformation potential constant charactering the electron-phonon scattering strength is an important paradigm for searching high thermoelectric performance materials.

Wasn’t I just talking about this?

Calling all capital!

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Hydrogenated Bismuthene Topological Quantum Atom Simulator

by Tommy on 28/09/2015

Topological Insulating Phases in Two-Dimensional Bismuth-Containing Single Layers Preserved by Hydrogenation, Rafael R. Queiroz Freitas, Roberto Rivelino, Fernando de Brito Mota, Caio Mário Castro de Castilho, Anelia Kakanakova-Georgieva, and Gueorgui K. Gueorguiev, J. Phys. Chem. C, Just Accepted Manuscript (22 September 2015), DOI: 10.1021/acs.jpcc.5b07961

Two-dimensional (2D) binary XBi compounds, where X belongs to group-III elements (B, Al, Ga, and In), in a buckled honeycomb structure may originate sizable-gap Z2 topological insulators (TIs). These are characterized by exhibiting single band inversion at the Γ point, as well as nontrivial edge states in their corresponding nanoribbons. By using first-principles calculations, we demonstrate that hydrogenation of XBi single layers leads to distinct and stable crystal structures, which can preserve their topological insulating properties. Moreover, hydrogenation opens a band gap in these new class of 2D Z2 TIs, with distinct intensities, exhibiting an interesting electronic behavior for viable room-temperature applications of these 2D materials. The nature of the global band gap (direct or indirect) and topological insulating properties depend on the X element type and spatial configuration of the sheet, as well as the applied strain. Our results indicate that the geometric configuration can be crucial to preserve totally the topological characteristics of the hydrogenated sheets. We identify sizable band inversions in the band structure for the relaxed hydrogenated GaBi and InBi in their chairlike configurations, and for hydrogenated BBi and AlBi under strain. Based on these findings, hydrogenation gives rise to a flexible chemical tunability and can preserve the band topology of the pristine XBi phases.

Wow, that was quick. Wasn’t I just talking about this? That is so weird.

This is a major piece of the puzzle right here.

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Quantum Astrophysics – The Sequel

by Tommy on 27/09/2015

Quantum astrophysics is defined as a new domain of science – bridging quantum and condensed matter physics with the astrobiological requirements of humanity and its cohabitating species, living together sustainably in the astrophysical universe, whether that be on earth or in deep space.

I point out that an enormous paradigmatic shift and technological revolution is now occurring in condensed matter physics, where modern computational and atomic and molecular simulations are converging to immediate spectroscopic verification of quantum theories of matter, which has suddenly revealed the rich and mathematically precise realm of novel topological physics and quantum critical behavior of matter, that apparently is now immediately available for technological exploitation.

I further point out that the single most valuable result coming out of this new perspective of quantum matter would be a ZT=3-4 room temperature thermoelectric device, since all quantum behavior of matter is temperature dependent, and cooling quantum devices will require lowering their temperature. A simple electrical device of this efficiency would also solve a number of pressing human habitability problems such as the boiling and freezing of water, humidity control of air, and the rejection and dissipation of waste heat. Cascading such devices down to cryogenic temperatures will solve a variety of astronautical problems in the area of cryogenic fuel and oxidizer storage in deep space and on planetary surfaces.

A new paradigmatic perspective of condensed matter systems is presented where the ‘atomenes’ of the Group IV elements, graphene, silicene, germanene and stanene, and the Group V elements, phosphorene, arsenene, antimonene and bismuthene, are considered to be at the low dimensional, weakly coupled, hexagonal limit of molecular matter, and the high pressure hydride phases of ammonia, phosphine, arsine, stibine and bismuthine are considered to be at the three dimensional, pseudo cubic limit of what is achievable in this realm of novel quantum physics.

Since the surfaces of the atomenes in their various polymorphic and allotropic forms are, for all practical purposes, essentially two dimensional atomic and molecular simulators, it is proposed that the thermodynamic, electrochemical and electronic transport properties and behaviors of various elemental configurations of atomic and molecular species can be quickly sorted through using them to produce and then host the most likely combination of elements and nanostructures necessary for high ZT thermoelectric effects.

Finally I point out that the most promising candidate for a ZT=4 room temperature thermoelectric energy conversion device would be bismuthene in the form of hydrogenated and/or iodated bismuthene at the low dimensional limit, and iodobismuthine under pressure at the high density limit, with any phosphorene based anolog of this system running a close second. Nanoengineered and nanostructured graphene appears to be viable up to about ZT=3, and doped stanene related compounds up to ZT=2.7.

Obviously this NASA NIAC submission refers to a ninth month $100,000 continuation of this effort to identify a suitable ZT=3-4 room temperature thermoelectric device, and the application of such a device to other more actively driven approaches to ZT efficiency enhancement using thermoelectrically cooled quantum physics, whatever those microscopic quantum processes turn out to be.

This is the summary. I already found a typo. It was #10 this time around.

Start here:

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Ab Initio Simulation of Bismuth Subhalides Bi4I4 and Bi4Br4

by Tommy on 25/09/2015
Bismuth Sub Halide Iodide Crystal Structure

Bismuth Sub Halide Iodide Crystal Structure

Weak Topological Insulators and Composite Weyl Semimetals: β-Bi4X4 (X = Br, I), Cheng-Cheng Liu, Jin-Jian Zhou, Yugui Yao and Fan Zhang

While strong topological insulators (STI) have been experimentally realized soon after their theoretical predictions, a weak topological insulator (WTI) has yet to be unambiguously confirmed. A major obstacle is the lack of distinct natural cleavage surfaces to test the surface selective hallmark of WTI. With a new scheme, we discover that Bi4X4 (X = Br, I), stable or synthesized before, can be WTI with two natural cleavage surfaces, where two anisotropic Dirac cones stabilize and annihilate, respectively. We further find four surface state Lifshitz transitions under charge doping and two bulk topological phase transitions under uniaxial strain. Near the WTI-STI transition, there emerges a novel Weyl semimetal phase, in which the Fermi arcs generically appear at both cleavage surfaces whereas the Fermi circle only appears at one selected surface.

It’s about time. Didn’t I say it was going to be a great week? Thinking about uniaxial strain on a one dimensional polymer van der Waals crystal has been a running 20 year nightmare though.

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Optical Exciton Bands of Molybdenum Diselenide – MoSe2

by Tommy on 23/09/2015

Excitonic band structure in thin films of MoSe2: From monolayer to bulk limit, Ashish Arora, Karol Nogajewski, Maciej Molas, Maciej Koperski and Marek Potemski

We present the micro-photoluminescence (μPL) and micro-reflectance contrast spectroscopy studies on thin films of MoSe2 with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe2 is tentatively attributed to the appearance of Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe2 with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe2.

Bismuth triiodide was a lot of fun back in the day.

This is a different animal.

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Real Space Electronic Structure Calculations Greatly Improved

by Tommy on 22/09/2015

RESCU: a Real Space Electronic Structure Method, Vincent Michaud-Rioux, Lei Zhang and Hong Guo

In this work we present RESCU, a powerful MATLAB-based Kohn-Sham density functional theory (KS-DFT) solver. We demonstrate that RESCU can compute the electronic structure properties of systems comprising many thousands of atoms using modest computer resources, e.g. 16 to 256 cores. Its computational efficiency is achieved from exploiting four routes. First, we use numerical atomic orbital (NAO) techniques to efficiently generate a good quality initial subspace which is crucially required by Chebyshev filtering methods. Second, we exploit the fact that only a subspace spanning the occupied Kohn-Sham states is required, and solving accurately the KS equation using eigensolvers can generally be avoided. Third, by judiciously analyzing and optimizing various parts of the procedure in RESCU, we delay the O(N3) scaling to large N, and our tests show that RESCU scales consistently as O(N2.3) from a few hundred atoms to more than 5,000 atoms when using a real space grid discretization. The scaling is better or comparable in a NAO basis up to the 14,000 atoms level. Fourth, we exploit various numerical algorithms and, in particular, we introduce a partial Rayleigh-Ritz algorithm to achieve efficiency gains for systems comprising more than 10,000 electrons. We demonstrate the power of RESCU in solving KS-DFT problems using many examples running on 16, 64 and/or 256 cores: a 5,832 Si atoms supercell; a 8,788 Al atoms supercell; a 5,324 Cu atoms supercell and a small DNA molecule submerged in 1,713 water molecules for a total 5,399 atoms. The KS-DFT is entirely converged in a few hours in all cases. Our results suggest that the RESCU method has reached a milestone of solving thousands of atoms by KS-DFT on a modest computer cluster.

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Theory of Thermoelectricity PhD Thesis of Shuang Tang at MIT

by Tommy on 22/09/2015

Materials Physics for Thermoelectric and Related Energetic Applications, Shuang Tang, PhD Thesis, Massachusetts Institute of Technology, Mildred S. Dresselhaus, Advisor (2015)

Thermoelectrics study the direct inter-conversion between heat flow and electrical power, which has a wide range of applications including power generation and refrigeration. The performance of thermoelectricity generation and the refrigeration is characterized by a dimensionless number called the Figure-of-Merit (ZT), defined as ZT = σ S2T/κ, where a is the electrical conductivity, κ is the thermal conductivity, S is the Seebeck coefficient, and T is the absolute temperature. Before 1993, the upper-limit of ZT was barely 1. After the efforts of more than twenty years, the upper-limit of ZT has been pushed up to ~ 2. However, for the thermoelectric technology to be commercially attractive, the value of ZT and the cost of production have to be further improved. Most of the ZT enhancing strategies that have been proposed since 1993 involve the changing and the controlling of the dimension of materials systems, the scattering mechanism(s) of carriers, the shape of the electronic band structure and the density of states, and the magnitude of the band gap. As further research is carried out, it is found that these strategies do not always work to enhance ZT. Even for a working materials system, the improvement margin of increasing ZT can be small. The balancing between σ and S2/κ has significantly limited the improvement margin for our ZT enhancing goal. Therefore, we have two problems to explore: (1) how can we deal with the strong correlation between σ and S2/κ, when trying to enhance ZT, and (2) how can we make the above mentioned strategies more convergent as we change the dimension of materials systems, the scattering mechanism(s) of carriers, the shape of electronic band structure, and the magnitude of the band gap? This thesis aims to explore the solutions to these two major problems at the research frontier of thermoelectric ZT enhancement. The first problem is discussed by providing a new framework of pseudo-ZTs, where the electronic contribution (zte) and the lattice contribution (ztL) to the overall ZT can be treated in a relatively separate manner. The second problem is discussed under this new framework of pseudo-ZTs, through four subsections: (i) scattering and system dimension; (ii) band structure; (iii) density of states; (iv) band gap. The one-to-one correspondence relation between the carrier scattering mechanism(s) and the maximum Seebeck coefficient is further studied. A new tool for scattering mechanism(s) inference and for the Seebeck coefficient enhancement is provided. For the band structure and the band gap part, advanced band engineering methods are provided to study nanostructured narrow-gap materials, the Dirac cone materials, and the anisotropic materials, which are historically found to be good thermoelectric materials. To further demonstrate the newly developed theories, this thesis has also illustrated the application of these models in some specific materials systems, including the graphene system, the transition metal dichalcogenides monolayer materials systems, the Bi1-xSbx alloys system, the In1-xGaxN alloys system, and the (Bi1-ySby)2(S1-xTex)3 alloys system.

There may be some typos in that last chemical formula, I thought it was selenium – Se.

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Phonon Drag and Phonon Filtering for Increasing ZT Efficiency

by Tommy on 22/09/2015

Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion, Jiawei Zhou, Bolin Liao, Bo Qiu, Samuel Huberman, Keivan Esfarjani, Mildred S. Dresselhaus and Gang Chen

While the thermoelectric figure of merit zT above 300K has seen significant improvement recently, the progress at lower temperatures has been slow, mainly limited by the relatively low Seebeck coefficient and high thermal conductivity. Here we report, for the first time, success in first-principles computation of the phonon drag effect – a coupling phenomenon between electrons and non-equilibrium phonons – in heavily doped region and its optimization to enhance the Seebeck coefficient while reducing the phonon thermal conductivity by nanostructuring. Our simulation quantitatively identifies the major phonons contributing to the phonon drag, which are spectrally distinct from those carrying heat, and further reveals that while the phonon drag is reduced in heavily-doped samples, a significant contribution to Seebeck coefficient still exists. An ideal phonon filter is proposed to enhance zT of silicon at room temperature by a factor of 20 to around 0.25, and the enhancement can reach 70 times at 100 K. This work opens up a new venue towards better thermoelectrics by harnessing non-equilibrium phonons.

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Carbon Thermoelecticity PhD Thesis of Jeong Yun Kim at MIT

by Tommy on 22/09/2015
Functionalized Graphene Thermoelectric ZT

Functionalized Graphene Thermoelectric ZT

Understanding and designing carbon-based thermoelectric materials with atomic-scale simulations, Jeong Yun Kim, PhD Thesis, Massachusetts Institute of Technology (MIT), Department of Materials Science and Engineering, Jeffrey C. Grossman, Advisor (2015)

Thermoelectric (TE) materials, which can convert unused waste heat into useful electricity or vice versa, could play an important role in solving the current global energy challenge of providing sustainable and clean energy. Nevertheless, thermoelectrics have long been too inefficient to be utilized due to the relatively low energy conversion efficiency of present thermoelectrics. One way to obtain improved efficiency is to optimize the so-called TE figure of merit, ZT = S2σ/κ, which is determined by the transport properties of the active layer material. To this end, higher-efficiency thermoelectrics will be enabled by a deep understanding of the key TE properties, such as thermal and charge transport and the impact of structural and chemical changes on these properties, in turn providing new design strategies for improved performance. To discover new classes of thermoelectric materials, computational materials design is applied to the field of thermoelectrics. This thesis presents a theoretical investigation of the influence of chemical modifications on thermal and charge transport in carbon-based materials (e.g., graphene and crystalline C60), with the goal of providing insight into design rules for efficient carbon-based thermoelectric materials. We carried out a detailed atomistic study of thermal and charge transport in carbon-based materials using several theoretical and computational approaches – equilibrium molecular dynamics (EMD), lattice dynamics (LD), density functional theory (DFT), and the semi-classical Boltzmann theory. We first investigated thermal transport in graphene with atomic-scale classical simulations, which has been shown that the use of two-dimensional (2D) periodic patterns on graphene substantially reduces the room-temperature thermal conductivity compared to that of the pristine monolayer. This reduction is shown to be due to a combination of boundary effects induced from the sharp interface between sp2 and sp3 carbon as well as clamping effects induced from the additional mass and steric packing of the functional groups. Using lattice dynamics calculations, we elucidate the correlation between this large reduction in thermal conductivity and the dynamical properties of the main heat carrying phonon modes. We have also explored an understanding of the impact of chemical functionalization on charge transport in graphene. Using quantum mechanical calculations, we predict that suitable chemical functionalization of graphene can enhance the room-temperature power factor of a factor of two compared to pristine graphene. Based on the understanding on both transport studies we have gained here, we propose the possibility of highly efficient graphene-based thermoelectric materials, reaching a maximum ZT ~ 3 at room temperature. We showed here that it is possible to independently control charge transport and thermal transport of graphene, achieving reduced thermal conductivity and enhanced power factor simultaneously. In addition, we discuss here the broader potential and understanding of the key thermoelectric properties in 2D materials, which could provide new design strategies for high efficient TE materials. Transport properties of crystalline C60 are investigated, and the results demonstrate that these properties can be broadly modified with metal atom intercalation in crystalline C60. In contrast to the case of graphene, where chemical modifications induce structural changes in graphene lattice (from sp2 C to sp3 C), intercalating metal atoms only modify van der Waals interactions between C60 molecules, but still having a huge impact on both thermal and charge transport. Taken both transport studies together, we suggest that the metal atom intercalation in crystalline C60 could be a highly appealing approach to improve both transports in solid C60, and with appropriate optimization of TE figure of merit, ZT value as large as 1 at room-temperature can be achieved. This dissertation consists of five chapters. Chapter 1 contains a brief review of thermoelectric materials. Chapter 2 introduces the theoretical approaches for computing both thermal (with molecular dynamics and lattice dynamics) and charge transport (with density functional theory and semi-classical Boltzmann approach) in materials. In Chapter 3, our study of thermal transport in functionalized graphene is presented. Chapter 4 describes our results on charge carrier transport in functionalized graphene. Combining these two works, we predict the full ZT values of functionalized graphene. Chapter 5 describes how to optimize ZT value in metal atom intercalated crystalline C60.

This has been published:

High-Efficiency Thermoelectrics with Functionalized Graphene, Jeong Yun Kim and Jeffrey C. Grossman, Nano Lett., 2015, 15 (5), 2830–2835 (13 April 2015), DOI: 10.1021/nl504257q

Graphene superlattices made with chemical functionalization offer the possibility of tuning both the thermal and electronic properties via nanopatterning of the graphene surface. Using classical and quantum mechanical calculations, we predict that suitable chemical functionalization of graphene can introduce peaks in the density of states at the band edge that result in a large enhancement in the Seebeck coefficient, leading to an increase in the room-temperature power factor of a factor of 2 compared to pristine graphene, despite the degraded electrical conductivity. Furthermore, the presence of patterns on graphene reduces the thermal conductivity, which when taken together leads to an increase in the figure of merit for functionalized graphene by up to 2 orders of magnitude over that of pristine graphene, reaching its maximum ZT ∼ 3 at room temperature according to our calculations. These results suggest that appropriate chemical functionalization could lead to efficient graphene-based thermoelectric materials.

ZT = 3. I’m getting that.

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Reversible Strain Induced Semiconductor to Metal Transition in Tin Sulfide – SnS2

by Tommy on 22/09/2015

From semiconductor to metal: A reversible tuning of electronic properties of mono to multilayered SnS2 under applied strain, Babu Ram, Aaditya Manjanath and Abhishek K. Singh

Controlled variation of the electronic properties of 2D materials by applying strain has emerged as a promising way to design materials for customized applications. Using first principles density functional theory calculations, we show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS2 exhibit a reversible semiconductor to metal transition (S-M) at strain values of 0.17, −0.26, and −0.24 under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve S-M transition in SnS2 under NC strain is much higher than for MoS2. The S-M transition for BT, BC, and NC strains is caused by the interaction between the S-pz and Sn-s, S-px/py and Sn-s, and S-pz and Sn-s orbitals, respectively.

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The Dielectric Functions of Graphene, Silicene and Arsenene

by Tommy on 21/09/2015

Temperature-dependent dielectric functions in atomically thin graphene, silicene, and arsenene, J. Y. Yang and L. H. Liu, Appl. Phys. Lett. 107, 091902 (2015), doi:10.1063/1.4930025

The dielectric functions of atomically thin graphene, silicene, and arsenene have been investigated as a function of temperature. With zero energy gap, more carriers in graphene and silicene are thermally excited as temperature increases and intraband transition strengthens, resulting in the strengthened absorption peak. Yet with large energy gap, interband transition dominates optical absorption of arsenene but it reduces as lattice vibration enhances, inducing the redshift and decreased absorption peak. To validate the theoretical method, the calculated optical constants of isolated graphene are compared with ellipsometry results and demonstrate good agreement.

This article is open and is probably the best place to start.

There is not too much in the literature on this yet.

I would rather have cheap and non-toxic.

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Anisotropic Lattice Thermal Conductivity of Arsenene Analyzed

by Tommy on 21/09/2015

Highly Anistoropic Thermal Conductivity of Arsenene: An ab initio Study, M. Zeraati, S. M. Vaez Allaei, I. Abdolhosseini Sarsari, M. Pourfath and D. Donadio

Elemental 2D materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity, of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 μm provide the main contribution to the large thermal conductivity in the zig-zag direction, mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained low and anisotropic thermal conductivity, and feasibility of synthesis, in addition to other reports on high electron mobility, make arsenene a promising material for a variety of applications, including thermal management and thermoelectric devices.

More of the start of it.

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Hydrogenated Arsenene and Antimonene Studied

by Tommy on 21/09/2015

Hydrogenated arsenenes as planar magnet and Dirac material, Shengli Zhang, Yonghong Hu, Ziyu Hu, Bo Cai and Haibo Zeng, Appl. Phys. Lett. 107, 022102 (13 July 2015), doi:10.1063/1.4926761

Arsenene and antimonene are predicted to have 2.49 and 2.28 eV band gaps, which have aroused intense interest in the two-dimensional (2D) semiconductors for nanoelectronic and optoelectronic devices. Here, the hydrogenated arsenenes are reported to be planar magnet and 2D Dirac materials based on comprehensive first-principles calculations. The semi-hydrogenated (SH) arsenene is found to be a quasi-planar magnet, while the fully hydrogenated (FH) arsenene is a planar Dirac material. The buckling height of pristine arsenene is greatly decreased by the hydrogenation, resulting in a planar and relatively low-mass-density sheet. The electronic structures of arsenene are also evidently altered after hydrogenating from wide-band-gap semiconductor to metallic material for SH arsenene, and then to Dirac material for FH arsenene. The SH arsenene has an obvious magnetism, mainly contributed by the p orbital of the unsaturated As atom. Such magnetic and Dirac materials modified by hydrogenation of arsenene may have potential applications in future optoelectronic and spintronic devices.

So what is the holdup here?

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