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In this work, molybdenum disulfide (MoS2) nanorods (NRs) were prepared by a simple hydrothermal method. A sensitive electrochemical glucose biosensor was developed based on the immobilization of glucose oxidase (GOx) on MoS2 NRs modified glassy carbon electrode (GCE). The SnO2 NRs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). SnO2 NRs have large specific area and can load large amounts of GOx molecules. The cyclic voltammetry (CV) of GOx/MoS2 NR/GCE exhibited a linear relationship between the peak current density of CV with glucose concentration in the range of 3.0 mM to 7.0 mM with the limit of detection (LOD) of 3.0 mM and high sensitivity of mA.mM. The parameters affecting the oxidation current density such as pH, temperature, GOx concentration were also investigated. This study demonstrates the feasibility of realizing inexpensive, reliable, and highly effective performance glucose biosensors using MoS2 nanorods.

Objectives: To describe the clinical and laboratory features of stage III empyema and to evaluate the effectiveness and safety of using VATS surgery to treat stage III empyema at the Vietnam National Lung hospital. Methods: Propective observational study. From 7/2019 to 01/2020 a total of 58 stage III empyema patients diagnosed who were treated at the Department of Thoracic Surgery – NLH were included in this study. Data were analyzed using SPSS software version 22.0. Results: Among 58 patients with stage III empyema, 39 were treated by total VATS surgery, 19 by VATS as supported procedures. The mean age was 45±19.5 (17-85) years old. Male/female ratio :3.83. The main clinical symptoms were chest pain (81.0%), shortness of breath (70.7%), cough (60.3%). Average surgical time: 127.2±41.6(60- 250) minutes, draining time: 7.5±3 days, hospitalization time 10.3±3.4 (5-22) day. 96.6% of patients discharged with well conditions, confirmed by lung well-expanded lungs on Xrays, the success rate was 94.8%, and there was no patient death. Conclusion: Diagnosing stage III empyema was mainly based on clinical symptoms, chest HRCT. VATS surgery is the safe and effective technique, which helps to reduce surgery time, draining, lying down and reducing pain for patients.

We present a theoretical model of composite excitonic states in doped semiconductors. Many-body interactions between a photoexcited electron-hole pair and the electron gas are integrated into a computationally tractable few-body problem, solved by the variational method. We focus on electron-doped ML-MoSe$_2$ and ML-WSe$_2$ due to the contrasting character of their conduction bands. In both cases, the core of the composite is a tightly-bound trion (two electrons and valence-band hole), surrounded by a region depleted of electrons. The composite in ML-WSe$_2$ further includes a satellite electron with different quantum numbers. The theory is general and can be applied to semiconductors with various energy-band properties, allowing one to calculate their excitonic states and to quantify the interaction with the Fermi sea.

Ho ra máu là máu khạc ra từ dưới thanh môn trở xuống, một triệu chứng thường gặp nhưng có rất nhiều nguyên nhân. Ho ra máu nặng được định nghĩa là khi lượng máu ho ra từ > 100 đến > 600 ml trong 24 giờ. Ho ra máu nặng thường là dấu hiệu của một bệnh nặng và có thể gây tử vong do ngạt và tắc nghẽn đường thở. Ho ra máu nặng chiếm khoảng 5% các trường hợp ho ra máu và tỷ lệ tử vong lên tới 80%. Tại Bệnh viện Phổi trung ương đã điều trị nhiều trường hợp ho ra máu nặng bằng phẫu thuật cùng với sự phối hợp của nhiều chuyên khoa khác nhau (Hồi sức tích cực, các phương pháp điều trị nội khoa mới nhất, nút mạch, nội soi can thiệp, …), đã cứu chữa thành công rất nhiều bệnh nhân ho ra máu nặng với nhiều nguyên nhân khác nhau như do di chứng lao phổi, u nấm phổi, giãn phế quản, ung thư phế quản, nhồi máu phổi, các dị tật bẩm sinh phổi, …). Tháng 02/2018 vừa qua, Bệnh viện tiếp nhận một trường hợp ho ra máu nặng có nguy cơ tử vong rất cao. Bệnh nhân được cứu chữa thành công nhờ sự chỉ đạo trực tiếp của Giám Đốc Bệnh viện, sự phối hợp tích cực của nhiều chuyên Khoa, kíp phẫu thuật và việc ứng dụng các kỹ thuật, công nghệ cao trong chẩn đoán, điều trị. Mặc dù tại bệnh viện chuyên khoa đầu ngành về hô hấp, ho ra máu nặng không quá hiếm song trường hợp này lại có rất đặc biệt.

46 trường hợp (18 nam, 28 nữ) chẩn đoán phổi biệt lập sau phẫu thuật giai đoạn 10/2009 – 07/2016 tại bệnh viện Phổi trung ương. Tuổi trung bình 33,3 ± 12,6 tuổi, 28 trường hợp có tiền sử điều trị các bệnh hô hấp khác nhau. Triệu chứng lâm sàng chủ yếu đau ngực và ho, sốt, khó thở, XQuang ngực thường quy chủ yếu với dạng tổn thương: Mờ không đồng nhất 31,82 %, mờ đồng nhất 27,27 %, kén dịch khí 20,45%, kén khí/ hang 18,18%, đa kén khí 2,27%. Chẩn đoán xác định dựa trên CT-Scanner ngực có dựng hình mạch máu phát hiện mạch bất thường 84,78%. Hình ảnh tổn thương: khối đặc: 13,04%, nang dịch 28,26%, mờ không đồng nhất 21,74%, kén dịch khí 15,21%, kén khí 17,39%, đa kén khí 4,35%. Tổn thương bên phải: 18 trường hợp, trái: 28 trường hợp nằm phân thùy sau dưới. Chẩn đoán đúng trước mổ 39/46 (84,78%) trường hợp. Điều trị với phổi biệt lập trong thùy (40 trường hợp) cắt thùy phổi 95 %, cắt phần phổi chứa phổi biệt lập 5 %. Phổi biệt lập ngoài thùy 2/6 trường hợp cắt nguyên phổi biệt lập, 3/6 trường hợp cắt phổi biệt lập và 1 phần thùy phổi lân cận, 1/6 trường hợp phải cắt cả thùy phổi. Thời gian nằm viện sau mổ khi không có biến chứng rò thực quản sau mổ trung bình 16,2 ± 12,84 ngày.

In this study, the effects of the morphological characteristics of MoS2 nanomaterials on the glucose sensing of electrochemical biosensors were explored. Nanostructured MoS2 materials, including nanoparticles (NPs), nanoflowers (NFs), and nanoplatelets (NPLs), were prepared via a simple hydrothermal method. The structure and morphological characteristics of MoS2 nanomaterials were examined through X-ray diffraction, field emission scanning electron microscopy, and Raman spectroscopy. Electrochemical properties were analyzed through cyclic voltammetry. Results showed that the obtained sensitivity was 64, 68.7, and 77.6 μAmM−1 cm−2 for MoS2 NP-, MoS2 NF-, and MoS2 NPL-based biosensors, respectively. The limit of detection (LOD) of all MoS2-based glucose biosensors was 0.081 mM. In addition, the pH, temperature, glucose oxidase (GOx) concentration, reproducibility, specificity, and stability of glucose biosensors with different MoS2 morphologies were also investigated and indicated the oxidation current response of the MoS2 NPL-based glucose biosensor was higher than that of MoS2 NF- and NP-based biosensors.

In the archetypal monolayer semiconductor WSe_{2}, the distinct ordering of spin-polarized valleys (low-energy pockets) in the conduction band allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities. We discuss magneto-optical measurements of electron-rich WSe_{2} monolayers and interpret the spectral lines that emerge at high electron doping as optical transitions of six-body exciton states ("hexcitons") and eight-body exciton states ("oxcitons"). These many-body states emerge when a photoexcited electron-hole pair interacts simultaneously with multiple Fermi seas, each having distinguishable spin and valley quantum numbers. In addition, we explain the relations between dark trions and satellite optical transitions of hexcitons in the photoluminescence spectrum.

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light. Optical excitation of transition metal dichalcogenide monolayers mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here, the authors demonstrate efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers.

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons, phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, and the resulting exciton complexes in the monolayer semiconductor WSe2. From Lande g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons., to appear in Nature Communications

In this review we discuss the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification. We first discuss the spin-orbit coupling induced by local $\sigma-\pi$ re-hybridization and ${\bf sp}^{3}$ C-H defect formation together with the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances, recently introduced \cite{Kochan2014} is revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space order N wavepacket propagation method. Typical spin lifetimes on the order of 1 nanosecond are obtained for 1 ppm of hydrogen impurities (corresponding to transport time about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that magnetism is the origin of the substantial spin polarization loss in the ultraclean graphene limit., Comment: 10 pages, 8 figures, accepted for publication in 2D Materials as a Topical Review

Open Access: ACS AuthorChoice License., Polycrystalline graphene is a patchwork of coalescing graphene grains of varying lattice orientations and size, resulting from the chemical vapor deposition (CVD) growth at random nucleation sites on metallic substrates. The morphology of grain boundaries has become an important topic given its fundamental role in limiting the mobility of charge carriers in polycrystalline graphene, as compared to mechanically exfoliated samples. Here we report new insights to the current understanding of charge transport in polycrystalline geometries. We created realistic models of large CVD-grown graphene samples and then computed the corresponding charge carrier mobilities as a function of the average grain size and the coalescence quality between the grains. Our results reveal a remarkably simple scaling law for the mean free path and conductivity, correlated to atomic-scale charge density fluctuations along grain boundaries. © 2013 American Chemical Society., J.K. acknowledges computational resources from the Vienna Scientific Cluster and funding from University of Helsinki Funds as well as the Austrian Science Fund (FWF): M 1481-N20. S.R. acknowledges the support from SAMSUNG within the Global Innovation Program.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We report an intriguing transition from the quantum spin Hall phase to the spin Hall effect upon segregation of thallium adatoms adsorbed onto a graphene surface. Landauer-Büttiker and Kubo-Greenwood simulations are used to access both edge and bulk transport physics in disordered thallium-functionalized graphene systems of realistic sizes. Our findings not only quantify the detrimental effects of adatom clustering in the formation of the topological state, but also provide evidence for the emergence of spin accumulation at opposite sample edges driven by spin-dependent scattering induced by thallium islands, which eventually results in a minimum bulk conductivity ~4e2/h, insensitive to localization effects., S. R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Severo Ochoa Program (MINECO SEV-2013-0295), and the Secretaria de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship.

The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe$_2$ and MoTe$_2$ wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe$_2$, WS$_2$ and likely MoS$_2$ wherein the Rashba-induced coupling is negligible., We would appreciate any feedback

Monolayer transition-metal dichalcogenides (ML-TMDs) offer exciting opportunities to test the manifestations of many-body interactions through changes in the charge density. Tuning the charge density by a gate voltage leads to profound changes in the optical spectra of excitons in ML-TMDs. We review the band-gap renormalization and dynamical screening as a function of charge density, and then incorporate these effects through various approximations that model long-wavelength charge excitations in the Bethe-Salpeter Equation (BSE). We then show that coupling between excitons and shortwave charge excitations is essential to resolve several experimental puzzles. Unlike ubiquitous and well-studied plasmons, driven by collective oscillations of the background charge density in the long-wavelength limit, we discuss the emergence of shortwave plasmons that originate from the short-range Coulomb interaction through which electrons transition between the $\mathbf{K}$ and $-\mathbf{K}$ valleys. We study the coupling between the shortwave plasmons and the neutral exciton through the self-energy of the latter. We then elucidate how this coupling as well as the spin ordering in the conduction band give rise to an experimentally observed optical sideband in electron-doped W-based MLs, conspicuously absent in electron-doped Mo-based MLs or any hole-doped ML-TMDs. While the focus of this review is on the optical manifestations of many-body effects in ML-TMDs, a systematic description of the dynamical screening and its various approximations allow one to revisit other phenomena, such as nonequilibrium transport or superconducting pairing, where the use of the BSE or the emergence of shortwave plasmons can play an important role., Comment: This revised version is far more extensive. Any feedback is welcome

Photoluminescence experiments from monolayer transition-metal dichalcogenides often show that the binding energy of trions is conspicuously similar to the energy of optical phonons. This enigmatic coincidence calls into question whether phonons are involved in the radiative recombination process. We address this problem, unraveling an intriguing optical transition mechanism. Its initial state is a localized charge (electron or hole) and delocalized exciton. The final state is the localized charge, phonon and photon. In between, the intermediate state of the system is a virtual trion formed when the localized charge captures the exciton through emission of the phonon. We analyze the difference between radiative recombinations that involve real and virtual trions (i.e., with and without a phonon), providing useful ways to distinguish between the two in experiment., Any Feedback is welcome. Enjoy the reading

Recently, the celebrated Keldysh potential has been widely used to describe the Coulomb interaction of few-body complexes in monolayer transition-metal dichalcogenides. Using this potential to model charged excitons (trions), one finds a strong dependence of the binding energy on whether the monolayer is suspended in air, supported on SiO$_2$, or encapsulated in hexagonal boron-nitride. However, empirical values of the trion binding energies show weak dependence on the monolayer configuration. This deficiency indicates that the description of the Coulomb potential is still lacking in this important class of materials. We address this problem and derive a new potential form, which takes into account the three atomic sheets that compose a monolayer of transition-metal dichalcogenides. The new potential self-consistently supports (i) the non-hydrogenic Rydberg series of neutral excitons, and (ii) the weak dependence of the trion binding energy on the environment. Furthermore, we identify an important trion-lattice coupling due to the phonon cloud in the vicinity of charged complexes. Neutral excitons, on the other hand, have weaker coupling to the lattice due to the confluence of their charge neutrality and small Bohr radius., A longer version

Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors, and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. In this work, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical sideband that is observed repeatedly in monolayers of WSe\(_2\) and WS\(_2\) but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.

We calculate the dynamical dielectric function of doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities \(n_1, n_2\), respectively, and a separation interlayer of \(d\) at finite temperature. The results are used to find the dispersion of plasmon modes. We study the temperature effects on the DLG plasmon modes in the case of symmetric system \((n_1=n_2)\), asymmetric system \((n_1\neq n_2)\) and no free carriers in the second layer \((n_2=0)\). We show that the effect of temperature on the plasmon dispersion is significant and can not be ignored in investigating graphene properties.

The understanding of spin dynamics and relaxation mechanisms in clean graphene, and the upper time and length scales on which spin devices can operate, are prerequisites to realizing graphene-based spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with Rashba spin-orbit fields as low as a few tens of μeV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in the presence of electron-hole puddles., This work has received funding from the European Union Seventh Framework Programme under grant agreement 604391 Graphene Flagship. S.R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluña and the Severo Ochoa Program (MINECO SEV-2013-0295). F.O. would like to acknowledge the Deutsche Forschungsgemeinschaft (grant OR 349/1-1).

Spin manipulation is one of the most critical challenges to realize spin-based logic devices and spintronic circuits. Graphene has been heralded as an ideal material to achieve spin manipulation, but so far new paradigms and demonstrators are limited. Here we show that certain impurities such as fluorine adatoms, which locally break sublattice symmetry without the formation of strong magnetic moment, could result in a remarkable variability of spin transport characteristics. The impurity resonance level is found to be associated with a long-range sublattice pseudospin polarization, which by locally decoupling spin and pseudospin dynamics provokes a huge spin lifetime electron-hole asymmetry. In the dilute impurity limit, spin lifetimes could be tuned electrostatically from 100 ps to several nanoseconds, providing a protocol to chemically engineer an unprecedented spin device functionality., This work has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. S. R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO SEV-2013-0295).

We report anomalous quantum transport features in bilayer graphene in the presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus nondimer vacancies is investigated in very large scale disordered models. While nondimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer band structure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a realization of Dirac semimetals in high dimension., S.R. acknowledges the Spanish Ministry of Economy and Competitiveness (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO SEV-2013-0295).

We report on spin transport features which are unique to high quality bilayer graphene, in the absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of a propagating wave packet is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy dependence of the spin lifetime is similar to monolayer graphene with an M-shaped profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hole puddles (characteristics of supported graphene either on SiO2 or hBN) surprisingly results in a large enhancement of the low-energy spin lifetime and a lowering of its high-energy values. Such a feature, unique to the bilayer, is explained in terms of a reinforced Dyakonov-Perel mechanism at the Dirac point, whereas spin relaxation at higher energies is driven by pure dephasing effects. This suggests further electrostatic control of the spin transport length scales in graphene devices., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 696656, Graphene Flagship. S.R. acknowledges Funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund [Project No. FIS2015- 67767-P (MINECO/FEDER)] and the Severo Ochoa Program (MINECO SEV-2013-0295). S.A. acknowledges support from the National Research Foundation of Singapore under its Fellowship programme (NRF-NRFF2012-01) and computational resources from the Centre for Advanced 2D Materials (an NRF midsized center).

Recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms have raised a fierce controversy. We apply numerically exact Kubo and Landauer-Büttiker formulas to realistic models of gold-decorated disordered graphene (including adatom clustering) to obtain the spin Hall conductivity and spin Hall angle, as well as the nonlocal resistance as a quantity accessible to experiments. Large spin Hall angles of ∼0.1 are obtained at zero temperature, but their dependence on adatom clustering differs from the predictions of semiclassical transport theories. Furthermore, we find multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, as well as a strong suppression of the SHE at room temperature. This motivates us to design a multiterminal graphene geometry which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 696656. S. R. acknowledges Funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (Project No. FIS2015-67767-P (MINECO/FEDER)), the Secretaria de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO, Grant No. SEV2013-0295). We acknowledge computational resources from PRACE and the Barcelona Supercomputing Center (Mare Nostrum), under Project No. 2015133194. J. M. M.-T. was supported as a Fulbright Colombia Scholar and by Colciencias (Departamento Administrativo de Ciencia, Tecnologia e Innovacion) of Colombia. B. K. N. was supported by United States National Science Foundation (NSF) Grant No. ECCS 1509094 and is grateful for the hospitality of Centre de Physique Théorique de Grenoble-Alpes where part of this work was done. S. O. V. acknowledges support from the European Research Council under Grant Agreement No. 308023 SPINBOUND. The supercomputing time was provided by Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant No. ACI-1053575.

Ideal crystalline graphene has exotic properties such as remarkably low dimensionality, high mobility and mechanical strength, tunable carrier type and density, etc. However, as with most other materials, defects are unavoidable during the preparation of graphene and can play a key role in many observables, and particularly electronic properties. The purpose of this chapter is to discuss the transport properties of realistic graphene with the increasing of disorder, beginning from single defects (vacancies) to line defects in Poly-G and finally to amorphous graphene, a strongly topological disordered graphene.

Quantum simulations are very important tools to study transport phenomena in the nanoscale. There are two numerical approaches for quantum transport simulations at the present, one is the widely used non-equilibrium Green’s function (NEGF) method, the other is the Kubo-Greenwood method.

Graphene has received a great attention since it was first isolated by Nobel Laureates Konstantin Novoselov and Andre K. Geim in 2004.

Carbon has a weak atomic SOC, so graphene is expected to have long spin relaxation time and phase coherence lengths. However, the spin injection measurements based on a non-local spin valve geometry [1, 2, 3] revealed surprisingly short spin relaxation times of only about 100–200 ps, being only weakly dependent on the charge density and temperature. The longest spin relaxation time has been measured up to now is also in the order of a few ns [4].

Esta tesis está enfocada en la modelización y simulación del transporte de carga y spin en materiales bidimensionales basados en Grafeno, así como en el impacto de la policristalinidad en el rendimiento de transistores de efecto campo diseñados con este tipo de materiales. Para este estudio se ha utilizado la metodología de transporte Kubo-Greenwood, la cual presenta grandes ventajas a la hora de realizar cálculos numéricos en sistemas microscópicos con el fin de obtener las propiedades de transporte de carga. Este trabajo cubre todos los tipos de desorden que pueden tener lugar en Grafeno, desde vacantes a la posible adsorción de especies químicas a lo largo de las fronteras de grano en el caso de Grafeno policristalino. Además tiene en cuenta importantes efectos cuánticos, como las interferencias cuánticas y los efectos debidos al acoplamiento spin-órbita intrínseco y extrínseco. Para el transporte de spin, se ha desarrollado un nuevo método basado en el formalismo de transporte en espacio real de orden O(N). Este nuevo método permite explorar y entender los mecanismos de relajación de spin en Grafeno y sus derivados. A partir de esta nueva metodología ha sido posible descubrir un nuevo mecanismo de relajación de spin basado en el acoplamiento entre spin y pseudospin (en presencia de un acoplamiento spin-órbita extrínseco o Rashba) que podría ser el mecanismo principal que gobierna la rápida relajación de spin observada experimentalmente en muestras de grafeno de alta calidad., This thesis is focused on modeling and simulation of charge and spin transport in two dimensional graphene-based materials as well as the impact of graphene polycrystallinity on the performance of graphene field-effect transistors. The Kubo-Greenwood transport approach has been used as the key method to carry out numerical calculations for charge transport properties. The study covers all kinds of disorder in graphene from vacancies to chemical adsorbates on grain boundaries of polycrystalline graphene and takes into account important quantum effects such as the quantum interferences and spin-orbit coupling effects. For spin transport, a new method based on the real space order O(N) transport formalism is developed to explore the mechanism of spin relaxation in graphene. A new spin relaxation phenomenon related to spin-pseudospin entanglement is unveiled and could be the main mechanism at play governing fast spin relaxation in ultra-clean graphene.

The prospect of transporting spin information over long distances in graphene, possible because of its small intrinsic spin-orbit coupling (SOC) and vanishing hyperfine interaction, has stimulated intense research exploring spintronics applications. However, measured spin relaxation times are orders of magnitude smaller than initially predicted, while the main physical process for spin dephasing and its charge-density and disorder dependences remain unconvincingly described by conventional mechanisms. Here, we unravel a spin relaxation mechanism for non-magnetic samples that follows from an entanglement between spin and pseudospin driven by random SOC, unique to graphene. The mixing between spin and pseudospin-related Berrya's phases results in fast spin dephasing even when approaching the ballistic limit, with increasing relaxation times away from the Dirac point, as observed experimentally. The SOC can be caused by adatoms, ripples or even the substrate, suggesting novel spin manipulation strategies based on the pseudospin degree of freedom., The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement number 604391 Graphene Flagship. This work was also funded by Spanish Ministry of Economy and Competitiveness under contracts MAT2012-33911 and MAT2010-18065. S.O.V. acknowledges ERC Grant agreement 308023 SPINBOUND.

Under the terms of the Creative Commons Attribution (CC BY) license to their work., We have used a multi-scale physics-based model to predict how the grain size and different grain boundary morphologies of polycrystalline graphene will impact the performance metrics of graphene field-effect transistors. We show that polycrystallinity has a negative impact on the transconductance, which translates to a severe degradation of the maximum and cutoff frequencies. On the other hand, polycrystallinity has a positive impact on current saturation, and a negligible effect on the intrinsic gain. These results reveal the complex role played by graphene grain boundaries and can be used to guide the further development and optimization of graphene-based electronic devices., We acknowledge support from SAMSUNG within the Global Innovation Program. The research leading to these results has received funding from Ministerio de Economía y Competitividad of Spain under the Project Nos. TEC2012-31330 and MAT2012-33911, and from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship.

Polycrystalline graphene is a patchwork of coalescing graphene grains of varying lattice orientations and size, resulting from the chemical vapor deposition (CVD) growth at random nucleation sites on metallic substrates. The morphology of grain boundaries has become an important topic given its fundamental role in limiting the mobility of charge carriers in polycrystalline graphene, as compared to mechanically exfoliated samples. Here we report new insights to the current understanding of charge transport in polycrystalline geometries. We created realistic models of large CVD-grown graphene samples and then computed the corresponding charge carrier mobilities as a function of the average grain size and the coalescence quality between the grains. Our results reveal a remarkably simple scaling law for the mean free path and conductivity, correlated to atomic-scale charge density fluctuations along grain boundaries.

The role of defect-induced zero-energy modes on charge transport in graphene is investigated using Kubo and Landauer transport calculations. By tuning the density of random distributions of monovacancies either equally populating the two sublattices or exclusively located on a single sublattice, all conduction regimes are covered from direct tunneling through evanescent modes to mesoscopic transport in bulk disordered graphene. Depending on the transport measurement geometry, defect density, and broken sublattice symmetry, the Dirac-point conductivity is either exceptionally robust against disorder (supermetallic state) or suppressed through a gap opening or by algebraic localization of zero-energy modes, whereas weak localization and the Anderson insulating regime are obtained for higher energies. These findings clarify the contribution of zero-energy modes to transport at the Dirac point, hitherto controversial. © 2013 American Physical Society., S. R. acknowledges the Spanish Ministry of Economy and Competitiveness for national project funding (MAT2012-33911), and SAMSUNG for support within the Global Innovation Program.

We investigate the charge transport properties of planar amorphous graphene that is fully topologically disordered, in the form of $s{p}^{2}$ threefold coordinated networks consisting of hexagonal rings but also including many pentagons and heptagons distributed in a random fashion. Using the Kubo transport methodology and the Lanczos method, the density of states, mean free paths, and semiclassical conductivities of such amorphous graphene membranes are computed. Despite a large increase in the density of states close to the charge neutrality point, all electronic properties are dramatically degraded, evidencing an Anderson insulating state caused by topological disorder alone. These results are supported by Landauer-B\"uttiker conductance calculations, which show a localization length as short as 5 nm.