The authors consider the simplest quantum mechanics model of solids, the tight binding model, and prove that in the continuum limit, the energy of tight binding model converges to that of the continuum elasticity mode...The authors consider the simplest quantum mechanics model of solids, the tight binding model, and prove that in the continuum limit, the energy of tight binding model converges to that of the continuum elasticity model obtained using Cauchy-Born rule. The technique in this paper is based mainly on spectral perturbation theory for large matrices.展开更多
Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this ...Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this purpose,efficient light emission/absorption and spectral tunability are necessary conditions.These are currently fulfilled by the direct bandgap materials.Si-QDs could offer the solution to major hurdles posed by these materials,namely,toxicity(e.g.,Cd-,Pb-or As-based QDs),scarcity(e.g.,QD with In,Se,Te)and/or instability.Here we show that by combining quantum confinement with dedicated surface engineering,the biggest drawback of Si—the indirect bandgap nature—can be overcome,and a‘direct bandgap’variety of Si-QDs is created.We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling.The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain‘direct bandgap-like’(phonon-less)character.This results in efficient fast emission,tunable within the visible spectral range by QD size.These findings are fully justified within a tight-binding theoretical model.When the C surface termination is replaced by oxygen,the emission is converted into the well-known red luminescence,with microsecond decay and limited spectral tunability.In that way,the‘direct bandgap’Si-QDs convert into the‘traditional’indirect bandgap form,thoroughly investigated in the past.展开更多
In spite of the last century of research into the physical properties of graphite,this material regularly displays new,unexpected features enabled by the variations of stacking between van der Waals coupled layers1–6...In spite of the last century of research into the physical properties of graphite,this material regularly displays new,unexpected features enabled by the variations of stacking between van der Waals coupled layers1–6.Here,we show that a stacking fault in bulk graphite hosts a band of two-dimensional electrons clearly distinguishable from the bulk carriers.Using a self-consistent tight-binding model of graphite,incorporating all Slonczewski-Weiss-McClure parameters,we compute the dispersion and quantum topological characteristics of the two dimensional band,we calculate the Landau level spectrum in magnetic field and the related Shubnikov-de Haas oscillation parameters,as well as the cyclotron mass of the two-dimensional carriers.We also show that most of the features of the faultbound states are inherited from another celebrated graphitic system,rhombohedral trilayer graphene7,which represents the central structural block of the stacking fault.展开更多
基金Project supported by the Natural Science Foundation(No. DMS 04-07866)the "Research Team on Complex Systems" of Chinese Academy of Sciences.
文摘The authors consider the simplest quantum mechanics model of solids, the tight binding model, and prove that in the continuum limit, the energy of tight binding model converges to that of the continuum elasticity model obtained using Cauchy-Born rule. The technique in this paper is based mainly on spectral perturbation theory for large matrices.
基金This work was financially supported by Stichting der Fundamenteel Onderzoek der Materie and Stichting voor de Technische Wetenschappen.Part of this work(CPU,JMJP andHZ)was financed by the Dutch Polymer Institute for funding of UCin Functional Polymer Systemsproject no.681,and(ANP andAAP)Russian Foundation for Basic Research and‘Dynasty’-Foundation of International Center for Fundamental Physics in Moscow.
文摘Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this purpose,efficient light emission/absorption and spectral tunability are necessary conditions.These are currently fulfilled by the direct bandgap materials.Si-QDs could offer the solution to major hurdles posed by these materials,namely,toxicity(e.g.,Cd-,Pb-or As-based QDs),scarcity(e.g.,QD with In,Se,Te)and/or instability.Here we show that by combining quantum confinement with dedicated surface engineering,the biggest drawback of Si—the indirect bandgap nature—can be overcome,and a‘direct bandgap’variety of Si-QDs is created.We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling.The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain‘direct bandgap-like’(phonon-less)character.This results in efficient fast emission,tunable within the visible spectral range by QD size.These findings are fully justified within a tight-binding theoretical model.When the C surface termination is replaced by oxygen,the emission is converted into the well-known red luminescence,with microsecond decay and limited spectral tunability.In that way,the‘direct bandgap’Si-QDs convert into the‘traditional’indirect bandgap form,thoroughly investigated in the past.
基金supported by EPSRC grants EP/S030719/1 and EP/V007033/1,Graphene-NOWNANO CDT,British Council Grant 1185409051International Science Partnerships Fund for supporting research collaboration between UK and Japan.
文摘In spite of the last century of research into the physical properties of graphite,this material regularly displays new,unexpected features enabled by the variations of stacking between van der Waals coupled layers1–6.Here,we show that a stacking fault in bulk graphite hosts a band of two-dimensional electrons clearly distinguishable from the bulk carriers.Using a self-consistent tight-binding model of graphite,incorporating all Slonczewski-Weiss-McClure parameters,we compute the dispersion and quantum topological characteristics of the two dimensional band,we calculate the Landau level spectrum in magnetic field and the related Shubnikov-de Haas oscillation parameters,as well as the cyclotron mass of the two-dimensional carriers.We also show that most of the features of the faultbound states are inherited from another celebrated graphitic system,rhombohedral trilayer graphene7,which represents the central structural block of the stacking fault.
