Point defects in two-dimensional hexagonal boron nitride: A perspective

Activating Basal Planes of NiPS 3 for Hydrogen Evolution by Nonmetal Heteroatom Doping

(2020) Jun Wang et al.   ADVANCED FUNCTIONAL MATERIALS

Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature

(2020) Andreas Gottscholl et al.   NATURE MATERIALS

Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy

(2020) Fariah Hayee et al.   NATURE MATERIALS

Growth of NiSe2, NiTe2 and alloy NiSe2− x Te x nanosheets with tunable shape evolution and chemical composition

(2020) Shijie He et al.   2D Materials

Gate-Tunable Topological Flat Bands in Trilayer Graphene Boron-Nitride Moiré Superlattices

(2019) Bheema Lingam Chittari et al.   PHYSICAL REVIEW LETTERS

Magnetic-field-dependent quantum emission in hexagonal boron nitride at room temperature

(2019) Annemarie L. Exarhos et al.   Nature Communications

Low temperature self-densification of high strength bulk hexagonal boron nitride

(2019) Haotian Yang et al.   Nature Communications

Two-dimensional magnetic crystals and emergent heterostructure devices

(2019) Cheng Gong et al.   SCIENCE

Observation of moiré excitons in WSe2/WS2 heterostructure superlattices

(2019) Chenhao Jin et al.   NATURE

Wide-Field Spectral Super-Resolution Mapping of Optically Active Defects in Hexagonal Boron Nitride

(2019) Jean Comtet et al.   NANO LETTERS

Phonon Polariton-assisted Infrared Nanoimaging of Local Strain in Hexagonal Boron Nitride

(2019) Bosai Lyu et al.   NANO LETTERS

Deterministic Quantum Emitter Formation in Hexagonal Boron Nitride via Controlled Edge Creation

(2019) Joshua Ziegler et al.   NANO LETTERS

Single Photon Sources in Atomically Thin Materials

(2019) Milos Toth et al.   Annual Review of Physical Chemistry

Doping Nanoscale Graphene Domains Improves Magnetism in Hexagonal Boron Nitride

(2019) Mengmeng Fan et al.   ADVANCED MATERIALS

Engineering and Tuning of Quantum Emitters in Few-Layer Hexagonal Boron Nitride

(2019) Noah Mendelson et al.   ACS Nano

Engineering Point-Defect States in Monolayer WSe2

(2019) Chendong Zhang et al.   ACS Nano

Epitaxial growth of a 100-square-centimetre single-crystal hexagonal boron nitride monolayer on copper

(2019) Li Wang et al.   NATURE

Magnetic 2D materials and heterostructures

(2019) M. Gibertini et al.   Nature Nanotechnology

Direct band-gap crossover in epitaxial monolayer boron nitride

(2019) C. Elias et al.   Nature Communications

High-entropy alloys

(2019) Easo P. George et al.   Nature Reviews Materials

Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes

(2018) Yongzhou Xue et al.   ACS Nano

Role of Hydrogen in High-Yield Growth of Boron Nitride Nanotubes at Atmospheric Pressure by Induction Thermal Plasma

(2018) Keun Su Kim et al.   ACS Nano

Quantum Emitters in Hexagonal Boron Nitride Have Spectrally Tunable Quantum Efficiency

(2018) Andreas W. Schell et al.   ADVANCED MATERIALS

Electronic structure and magnetism in transition metal doped InSe monolayer: A GGA + U study

(2018) Zhi Xie et al.   CERAMICS INTERNATIONAL

Exploring Two-Dimensional Materials toward the Next-Generation Circuits: From Monomer Design to Assembly Control

(2018) Mengqi Zeng et al.   CHEMICAL REVIEWS

Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures

(2018) Zhengyang Cai et al.   CHEMICAL REVIEWS

Recent progress in the tailored growth of two-dimensional hexagonal boron nitride via chemical vapour deposition

(2018) Jingyu Sun et al.   CHEMICAL SOCIETY REVIEWS

Two-dimensional transition metal dichalcogenides: interface and defect engineering

(2018) Zehua Hu et al.   CHEMICAL SOCIETY REVIEWS

Adsorption of 3d, 4d, and 5d transition-metal atoms on single-layer boron nitride

(2018) Shujing Li et al.   JOURNAL OF APPLIED PHYSICS

Imaging of Optically Active Defects with Nanometer Resolution

(2018) Jiandong Feng et al.   NANO LETTERS

Stark Tuning of Single-Photon Emitters in Hexagonal Boron Nitride

(2018) Gichang Noh et al.   NANO LETTERS

Single photon emission from plasma treated 2D hexagonal boron nitride

(2018) Zai-Quan Xu et al.   Nanoscale

Unconventional superconductivity in magic-angle graphene superlattices

(2018) Yuan Cao et al.   NATURE

Electric-field switching of two-dimensional van der Waals magnets

(2018) Shengwei Jiang et al.   NATURE MATERIALS

Controlling magnetism in 2D CrI3 by electrostatic doping

(2018) Shengwei Jiang et al.   Nature Nanotechnology

Electric-field control of magnetism in a few-layered van der Waals ferromagnetic semiconductor

(2018) Zhi Wang et al.   Nature Nanotechnology

Infrared hyperbolic metasurface based on nanostructured van der Waals materials

(2018) Peining Li et al.   SCIENCE

Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling

(2018) D. R. Klein et al.   SCIENCE

Anomalously low dielectric constant of confined water

(2018) L. Fumagalli et al.   SCIENCE

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal

(2018) Sanfeng Wu et al.   SCIENCE

Functionalized boron nitride membranes with ultrafast solvent transport performance for molecular separation

(2018) Cheng Chen et al.   Nature Communications

All-optical control and super-resolution imaging of quantum emitters in layered materials

(2018) Mehran Kianinia et al.   Nature Communications

Enhancement of Out-of-Plane Charge Transport in a Vertically Stacked Two-Dimensional Heterostructure Using Point Defects

(2018) Yanran Liu et al.   ACS Nano

Introduction of Protonated Sites on Exfoliated, Large-Area Sheets of Hexagonal Boron Nitride

(2018) Kabeer Jasuja et al.   ACS Nano

Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

(2018) Mingsheng Long et al.   ADVANCED FUNCTIONAL MATERIALS

Electronic and Optical Properties of 2D Materials Constructed from Light Atoms

(2018) Qunhong Weng et al.   ADVANCED MATERIALS

Photoluminescence upconversion by defects in hexagonal boron nitride

(2018) Qixing Wang et al.   NANO LETTERS

Strain engineering in two-dimensional nanomaterials beyond graphene

(2018) Shikai Deng et al.   Nano Today

Dynamic band-structure tuning of graphene moiré superlattices with pressure

(2018) Matthew Yankowitz et al.   NATURE

Heterointerface effects in the electrointercalation of van der Waals heterostructures

(2018) D. Kwabena Bediako et al.   NATURE

Correlated insulator behaviour at half-filling in magic-angle graphene superlattices

(2018) Yuan Cao et al.   NATURE

Twistable electronics with dynamically rotatable heterostructures

(2018) Rebeca Ribeiro-Palau et al.   SCIENCE

Direct electric field imaging of graphene defects

(2018) Ryo Ishikawa et al.   Nature Communications

Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2

(2018) Yujun Deng et al.   NATURE

Magnetism in two-dimensional van der Waals materials

(2018) Kenneth S. Burch et al.   NATURE

Wafer-scale single-crystal hexagonal boron nitride film via self-collimated grain formation

(2018) Joo Song Lee et al.   SCIENCE

Deeply subwavelength phonon-polaritonic crystal made of a van der Waals material

(2018) F. J. Alfaro-Mozaz et al.   Nature Communications

Anisotropic Etching of Hexagonal Boron Nitride and Graphene: Question of Edge Terminations

(2017) Yijing Y. Stehle et al.   NANO LETTERS

Deterministic Coupling of Quantum Emitters in 2D Materials to Plasmonic Nanocavity Arrays

(2017) Toan Trong Tran et al.   NANO LETTERS

Engineering two-dimensional electronics by semiconductor defects

(2017) Dan Wang et al.   Nano Today

First-principles investigation of quantum emission from hBN defects

(2017) Sherif Abdulkader Tawfik et al.   Nanoscale

Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

(2017) Cheng Gong et al.   NATURE

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

(2017) Bevin Huang et al.   NATURE

Ultralow-loss polaritons in isotopically pure boron nitride

(2017) Alexander J. Giles et al.   NATURE MATERIALS

Ligand-field helical luminescence in a 2D ferromagnetic insulator

(2017) Kyle L. Seyler et al.   Nature Physics

First-Principles-Based Method for Electron Localization: Application to Monolayer Hexagonal Boron Nitride

(2017) C. E. Ekuma et al.   PHYSICAL REVIEW LETTERS

Spin-Mechanical Scheme with Color Centers in Hexagonal Boron Nitride Membranes

(2017) Mehdi Abdi et al.   PHYSICAL REVIEW LETTERS

Optical Absorption and Emission Mechanisms of Single Defects in Hexagonal Boron Nitride

(2017) Nicholas R. Jungwirth et al.   PHYSICAL REVIEW LETTERS

Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit

(2017) I. Lovchinsky et al.   SCIENCE

Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor

(2017) Artur Branny et al.   Nature Communications

Large-scale quantum-emitter arrays in atomically thin semiconductors

(2017) Carmen Palacios-Berraquero et al.   Nature Communications

Tunable and high-purity room temperature single-photon emission from atomic defects in hexagonal boron nitride

(2017) Gabriele Grosso et al.   Nature Communications

Taming interfacial electronic properties of platinum nanoparticles on vacancy-abundant boron nitride nanosheets for enhanced catalysis

(2017) Wenshuai Zhu et al.   Nature Communications

Recent Development of Boron Nitride towards Electronic Applications

(2017) Natalia Izyumskaya et al.   Advanced Electronic Materials

Fluorinated h-BN as a magnetic semiconductor

(2017) Sruthi Radhakrishnan et al.   Science Advances

Robust Multicolor Single Photon Emission from Point Defects in Hexagonal Boron Nitride

(2016) Toan Trong Tran et al.   ACS Nano

Defect Graphene as a Trifunctional Catalyst for Electrochemical Reactions

(2016) Yi Jia et al.   ADVANCED MATERIALS

Functionalized hexagonal boron nitride nanomaterials: emerging properties and applications

(2016) Qunhong Weng et al.   CHEMICAL SOCIETY REVIEWS

Bright UV Single Photon Emission at Point Defects in h-BN

(2016) Romain Bourrellier et al.   NANO LETTERS

Structural Attributes and Photodynamics of Visible Spectrum Quantum Emitters in Hexagonal Boron Nitride

(2016) Nathan Chejanovsky et al.   NANO LETTERS

Formation and Dynamics of Electron-Irradiation-Induced Defects in Hexagonal Boron Nitride at Elevated Temperatures

(2016) Thang Pham et al.   NANO LETTERS

Atomic-Scale Structure and Local Chemistry of CoFeB–MgO Magnetic Tunnel Junctions

(2016) Zhongchang Wang et al.   NANO LETTERS

Temperature Dependence of Wavelength Selectable Zero-Phonon Emission from Single Defects in Hexagonal Boron Nitride

(2016) Nicholas R. Jungwirth et al.   NANO LETTERS

Hexagonal boron nitride is an indirect bandgap semiconductor

(2016) G. Cassabois et al.   Nature Photonics

Gaps induced by inversion symmetry breaking and second-generation Dirac cones in graphene/hexagonal boron nitride

(2016) Eryin Wang et al.   Nature Physics

Chromatic Aberration Correction for Atomic Resolution TEM Imaging from 20 to 80 kV

(2016) Martin Linck et al.   PHYSICAL REVIEW LETTERS

Atomic-scale control of graphene magnetism by using hydrogen atoms

(2016) H. Gonzalez-Herrero et al.   SCIENCE

2D materials and van der Waals heterostructures

(2016) K. S. Novoselov et al.   SCIENCE

Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

(2016) Johannes Jobst et al.   Nature Communications

Defect engineering of two-dimensional transition metal dichalcogenides

(2016) Zhong Lin et al.   2D Materials

The intrinsic defect structure of exfoliated MoS2 single layers revealed by Scanning Tunneling Microscopy

(2016) Péter Vancsó et al.   Scientific Reports

Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal Morphology

(2015) Yijing Stehle et al.   CHEMISTRY OF MATERIALS

Large-Scale Synthesis of Few-Layer F-BN Nanocages with Zigzag-Edge Triangular Antidot Defects and Investigation of the Advanced Ferromagnetism

(2015) Haibin Si et al.   NANO LETTERS

Tunable ferromagnetic ordering in MoS2 nanosheets with fluorine adsorption

(2015) Daqiang Gao et al.   Nanoscale

Controlling many-body states by the electric-field effect in a two-dimensional material

(2015) L. J. Li et al.   NATURE

Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies

(2015) Hong Li et al.   NATURE MATERIALS

Light-emitting diodes by band-structure engineering in van der Waals heterostructures

(2015) F. Withers et al.   NATURE MATERIALS

Single quantum emitters in monolayer semiconductors

(2015) Yu-Ming He et al.   Nature Nanotechnology

Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunnelling microscopy

(2015) Dillon Wong et al.   Nature Nanotechnology

Quantum emission from hexagonal boron nitride monolayers

(2015) Toan Trong Tran et al.   Nature Nanotechnology

Nature of the quantum metal in a two-dimensional crystalline superconductor

(2015) A. W. Tsen et al.   Nature Physics

Structure and Local Chemical Properties of Boron-Terminated Tetravacancies in Hexagonal Boron Nitride

(2015) Ovidiu Cretu et al.   PHYSICAL REVIEW LETTERS

Tunable Magnetism and Half-Metallicity in Hole-Doped Monolayer GaSe

(2015) Ting Cao et al.   PHYSICAL REVIEW LETTERS

Recent development in 2D materials beyond graphene

(2015) Ankur Gupta et al.   PROGRESS IN MATERIALS SCIENCE

Misfit accommodation mechanism at the heterointerface between diamond and cubic boron nitride

(2015) Chunlin Chen et al.   Nature Communications

Carbon-doped BN nanosheets for metal-free photoredox catalysis

(2015) Caijin Huang et al.   Nature Communications

Synthesis of large single-crystal hexagonal boron nitride grains on Cu–Ni alloy

(2015) Guangyuan Lu et al.   Nature Communications

Atomistic mechanisms of nonstoichiometry-induced twin boundary structural transformation in titanium dioxide

(2015) Rong Sun et al.   Nature Communications

Exploring atomic defects in molybdenum disulphide monolayers

(2015) Jinhua Hong et al.   Nature Communications

Synthesis of large-area multilayer hexagonal boron nitride for high material performance

(2015) Soo Min Kim et al.   Nature Communications

An Open Canvas—2D Materials with Defects, Disorder, and Functionality

(2014) Xiaolong Zou et al.   ACCOUNTS OF CHEMICAL RESEARCH

Carbon-Doped Boron Nitride Nanosheets with Ferromagnetism above Room Temperature

(2014) Chong Zhao et al.   ADVANCED FUNCTIONAL MATERIALS

One-Step Exfoliation and Fluorination of Boron Nitride Nanosheets and a Study of Their Magnetic Properties

(2014) Miao Du et al.   ANGEWANDTE CHEMIE-INTERNATIONAL EDITION

Band gap effects of hexagonal boron nitride using oxygen plasma

(2014) Ram Sevak Singh et al.   APPLIED PHYSICS LETTERS

Evidence for Active Atomic Defects in Monolayer Hexagonal Boron Nitride: A New Mechanism of Plasticity in Two-Dimensional Materials

(2014) Ovidiu Cretu et al.   NANO LETTERS

Highly confined low-loss plasmons in graphene–boron nitride heterostructures

(2014) Achim Woessner et al.   NATURE MATERIALS

Two-dimensional material nanophotonics

(2014) Fengnian Xia et al.   Nature Photonics

Spin-polarized quantum confinement in nanostructures: Scanning tunneling microscopy

(2014) Hirofumi Oka et al.   REVIEWS OF MODERN PHYSICS

Doped h-BN monolayer as efficient noble metal-free catalysts for CO oxidation: the role of dopant and water in activity and catalytic de-poisoning

(2014) S. Sinthika et al.   Journal of Materials Chemistry A

Strain engineering of magnetic states of vacancy-decorated hexagonal boron nitride

(2013) Bin Ouyang et al.   APPLIED PHYSICS LETTERS

Nano boron nitride flatland

(2013) Amir Pakdel et al.   CHEMICAL SOCIETY REVIEWS

Tuning the electronic and magnetic properties in zigzag boron nitride nanoribbons with carbon dopants

(2013) L.L. Song et al.   COMPUTATIONAL MATERIALS SCIENCE

First-Principles Investigations of Metal (Cu, Ag, Au, Pt, Rh, Pd, Fe, Co, and Ir) Doped Hexagonal Boron Nitride Nanosheets: Stability and Catalysis of CO Oxidation

(2013) Sen Lin et al.   Journal of Physical Chemistry C

Atomic Resolution Imaging of Grain Boundary Defects in Monolayer Chemical Vapor Deposition-Grown Hexagonal Boron Nitride

(2013) Ashley L. Gibb et al.   JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

Ultrahard nanotwinned cubic boron nitride

(2013) Yongjun Tian et al.   NATURE

Electronic structure of boron nitride sheets doped with carbon from first-principles calculations

(2013) Natalia Berseneva et al.   PHYSICAL REVIEW B

Strong Renormalization of the Electronic Band Gap due to Lattice Polarization in theGWFormalism

(2013) Silvana Botti et al.   PHYSICAL REVIEW LETTERS

Semihydrogenated BN Sheet: A Promising Visible-light Driven Photocatalyst for Water Splitting

(2013) Xingxing Li et al.   Scientific Reports

Mechanics and Mechanically Tunable Band Gap in Single-Layer Hexagonal Boron-Nitride

(2013) Jiangtao Wu et al.   Materials Research Letters

Tunable magnetic states in hexagonal boron nitride sheets

(2012) Eduardo Machado-Charry et al.   APPLIED PHYSICS LETTERS

Magnetic states and optical properties of single-layer carbon-doped hexagonal boron nitride

(2012) Hyoungki Park et al.   APPLIED PHYSICS LETTERS

Structure, stability and defects of single layer hexagonal BN in comparison to graphene

(2012) G J Slotman et al.   JOURNAL OF PHYSICS-CONDENSED MATTER

Strain-Engineering of Band Gaps in Piezoelectric Boron Nitride Nanoribbons

(2012) Jingshan Qi et al.   NANO LETTERS

Spin-half paramagnetism in graphene induced by point defects

(2012) R. R. Nair et al.   Nature Physics

Band gap engineering by functionalization of BN sheet

(2012) A. Bhattacharya et al.   PHYSICAL REVIEW B

Effective Control of the Charge and Magnetic States of Transition-Metal Atoms on Single-Layer Boron Nitride

(2012) Bing Huang et al.   PHYSICAL REVIEW LETTERS

Subangstrom Edge Relaxations Probed by Electron Microscopy in Hexagonal Boron Nitride

(2012) Nasim Alem et al.   PHYSICAL REVIEW LETTERS

Core-Level Spectroscopy of Point Defects in Single Layerh-BN

(2012) Kazu Suenaga et al.   PHYSICAL REVIEW LETTERS

Dislocation-Driven Deformations in Graphene

(2012) J. H. Warner et al.   SCIENCE

Electron-Beam-Induced Substitutional Carbon Doping of Boron Nitride Nanosheets, Nanoribbons, and Nanotubes

(2011) Xianlong Wei et al.   ACS Nano

Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition

(2011) Ki Kang Kim et al.   NANO LETTERS

Atom-resolved imaging of ordered defect superstructures at individual grain boundaries

(2011) Zhongchang Wang et al.   NATURE

Probing the Out-of-Plane Distortion of Single Point Defects in Atomically Thin Hexagonal Boron Nitride at the Picometer Scale

(2011) Nasim Alem et al.   PHYSICAL REVIEW LETTERS

Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

(2011) J. N. Coleman et al.   SCIENCE

Unusual magnetic properties of graphene and related materials

(2011) C. N. R. Rao et al.   Chemical Science

X-ray Excited Optical Luminescence from Hexagonal Boron Nitride Nanotubes: Electronic Structures and the Role of Oxygen Impurities

(2010) Lijia Liu et al.   ACS Nano

Structural Defects in Graphene

(2010) Florian Banhart et al.   ACS Nano

Graphene Analogues of BN: Novel Synthesis and Properties

(2010) Angshuman Nag et al.   ACS Nano

Boron Nitride Nanotubes and Nanosheets

(2010) Dmitri Golberg et al.   ACS Nano

Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition

(2010) Yumeng Shi et al.   NANO LETTERS

Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers

(2010) Li Song et al.   NANO LETTERS

Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy

(2010) Ondrej L. Krivanek et al.   NATURE

Atom-by-atom spectroscopy at graphene edge

(2010) Kazu Suenaga et al.   NATURE

Electronic and magnetic properties of a BN sheet decorated with hydrogen and fluorine

(2010) Jian Zhou et al.   PHYSICAL REVIEW B

Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties

(2009) Chunyi Zhi et al.   ADVANCED MATERIALS

First-principles simulations on bulk Ta2O5 and Cu/Ta2O5/Pt heterojunction: Electronic structures and transport properties

(2009) Tingkun Gu et al.   JOURNAL OF APPLIED PHYSICS

The Nature of Radiative Transitions in O-Doped Boron Nitride Nanotubes

(2009) Gaoyang Gou et al.   JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

Dots versus Antidots: Computational Exploration of Structure, Magnetism, and Half-Metallicity in Boron−Nitride Nanostructures

(2009) Aijun Du et al.   JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

Selective Sputtering and Atomic Resolution Imaging of Atomically Thin Boron Nitride Membranes

(2009) Jannik C. Meyer et al.   NANO LETTERS

Atomically thin hexagonal boron nitride probed by ultrahigh-resolution transmission electron microscopy

(2009) Nasim Alem et al.   PHYSICAL REVIEW B

Monolayer honeycomb structures of group-IV elements and III-V binary compounds: First-principles calculations

(2009) H. Şahin et al.   PHYSICAL REVIEW B

First-principles study of two- and one-dimensional honeycomb structures of boron nitride

(2009) M. Topsakal et al.   PHYSICAL REVIEW B

Fabrication of a Freestanding Boron Nitride Single Layer and Its Defect Assignments

(2009) Chuanhong Jin et al.   PHYSICAL REVIEW LETTERS

Graphene at the Edge: Stability and Dynamics

(2009) C. O. Girit et al.   SCIENCE

Excess-silver-induced bridge formation in a silver sulfide atomic switch

(2008) Zhongchang Wang et al.   APPLIED PHYSICS LETTERS

Direct Imaging of Lattice Atoms and Topological Defects in Graphene Membranes

(2008) Jannik C. Meyer et al.   NANO LETTERS